MIC2871
1.2A High-Brightness LE D Fl ash Driver with
Single-Wire Serial Interface
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
May 29, 2013
052913-1.0 (while in progress)
Revision 1.0 (final document)
General Description
The MIC2871 is a high-current, high-efficiency flash LED
driver. The LED driver current is generated by an
integrate d i nductive boos t conver ter w ith a 2MHz switchin g
frequency which allows the use of very small inductor and
output capacitor. These features make the MIC2871 an
ideal solution for high-resolution camera phone LED flash
light driver applications.
The MIC2871 operates in either flash or torch modes that
can be controlled through the single-wire serial interface
and/or external control pins. Default flash and torch
brightness can be adjusted via an external resistor. A
robust single-wire serial interface allows simple control by
the host processor to support typical camera functions
such as auto-focus, white balance, and image capture
(flash mode).
The MIC2871 is available in a 14-pin 3mm × 2mm LDFN
package with a junction temperature range of −40°C to
+125°C.
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
Features
• Up to 1.2A flash LED driving current
• Highly-efficient, synchronous boost driver (up to 94%)
• ±5% LED current accuracy
• Control through single-wire serial interface or external
control pins
• Input voltage range: 2.7V to 5.5V
• True load disconnect
• Configurable safety time-out protection
• Output overvoltage protection (OVP)
• LED short detection and protection.
• 1µA shutdown current
• Available in 14-p in 3m m × 2mm LDFN package
Applications
• Camera phones/mobile handsets
• Cell phones/smartphones
• LED light for image capture/auto focus/white balance
• Handset video light (torch light)
• Digital cameras
• Portable applications
Typical Application
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MIC2871
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Ordering Information
Part Number Marking Code Temperature Range Package
(1)
Lead Finish
MIC2871YMK 2871 –40°C to +125°C 14-pin 3mm × 2mm LDFN Pb-Free
Note:
1. Package is a GREEN RoHS-compliant pack age. Lead finish is Pb-Free. Mold compound is Halogen Free.
Pin Configuration
14-Pin 3mm × 2mm LDFN (MK)
(Top View)
Pin Description
Pin
Number Pin Name Pin Function
1 AGND1 Analog Ground. LED current return path.
2 DC
Single-wire serial interface control input.
3 LED
LED Current Sink Pin. Connect the LED anode to OUT and cathode to this pin.
4 FEN1 Flash Mode Enable Pin. Toggling FEN1 from LO W to HIGH enables MIC2871 into the flash
mode. FEN 1 is logic-OR with FEN2. If this pin is left floating, it is pulled-down internally by a built-
in 1µA current source when the device is enabled.
5 AGND2 Analog Ground. Reference ground of the FRSET pin.
6 VIN Supply Input Pin. Connect a low-ESR ceramic capacitor of at least 2.2µF to AGND2.
7 PGND1
Power Ground. Inductor current return path.
8 OUT Boost Converter Output Pin. This is connected to the anode of the LED. Connect a low ESR
ceramic capacitor of at least 4.7µF to PGND1.
9, 12 NC No Connect. Connect this pin to AGND or leave it floating.
10 SW
Inductor Connection Pin. It is connected to the internal power MOSFETs.
11 FEN2 Additional Flash Mode Enable Pin. FEN2 is logic-OR with FEN1. If this pin is left floating, it is
pulled-down internally by a built-in 1µA current source when the device is enabled.
13 PGND2
Power Ground.
14 FRSET
Flash Mode Current-Level Programming Pin. Connect a resistor from this pin to AGND2 to set
the maximum current in the flash mode. This pin may be grounded if the default flash mode
current (1A) is desired. This pin cannot be left floating and the recommended resistance range is
from 17kΩ t o 60k Ω.
EP ePad Exposed Heat Sink Pad. Connect to ground for best thermal performance.
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Absolute Maximum Ratings(2)
Input Voltage (VIN) ........................................ −0.3V to +6.0V
General I/O Voltage (VFEN1, VFEN2) .................... −0.3V to VIN
VOUT and VLED Voltage .................................. −0.3V to +6.0V
Single-Wire I/O Voltage (VDC) ........................... −0.3V to VIN
VFRSET Voltage ................................................... −0.3V to VIN
VSW Voltage .................................................. −0.3V to +6.0V
Lead Temperature (soldering, 10s) .......................... +260°C
Junction Temperature ................................... 0°C to +150°C
Storage Temperature (Ts) ......................... −40°C to +150°C
ESD Rating(5) ............................... 2kV, HBM and 200V, MM
Operating Ratings(3)
Input Voltage (VIN) .......................................... 2.7V to +5.5V
Enable Input Voltage (VFEN1, VFEN2) ....................... 0V to VIN
Single-Wire I/O Voltage (VDC) ................................ 0V to VIN
Junction Temperature (TJ) ........................ −40°C to +125°C
Power Dissipation (PD) ........................... Interna lly Limited(4)
Package Thermal Resistance
3mm × 2mm LDFN (θJA)(4) ............................ 65.83°C/W
3mm × 2mm LDFN (θJC)(4) .............................................. 38.9°C/W
Electrical Characteristics(6)
VIN = 3.6V, L = 1µH, COUT = 4.7µF, RFRSET = 20.5kΩ, IOUT = 100mA, TA = 25°C, bold values indicate -40°C ≤ TJ ≤ +125°C,
unless otherw ise not ed..
Symbol Parameter Condition Min. Typ. Max. Units
Power Supply
VIN Supply Voltage Range 2.7 5.5 V
VSTART Start-Up Voltage 2.65 2.95 V
VUVLO UVLO Threshold (falling) 2.30 2.5 V
ISTB Standby Current VDC = HIGH, boost regulator and LED current
driver both OFF. 230 µA
ISD Shutdown Current VDC = 0V 1 2 µA
VOVP Overvoltage Protection (OVP)
Threshold 5.2 5.37 5.55 V
OVP Hysteresis 60 mV
OVP Blanking Time 23 µs
DMAX Maximum Duty Cycle 82 86 90 %
ISW Swit ch Current Limit VIN = VOUT = 2.7V 3.5 4.5 5.5 A
DMIN Minimum Duty Cycle 4 6.4 9 %
PMOS Switch On-Resistance ISW = 100mA 100 mΩ
NMOS ISW = 100mA
ISW Switch Leakage Current VDC = 0V, VSW = 5.5V 0.01 1 µA
FSW Oscillator Frequency 2 MHz
Oscillator Frequency Variation −10 10 %
Notes:
2. Exceeding the absolute maximum rating may damage the device.
3. The device is not guarant eed to function outside its operat i ng rating.
4. The maxim um allowable power dissi pation of any TA (ambient temperature) is PD(max) = (TJ(max) – TA) / θJA.
Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go int o thermal shutdown.
5. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.
6. Specific at i on for packaged product only.
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Electrical Characteristics(6)
VIN = 3.6V, L = 1µH, COUT = 4.7µF, RFRSET = 20.5kΩ, IOUT = 100mA, TA = 25°C, bold values indicate -40°C ≤ TJ ≤ +125°C,
unless otherw ise not ed.
Symbol Parameter Conditions Min. Typ. Max. Units
TSD Overtemperature Shutdown
Threshold 155 °C
Overtemperature Shutdown
Hysteresis 15 °C
TTO Safety Timeout Shutdown Default timer setting 1.25 s
ITO Safety Timer Current Threshold Default current threshold setting 250 mA
Safety Timer Current Resolution 50 mA
Safety Timer Current-Threshold
Accuracy 25 mA
VLBVD Low-Battery Voltage Detection
Threshold
Default LBVD threshold setting 3.6 V
Low-Battery Voltage Detection
Threshold Accuracy 50 mV
VSHORT LED Short-Circuit Detection
Voltage Threshold VOUT − VLED 1.7 V
ITEST LED Short-Circuit Detection
Test Current 1 2 3 mA
Current Sink Channels
Channel Current Accuracy 3.5V < VIN <4.2V, ILED = 1A −5 5 %
VLED Current Sink Voltage Dropout
Boost regulator ON, ILED = 1A
160
mV
FEN1, FEN2 Control Pins
FEN1/FEN2 Threshold Voltage FLASH ON
1.5 V
FLASH OFF
0.4
FEN1/FEN2 Pull-down Current FEN1 = FEN2 = 5.5V 1 5 µA
Electrical Characteristics – Single-Wire Interface (Guaranteed by Design)
VIN = 3.6V, L = 1µH, COUT = 4.7µF, IOUT= 100m A, TA = 25°C, bold v alu es indi cat e -40°C ≤ TJ ≤ +125°C, unless otherwise noted.
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Units
LOW-Level Input Voltage 0.4 V
HIGH-Level Input Voltage 1.5 V
DC Pull-Down Current DC = 5.5V 2.5 5 µA
TON ON Time 0.1 72 µs
TOFF OFF Time
0.1 72 µs
TLAT Latch Time
97 324 µs
TEND END Time 405 µs
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Typical Characteris tics
Shut dow n Current
vs. Temperature
0.0
0.1
0.2
0.3
0.4
0.5
0.6
-40 -20 020 40 60 80 100 120
TEM PERATURE ( °C)
SHUTDOWN CURRENT ( µA)
St andby Current
vs. Temperature
220
225
230
235
240
245
-40 -20 020 40 60 80 100 120
TEM PERATURE ( °C)
STANDBY CURRENT (µA)
Boost Sw itching Frequency
vs. Input V oltage
1.80
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.20
2.5 3.0 3.5 4.0 4.5
INPUT V OLTAGE (V)
SWITCHI NG F RE QUENCY ( MHz)
-40°C
125°C
75°C
25°C
L = 1 µH
C
OUT
= 1 µF
I
LED
= 1A
50
60
70
80
90
100
2.6 3.0 3.4 3.8 4.2 4.6 5.0
EFFICIENCY (%)
INPUT VOLTAGE (V)
WLED Power Efficiency
vs. Input Voltage
L = 1µH
C
OUT
= 4.7µF I
LED
= 100mA
I
LED
= 250mA
I
LED
= 400mA
I
LED
= 640mA
I
LED
= 1.2A
I
LED
= 1.0A
230
235
240
245
250
255
260
265
270
-40 -20 020 40 60 80 100 120
TORCH MODE LED CURRENT (mA)
TEMPERATURE (°C)
Torch Mode LED Current
vs. Temperature
L = 1µH
C
OUT
= 4.7µF
I
LED
= 250mA
R
FRSET
= 20kΩ
0.80
0.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
-40 -20 020 40 60 80 100 120
FLASH MODE LED CURRENT (A)
TEMPERATURE (°C)
Flash Mode LED Current
vs. Tempera ture
L = 1µH
C
OUT
= 4.7µF
I
LED
= 1A
R
FRSET
= 20kΩ
Fl ash Mode ILED(MA X)
vs. FRSET R esisto r
0
200
400
600
800
1000
1200
010 20 30 40 50 60
FRSET RESISTOR (kΩ)
FLASH MO DE ILED(MAX) (mA)
L = 1 µH
C
OUT
= 4. 7µF
Torch Mode ILED(MA X)
vs. FRSET R esist or
0
50
100
150
200
250
300
010 20 30 40 50 60
FRSET RESI STOR (kΩ)
L = 1 µH
C
OUT
= 4.7µF
TORCH MODE IL ED(MAX ) (mA)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
3.5 3.7 3.9 4.1 4.3
FLASH MODE ILED(MAX) ACCURACY ( %)
INPUT VOLTAGE (V)
Flash Mode ILED(MAX)
Accuracy vs. Input Voltage
R
FRSET = 17kΩ
RFRSET = 20kΩ
RFRSET = 30kΩ
RFRSET = 39kΩ
RFRSET = 62kΩ
RFRSET = 51kΩ
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Typical Characteristics (Continued)
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
3.5 3.7 3.9 4.1 4.3
TORCH MODE ILED(MAX) ACCURACY (%)
INPUT VOLTAGE (V)
Torch Mode ILED(MAX)
Accuracy vs. Input Voltage
R
FRSET = 51kΩ
RFRSET = 62kΩ
RFRSET = 39kΩ
RFRSET = 30kΩ
RFRSET = 20kΩ
RFRSET = 17kΩ
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Functional Characteristics
Micrel, Inc.
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Functional Diagram
Figure 1. Simplified MIC2871 Functional Block Diagram
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Functional Description
VIN
The input supply provides power to the internal
MOSFETs gate drive an d contr ols circu itry for the s witch-
mode r egulator. The operat ing input volta ge rang e is f rom
2.7V to 5.5V. A 2.2µF low-ESR ceramic input capacitor
should be conn ected from VIN to AGND2 as close to the
MIC2871 as pos sib le to ens ure a clean s uppl y voltage for
the device. The minimum voltage rating of 10V is
recom m ended for the input c apac itor.
SW
The MIC2871 has internal low-side and synchronous
MOSFET switches. The switch node (SW) between the
internal MOSFET switches connects directly to one end
of the inductor and provides the current paths during
switching cycles.
The other end of the inductor is connected to the input
supply voltage. Due to the high-speed switching on this
pin, the switch node should be routed away from
sensitive nod es wherever possible.
AGND1
This is the ground path of the LED current sink . It should
be connec ted to the AGND 2, but not via ex posed pad , on
the PCB. The current loop of the analog ground should
be separ ated from that of the power gro und (PGN D1 and
PGND2). AGND1 and AGND2 should be connected to
PGND1 and PGND2 at a single point.
AGND2
This is the gr ound path f or the int erna l biasi ng an d c ont r ol
circuitr y. AG ND2 s hou ld be c onnected to the PCB pad f or
the package exposed pad. AGND2 should be connected
to the AGND1 direc tly without go ing through the expos ed
pad. The current loop of the analog ground should be
separated from that of the power ground (PGND1 and
PGND2). The AGND2 and AGND1 should be connected
to PGND1 and PGND2 at a single point.
PGND1 and PGND2
The power ground pins are the ground path for the high
current in the boost switch and they are internally
connected together. The current loop for the power
ground s hould be as sm all as possib le and s eparat e f rom
the analog ground (AGND) loop as applicable.
OUT
Boost converter output pin which is connected to the
anode of t he LED. A lo w-ESR ceram ic capac itor of 4.7µF
or larger should be connected from OUT to PGND1 as
close as possible to the MIC2871. The minimum voltage
rating of 10V is recommended for the output capacitor.
LED
The current sink pin for the LED. The LED anode is
connected to the OUT pin and the LED cathode is
connecte d to this pin.
DC
The DC is a single multiplexed device enable and serial
data control pin used for functional control and
communication in GPIO-limited applications. When the
DC pin is used as a hardware device enable pin, a logic
high sign al on the DC pin enab les the device , and a lo gic
low signal on the DC pin disables the device. When the
DC pin is used as the single-wire serial interface digital
control pin, a combination of bit edges and the period
between edges is used to communicate a variable length
data word across the single wire. Each word is
transmitted as a series of pulses, each pulse
incrementing an internal data counter. A stop sequence
consisting of an inactive period is used to latch the data
word internally. The data word received is then used to
set the value of the corresponding register for controlling
specific function. The MIC2871 supports five writeable
registers for controlling flash mode, torch mode, safety
timer duration, safety timer threshold current, and low-
battery threshold.
An address/data frame is used to improve protection
against erroneous writes where communications are in
error. W hen DC is in a low state and no data is detec ted
for an extended period of time, the MIC2871 will
automatically go into a low-power SHUTDOWN state,
simultaneously resetting all internal registers to their
default states.
FEN1 and FEN2
FEN1 and FEN2 are hardware enable pins for flash
mode. FEN1 is logic-OR with FEN2. A logic low-to-high
transition on either FEN1 pin or FEN2 pin can initiate the
MIC2871 in f lash mode. If F EN1 or F E N2 is l ef t f loatin g, it
is pulled down internally by a built-in 1µA current source
when the device is enabled. Flash mode is terminated
when both F EN1 and FE N2 are pull ed low or lef t floating,
and the flash register is cleared.
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FRSET
The flash mode maximum LED current level is
programmed through the FRSET pin. A resistor
connected from the FRSET pin to AGND2 set the
maximum current in the flash mode. This pin can be
grounded if the default flash mode current of 1A is
desired. For the best current accuracy, 0.1% or smaller
tolerance resistor for setting the maximum flash mode
LED current is recommended. This pin cannot be left
floating and the minimum resistance is limited to 17kΩ.
The maximum flash mode current to maximum Torch
mode current ratio is internally fixed as 4 to1.
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Application Information
The MIC2871 can drive a high-current flash WLED in
either flash mode or torch mode.
Boost Converter
The intern al bo os t c on vert e r is tur n ed on/ of f autom atic all y
when the LED dri ver is act ivat ed/de-ac tivated with out any
exception.
The boost converter is an internally-compensated
current-mode PWM boost converter running at 2MHz. It
is for stepping up the supply voltage to a high enough
value at the OUT pin to drive the LED current. If the
suppl y vo ltage is high enough, the s ynchronous switch of
the converter is then fully turned on. In this case, all the
excessive voltage is dropped over the linear LED driver.
Flash Mode
The maximum current level in the flash mode is 1.2A.
This current level can be adjusted through an external
resistor connecting to the FRSET pin according to
Equation 1:
FRSET
LED(MAX) R
20500
I
=
Eq. 1
Alternatively, the default value of 1A is used when the
FRSET pin is grounded.
The flash mode current can be initiated at this preset
FRSET brightness level by asserting FEN1 or FEN2 pin
HIGH, or by setting the flash control register (address 1),
for the desired flash duration, subjected to the safety
timeout setting. The flash mode current is terminated
when the FE N1 and F EN2 pins are broug ht LOW and the
flash register is cleared.
Flash mode current can be adjusted to a fraction of the
maximum flash mode current level by selecting the
desired percentage in the flash control register through
the single-wire serial interface. The flash current is the
product of the maximum flash current setting and the
percentage selected in the flash register.
Torch Mode
By default, the maximum torch mode level is one-fourth
(1/4) of the maximum flash mode current. The torch
mode operation is activated by setting the torch control
register (address 2) for the desired duration. The torch
mode current is terminated when the torch register is
cleared or when the configurable safety timer expires.
Like the flash mode current, the torch mode current can
be tuned to a fraction of the maximum torch mode level
by selecting the desired torch current level percentage in
the torch control register (address 2) through the single-
wire serial interface.
The torch current is the product of the maximum Torch
current setting and the percentage selected in the torch
register.
Configurable Safety Timer
The f lash s af et y timeout f eatur e aut omaticall y shu ts d o wn
the LED current after the safety timer duration is expired
if the programmed LED c urrent exceeds a certain current
threshold. Both the current threshold and the timer
duration are programmable via the safety timer registers
(addresses 3 and 5).
Low-Battery Voltage Detection (LBVD)
When the VIN voltage drops below the LBVD threshold
(default = 3.6V) in flash or torch mode, the LED current
driver is disabled. The LED driver can be resumed by
toggling the corresponding input control signal. The
LBVD threshold is adjustable thru the LBVD control
register (address 4).
Overvoltage Pr o tection
When the output voltage rises above the OVP threshold,
MIC2871 is latched off automatically to avoid permanent
damage to the IC. To clear the latched off condition,
either power cycle the MIC2871 or assert the DC pin
LOW.
Short-Circuit Detection
Each time before enabling the LED driver, the MIC2871
performs the short circuit test by driving the flash LED
with a small (2mA typical) current for 200µs. If (VOUT –
VLED) < 1.7V at the end of the s hor t-c irc uit test, t he L E D is
considered to be shorted and MIC2871 will ignore the
flash and/or torch mode command. Note that the short-
circuit test is carried out every time prior to flash and
torch mode but the result is not latched.
Thermal Shu td o wn
When the internal die temperature of MIC2871 reaches
155°C, the LED driver is disabled until the die
temperature falls below 140°C.
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Single-Wire Interface
The single-wire interface allows the use of a single
multiplexed enable and data pin (DC) for control and
communication in GPIO limited applications. The
interface is implemented using a simple mechanism
allowing an y open drain or directl y driven GPIO to c ontrol
the MIC2871.
The MIC2871 uses the single-wire interface for simple
command and control functions. The interface provides
fast ac cess to write onl y register s with protec tion featu res
to avoid potentially erroneous data writes and improve
robustness. When DC is in a low state and no data is
detected f or an ex tend ed period of tim e, the MIC 28 71 w ill
automatically go into a low-power SHUTDOWN state,
simultaneously resetting internal registers to default
states.
Overview
The single-wire interface relies on a combination of bit
edges and the period between edges in order to
communicate across a single wire. Each word is
transmitted as a series of pulses, with each pulse
incrementing an internal data counter. A stop sequence
consisting of an inactive period is used to latch the data
word internally. An address and data framing format is
used to improve protection against erroneous writes by
enforcing address and data field lengths as well as the
timing duration between them.
Tim ing is designed s uch that when comm unicating with a
device using a low cost on chip oscillator, the worst case
minimum and maximum conditions can be easily met
within the wide operating range of the oscillator. Using
this m ethod g uar a ntees t ha t the de vice c an a l wa ys d et ec t
the delay introduced by the communication master.
Idle States and Error Condition s
In shutdown mode, the MIC2871 is in a reset condition
with all functions off while consuming minimal power.
Register settings are reset to default state when coming
out of shutdown state. In idle mode, all register settings
persist and a ll MIC2871 functions c onti nue in the ir c ur rent
state. Table 1 summarises the difference between the
two idle modes:
Table 1. Differen ces b et wee n Idle Modes
DC Shutdown Idle
Low High
ISUPPLY
(all functions off) 1μA 230μA
Register State Default Persist
Start-Up Time 1μs 100ns
Idle mode is entered automatically at the end of a
communication frame by holding DC high for ≥TEND, by
enabling the device by bringing DC high when in
shutdown mode, or when an error is detected by the
single-wire interface logic.
Shutdown mode can be entered at any time by pulling
down DC for ≥TEND, discarding any current
communication and resetting the internal registers. If a
communication is received before the shutdown period
but after the T LAT period, t he comm unication is disc arded.
This state is also used to create an internal error state to
avoid erro neo us ly latc h in g data wher e th e comm unication
process cannot be serviced in time. Additionally, each
register has a maximum value associated with it. If the
number of bits cloc ked in exc eeds the maximum value f or
the register, the data is assumed to be in error and the
data is discarded.
Figure 2. Abort, Shutdown, and Idle Timing Waveforms
Communication Details
The serial interface requires delimiters to indicate the
start of frame, data as a series of pulses, and end of
fram e indicated b y a lac k of activit y. T he st art of f rame is
the first high to low transition of DC when in idle mode.
The first rising edge resets the intern al dat a cou nter to 0.
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Figure 3. Data Word Pulse Timing
A pulse is del im ited b y the s ignal f irst go ing b elo w VL and
then above VH within the latch timeout TLAT. During this
transition, minimum on (TON) and off (TOFF) periods are
observed to improve tolerance to glitches. Each rising
edge increments the internal data register. Data is
automatically latched into internal shadow address and
data registers after an inactivity period of >TLAT.
To send register write commands, the address and data
are entered in series as two data words using the above
pattern, with the second word starting after the first latch
period h as expired. Af ter the second word is entered, the
IDLE command should be issued by leaving the DC pins
high for ≥ TEND.
After receiving the stop sequence, the internal registers
decode and update cycle is started, with the shadow
register valu es being transf erred to the decoder. Figur e 4
shows an example of entering a write of data 5 to
address 3.
Figure 4. Communication Timing Example of Entering Write
for Data 5 to Address 3
Only corr ectl y form atted ad dress /data c om bination will be
treated as a valid frame and processed by the MIC2871.
Any other input, such as a single data word followed by
TEND, or three suc cess ive d ata words will b e disc arded b y
the target hardware as an erroneous entry. Additionally,
any register write to either an invalid register or with
invalid register data will also be discarded.
MIC2871 Registers
The MIC2871 supports five writeable registers for
controlling the torch and the flash modes of operation as
shown in Table 2. Note that register addressing starts at
1. Wr iting any v alue a bove the max imum valu e sho wn f or
each registers will cause an invalid data error and the
frame will be discarded.
Table 2. Five Writable Registers of MIC2871
Address Name Max.
Value Description
1 FEN/FCUR 31 Flash Enable/Current
2 TEN/TCUR 31 Torch Enable/Current
3 STDUR 7 Safety Ti m er Duration
4 LB_TH 9 Low Battery Voltage
Detection Threshold
5 ST_TH 5 Safety Timer Threshold
Flash Current Register (FEN/FCUR: default 0)
The flash current register enables and sets the flash
mode current level. Valid values are 0 to 31; values 0 −
15 will set the flash current without enabling the flash
(such that it can be triggered externally), values 16 − 31
will set the flash current and enable the flash. The flash
current register maps into the internal FEN and FCUR
registers as shown in the table below. Table 3 describes
the relationship between the flash current as a
percentage of maximum current, and the FCUR register
setting.
Micrel, Inc.
MIC2871
14 052913-1.0 (while in progress)
Table 3. Flash Current Register Mapping into Internal
FEN and FCUR Registers, and Relationship between
Flash Current as % of Maximum Current and the
FCUR Register Setting
Value FEN/FCUR[4:0]
Dec. Binary FEN[4] FCUR[3:0] % of IMAX
0 00000 0 100.00
1 00001 0 88.96
2 00010 0 79.04
3 00011 0 70.72
4 00100 0 63.04
5 00101 0 56.00
6 00110 0 49.92
7 00111 0 44.64
8 01000 0 39.68
9 01001 0 35.52
10 01010 0 31.68
11 01011 0 28.16
12 01100 0 25.12
13 01101 0 22.40
14 01110 0 20.00
15 01111 0 17.92
16 10000 1 100.00
17 10001 1 88.96
18 10010 1 79.04
19 10011 1 70.72
20 10100 1 63.04
21 10101 1 56.00
22 10110 1 49.92
23 10111 1 44.64
24 11000 1 39.68
25 11001 1 35.52
26 11010 1 31.68
27 11011 1 28.16
28 11100 1 25.12
29 11101 1 22.40
30 11110 1 20.00
31 11111 1 17.92
Torch Current Register (TEN/TCUR: default 0)
The torch current register enables and sets the torch
mode current level. Valid values are 0 to 31; values 0 −
15 will set the torch current without enabling the torch
(such that it can be triggered by setting the internal TEN
register value to 1), values 16 − 31 will set the torch
current an d enable th e torch. A value of 0 at the internal
TEN register will disable the torch. The torch current
register maps into the internal TEN and TCUR registers
as shown in Table 4. The table also describes the
relationsh ip between t he torc h current as a per centag e of
maximum current, and the TCUR register setting.
Micrel, Inc.
MIC2871
15 052913-1.0 (while in progress)
Table 4. Torch Current Register Mapping into Internal
TEN and TCUR Registers, and Relationship between
Torch Current as % of Maximum Current and the
TCUR Register Setting
Value TEN/TCUR[4:0]
Dec. Binary TEN[4] TCUR[3:0] % of IMAX
0 00000 0 100.00
1 00001 0 88.96
2 00010 0 79.04
3 00011 0 70.72
4 00100 0 63.04
5 00101 0 56.00
6 00110 0 49.92
7 00111 0 44.64
8 01000 0 39.68
9 01001 0 35.52
10 01010 0 31.68
11 01011 0 28.16
12 01100 0 25.12
13 01101 0 22.40
14 01110 0 20.00
15 01111 0 17.92
16 10000 1 100.00
17 10001 1 88.96
18 10010 1 79.04
19 10011 1 70.72
20 10100 1 63.04
21 10101 1 56.00
22 10110 1 49.92
23 10111 1 44.64
24 11000 1 39.68
25 11001 1 35.52
26 11010 1 31.68
27 11011 1 28.16
28 11100 1 25.12
29 11101 1 22.40
30 11110 1 20.00
31 11111 1 17.92
Safety Time r Duration Register (STDUR: default 7)
The saf ety tim er duration r egister sets the duration of the
flash and torch mode when the LED current exceeds the
programm ed threshold current. Valid values are 0 for the
minimum timer duration to 7 for the maximum duration.
Table 5. Safety Timer Duration Register Setting and Safety
Timer Duration
Value FDUR[2:0]
(binary) Timeout (ms)
Dec.
Binary
0
000
000
156.25
1 001 001 312.5
2 010 010 468.75
3 011 011 625
4 100 100 781.25
5
101
101
937.5
6
110
110
1093.75
7 111 111 1250
Low-Battery Threshold Register (LB_TH: default 7)
The LB_TH register sets the supply threshold voltage
below which the internal low battery flag is asserted and
flash f unctions are inhib ited. T able 6 shows the threshold
values that correspond to the register settings. Setting 0
is reserved for disabling the function, and settings
between 1 and 9 inclusively enable and set the LB_TH
value between 3.0V and 3. 8V with 100mV resolut ion.
Table 6. Low-Battery Threshold Register Setting and
Supply Threshold Voltage
Value LB_TH[3:0] VBAT Threshold (V)
Dec. Binary
0 0000 0000 Disabled
1 0001 0001 3.0
2 0010 0010 3.1
3 0011 0011 3.2
4 0100 0100 3.3
5 0101 0101 3.4
6 0110 0110 3.5
7 0111 0111 3.6
8 1000 1000 3.7
9 1001 1001 3.8
Micrel, Inc.
MIC2871
16 052913-1.0 (while in progress)
Safety Timer Threshold Current Register
(ST_TH: defaul t 4)
Safety timer threshold current determines the amount of
LED current flowing through the external LED before the
internal LED safety tim er is activated. Sett ing ST_T H to 0
disables th e safet y timer function , and setti ng the regi ster
to values 1 to 5 set the safety time threshold current
100mA to 300mA in 50mA steps.
Table 7. Safety Timer Threshold Current Register Setting
and Safety Timer Threshold Current
Value ST_TH[2:0] Safety Timer Threshold
Current (mA)
Dec. Binary
0 000 000 Disabled
1 001 001 100mA
2 010 010 150mA
3 011 011 200mA
4 100 100 250mA
5 101 101 300mA
Micrel, Inc.
MIC2871
17 052913-1.0 (while in progress)
Component Selection
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. Since the boost
converter is compensated internally, the recommended
inductance of L is limited from 1µH to 2.2µH to ensure
system stability. It is usually a good balance between
these considerations.
A large inductance value reduces the peak-to-peak
inductor ripple current hence the output ripple voltage and
the LED ripp le current. This also reduc es both the DC loss
and the transition loss at the same inductor’s DC
resistance (DCR). However, the DCR of an inductor
usuall y increases with the induc ta nc e in the s ame pac kage
size. This is due to the longer windings required for an
increase in inductance. Since the majority of the input
current passes through the inductor, the higher the DCR
the lower the efficiency is, and more significantly at higher
load currents. On the other hand, inductor with smaller
DCR but the same inductance usually has a larger size.
The saturation current rating of the selected inductor m ust
be higher than the maximum peak inductor current to be
encountered and should be at least 20% to 30% higher
than the average inductor current at maximum output
current.
Input Capacitor
A ceramic capacitor of 2.2µF or larger with low ESR is
recommended to reduce the input voltage ripple to ensure
a clean supply voltage for the device. The input capacitor
should b e placed as c lose as pos sible t o the MIC2 871 VIN
pin with short trace for good noise performance. X5R or
X7R type ceramic capacitors are recommended for better
tolerance over temperature. The Y5V and Z5U type
temperature rating ceramic capacitors are not
recommended due to their large reduction in capacitance
over temperature and increased resistance at high
frequencies. These reduce their ability to filter out high-
frequency noise. The rated voltage of the input capacitor
should be at least 20% higher than the maximum
operating input voltage over the operating temperature
range.
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing output capacitor
will lead to an improved transient response, however, the
size and cost also increase. The output capacitor is
preferred in the range of 2.2µF to 10µF with ESR from
10mΩ to 50mΩ. X5R or X7R type ceramic capacitors are
recommended for better tolerance over temperature.
The Y5V and Z5U type ceramic capacitors are not
recommended due to their wide variation in capacitance
over temperature and increased resistance at high
frequencies. The rated voltage of the output capacitor
should be at least 20% higher than the maximum
operating output voltage over the operating temperature
range.
FRSET Resistor
Since FRSET resistor is used for setting the maximum
LED current, resistor type with 0.1% tolerance is
recommended for more accurate maximum LED current
setting.
Micrel, Inc.
MIC2871
18 052913-1.0 (while in progress)
Power Dissipation Consideration
As with a ll po wer dev ices, t he ult im ate curr ent rating of the
output is limited by the thermal properties of the device
package and the PCB on which the device is mounted.
There is a simple, Ω’s law type relationship between
thermal resistance, power dissipation and temperature
which are analogous to an electrical circuit:
Figure 5. Series Electrical Resistance Circuit
From this simple circuit we can calculate VX if we know
ISOURCE, VZ and the resistor values, RXY and RYZ using
Equation 2:
ZYZXYSOURCEXV)R(RIV ++×=
Eq. 2
Thermal circuits can be considered using this same rule
and can be drawn similarly by replacing current sources
with power dissipation (in watts), resistance with thermal
resistanc e (in °C/W ) and volta ge sour ces with tem perature
(in °C).
Figure 6. Series Thermal Resistance Circuit
Now replac ing the variable s in Equation 2, we can f ind the
junction temperature (TJ) from the power dissipation,
ambient temperature and the known thermal resistance of
the PCB (θCA) and the package (θJC).
ACAJCDISSJ
T ) (PT +θ+θ×=
Eq. 3
As can be seen in the diagram, total thermal resistance
θJA = θJC + θCA. Hence this can also be written as in
Equation 4:
AJADISSJ
T )(PT +θ×=
Eq. 4
Since effectively all of the power losses (minus the
inductor losses) in the converter are dissipated within the
MIC2871 package, PDISS can be calculated thus:
Linear Mode:
DCR
2
I]1
η
1
[PP
OUTOUTDISS
×−−×=
Eq. 5
Boost Mode:
DCR
2
D1
I
]1
η
1
[PP OUT
OUTDISS ×
−
−−×=
Eq. 6
Duty Cycle in Boost Mode:
OUT
INOUT
V
VV
D−
=
Eq. 7
where:
η = Efficiency taken from efficiency curves and DCR =
inductor DCR. θJC and θJA are found in the operating
ratings section of the datasheet.
Micrel, Inc.
MIC2871
19 052913-1.0 (while in progress)
Where the real board area differs from 1” square, θCA (the
PCB thermal resistance) values for various PCB copper
areas can be taken from Figure 7. Figure 7 is taken from
Designing with Low Dropout Voltage Regulators available
from the Micrel website (“LDO Application Hints”).
Figure 7. Graph to Determine PC Board Area for a Given
PCB Thermal Resistance
Figure 7 shows the total area of a round or square pad,
centered on the device. The solid trace represents the
area of a square, single-sided, horizontal, solder-masked,
copper PC board trace heat sink, measured in square
millim eters . No airflo w is as sum ed. T he dashe d line s ho ws
PC boards trace heat s ink covered in black oil-based paint
and with 1.3m/sec (250 feet per minute) airflow. This
approaches a “best case” pad heat sink. Conservative
design dictates using the solid trace data, which indicates
a max im um pad s i ze of 5 000 mm2 is need ed. T his is a pa d
71mm by 71mm (2.8 inches per side).
Micrel, Inc.
MIC2871
20 052913-1.0 (while in progress)
PCB Layout Guidelines
PCB layout is critical to achieve reliable, stable and
eff icient performanc e. A ground pl ane is r equ ired t o c ontr ol
EMI and minimize the inductance in power, signal and
return paths. The following guidelines should be followed
to ensure proper operation of the device:
IC (Integrated Circuit)
• Place the IC c lose to t he po int-of-load (in th is cas e, the
flash LED).
• Use fat traces to route the input and output power
lines.
• Analog grounds (AGND1 and AGND2) and power
grounds (PGND1 and PGND2) should be kept
separate an d connec ted at a single location.
• The exposed pad (EPAD) on the bottom of the IC must
be connected to the analog grounds AGND2 of the IC.
• 8 to 12 thermal vias must be placed on the PCB pad
for exposed pad and connected it to the ground plane
to ensure a good PCB thermal resistance can be
achieved.
VIN Decoupling Capacitor
• The VIN d ecoupling capac itor mus t be placed close t o
the VIN pin of the IC and p referabl y connected direc tly
to the pin and not through any v ia. The c apacitor must
be located right at the IC.
• The VIN decoupling capacitor should be connected to
analog ground (AGND2).
• The VIN term inal is noise sensitive and the placement
of capacitor is very critical.
Inductor
• Keep both t he induc tor connec tions to the s witch no de
(SW) and input power line short and wide enough to
handle the switching current. Keep the areas of the
switching current loops small to minimize the EMI
problem.
• Do not route any digital lines underneath or close to
the inductor.
• Keep the switch node (SW) away from the noise
sensitive pi ns.
• To minimize noise, place a ground plane underneath
the inductor.
Output Capacitor
• Use wide and short traces to connect the output
capacitor to the OUT and PGND1 pins.
• Place several vias to the ground plane close to the
output capacitor ground terminal.
• Use either X5R or X7R temperature rating ceramic
capacitors. Do not use Y5V or Z5U type ceramic
capacitors.
Flash LED
• Use wide and s hort trace to connec t the LED anode t o
the OUT pin.
• Use wide and short trace to connect the LED cathode
to the LED pin.
• Make sure that the LED’s PCB land pattern can
provide sufficient PCB pad heat sink to the flash LED.
FRSET Resistor
The FRSET resistor should be placed close to the FRSET
pin and connected to AGND2.
Micrel, Inc.
MIC2871
21 052913-1.0 (while in progress)
Typical Application Schematic
Bill of Materials
Item Part Number Manufacturer Description Qty.
C1 GRM188R61A225KE34D Murata(7) 2.2µF, 10V, 10%, X5R, 0603 Capacitor 1
C4 LMK107BJ475KA-T Taiyo Yuden(8) 4.7µF, 10V, 10%, X5R, 0603 Capacitor 1
L1 PIFE25201B-1R0MS-39 Cyntec(9) 1.0µH, 3.55A, 2.5mm × 2.0mm × 1.2mm Inductor 1
LED1 SLSW6R007 Samsung
(10)
4mm × 4mm × 2.2mm High-Power Flash LED 1
LXCL-MN06-3002 Philips(11) LUXEON Flash 6 Module, 4mm × 4mm × 2.2mm,
180lux @ ILED = 1A LED
R4 ERA3AEB2052V Panasonic
(12)
20.5kΩ, 1/10W, 0.1%, 0603 Resistor 1
U1 MIC2871YMK Micrel, Inc.(13) 1.2A High-Brightness LED Fla sh Driver with Single-Wire Serial
Interface 1
Notes:
7. Murata: www.murata.com.
8. Taiyo Yuden: www.t-yuden.com.
9. Cyntec: www.cyntec.com.
10. Samsung: www.samsung.com.
11. Philips: www.philipslumileds.com.
12. Panasonic : www.panasonic.com.
13. Micrel, Inc.: www.micrel.com.
Micrel, Inc.
MIC2871
22 052913-1.0 (while in progress)
PCB Layout Recommendations
Top Layer
Bottom Layer
Micrel, Inc.
MIC2871
23 052913-1.0 (while in progress)
Package Information and Recommended Landing Pattern(14, 15)
14-Pin 3mm × 2mm LDFN (MK)
Notes:
14. Package information is correct as of the publication date. For updates and most current inform ation, go to www.micrel.com.
15. Disclaimer: This is only a recommendation based on information available t o Micrel from its suppli ers. Actual l and pattern may have to be
signific antly di fferent due to various materials and processes used in PCB assembly. Micrel makes no representation or warranty of performance
based on the recommended land pattern.
Micrel, Inc.
MIC2871
24 052913-1.0 (while in progress)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
Micrel makes no represent ations or warranties with respec t t o the accuracy or completeness of the information furnis hed in t his data sheet. This
information is not intended as a warranty and Micrel does not assume responsibili t y for its use. Micrel reserves the right to change circuitry,
specificat i ons and descript i ons at any time without notice. No license, whether express , im plied, arisi ng by estoppel or ot herwise, t o any intellectual
property rights is granted by this document. Except as provided in Micrel’s term s and condit i ons of sale for such products, Micrel assumes no liabil ity
whatsoever, and Micrel disclaims any express or impli ed warranty relati ng to t he sale and/or use of Micrel products including liabil ity or warranties
relating to fitness for a partic ular purpose, merchant abi lit y, or inf ri ngem ent of any patent, copyright or other intel l ect ual propert y right.
Micrel Products are not designed or authorized for use as components in life support applianc es, devic es or syst ems where malfuncti on of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or syst ems that (a) are intended for surgical
implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant inj ury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devic es or syst ems is a Purchaser’s own risk and Purchaser agrees to fully
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