MIC3205
High-Brightness LED Driver Controller
with Fixed-Frequency Hysteretic Control
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
General Description
The MIC3205 is a hysteretic, step-down, high-brightness
LED (HB LED) driver with a patent pending frequency
regulation scheme that maintains a constant operating
frequency over input voltage range. It provides an ideal
solution for interior/exterior lighting, architectural and
ambient lighting, LED bulbs, and other general illumination
applications.
The MIC3205 is well suited for lighting applications
requiring a wide input voltage range. The hysteretic control
provides good supply rejection and fast response during
load transients and PWM dimming. The high-side current
sensing and on-chip current-sense amplifier deliver LED
current with 5% accuracy. An external high-side current-
sense resistor is used to set the output current.
The MIC3205 offers a dedicated PWM input (DIM) which
enables a wide range of pulsed dimming. A high-frequency
switching operation up to 1.5MHz allows the use of smaller
external components minimizing space and cost.
The MIC3205 operates over a junction temperature from
–40°C to +125°C and is available in a 10-pin 3mm x 3mm
MLF® package.
Data sheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
Features
4.5V to 40V input voltage range
Fixed operating frequency over input voltage range
High efficiency (90%)
5% LED current accuracy
High-side current sense
Dedicated dimming control input
Hysteretic control (no compensation!)
Up to 1.5MHz switching frequency
Adjustable constant LED current
Over-temperature protection
–40C to 125C junction temperature range
Applications
Architectural, industrial, and ambient lighting
LED bulbs
Indicators and emergency lighting
Street lighting
Channel letters
12V lighting systems (MR-16 bulbs, under-cabinet
lighting, garden/pathway lighting)
_________________________________________________________________________________________________________________________
Typical Application
0.0
0.5
1.0
1.5
2.0
0 9 18 27 36 45
NORMALIZED FREQUENCY
INPUT VOLTAGE (V)
Normalized Switching Frequency
vs. Input Voltage
1 LED
L = 22µH
I
LED
= 1A
R
CS
= 0.2
4 LED
L = 47µH
6 LED
L = 68µH
10 LED
L = 33µH
MIC3205 Buck LED Driver
October 2012 M9999-102312-A
Micrel, Inc. MIC3205
October 2012 2 M9999-102312-A
Ordering Information
Part Number Junction Temperature Range Package(1)
MIC3205YML 40°C to 125°C 10-Pin 3mm x 3mm MLF
Note:
1. MLF is a GREEN RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
Pin Configuration
10-Pin 3mm x 3mm MLF (ML)
Top View
Pin Description
Pin Number Pin Name Pin Function
1 VCC
Voltage Regulator Output. The VCC pin is the output of a linear regulator powered from VIN, which
supplies power to the internal circuitry. A 4.7µF ceramic capacitor is recommended for bypassing. Place
it as close as possible to the VCC and AGND pins. Do not connect to an external load.
2 CS
Current Sense Input. Negative input to the current sense comparator. Connect an external sense
resistor to set the LED current. Connect the current sense resistor as close as possible to the chip.
3 VIN
Input Power Supply. VIN is the input supply pin to the internal circuitry. Due to high frequency switching
noise, a 10µF ceramic capacitor is recommended for bypassing and should be placed as close as
possible to the VIN and PGND pins. See “PCB Layout Guidelines.”
4 VINS
VIN Sense. Positive input to the current sense comparator. Connect as close as possible to the current
sense resistor.
5 AGND Analog Ground. Ground for all internal low-power circuitry.
6 EN
Enable Input. Logic high (greater than 2V) powers up the regulator. A logic low (less than 0.4V) powers
down the regulator and reduces the supply current of the device to less than 2µA. A logic low pulls down
the DRV pin turning off the external MOSFET. Do not drive the EN pin above VIN. Do not leave floating.
7 DIM
PWM Dimming Input. A PWM input can be used to control the brightness of the LED. Logic high (greater
than 2V) enables the output. Logic low (less than 0.4V) disables the output regardless of the EN state.
Do not drive the DIM pin above VIN. Do not leave floating.
8 CTIMER
Timer Capacitor. A capacitor is required from CTIMER to ground sets the target switching frequency
using the equation CTIMER=2.22*10-4 / FSW
9 PGND
Power Ground. Ground for the power MOSFET gate driver. The current loop for the power ground
should be as small as possible and separate from the analog ground loop. See “PCB Layout
Recommendations.”
10 DRV
Gate Drive Output. Connect to the gate of an external N-channel MOSFET. The drain of the external
MOSFET connects directly to the inductor and provides the switching current necessary to operate in
hysteretic mode.
EP ePAD Exposed Pad. Must be connected to a GND plane for best thermal performance.
Micrel, Inc. MIC3205
October 2012 3 M9999-102312-A
Absolute Maximum Ratings (1)
VIN to PGND .................................................. 0.3V to 42V
VINS to PGND......................................... 0.3V to (VIN+0.3V)
VCC to PGND ................................................ 0.3V to 6.0V
CS to PGND........................................ 0.3V to (VIN 0.3V)
EN to AGND........................................ 0.3V to (VIN 0.3V)
DIM to AGND ...................................... 0.3V to (VIN 0.3V)
CTIMER to AGND .............................. 0.3V to (VCC 0.3V)
DRV to PGND .................................... 0.3V to (VCC 0.3V)
PGND to AGND .......................................... 0.3V to 0.3V
Junction Temperature ................................................ 150C
Storage Temperature Range .................... 60°C to 150C
Lead Temperature (Soldering, 10sec) ....................... 260C
ESD Ratings (3)
HBM...................................................................... 1.5kV
MM.........................................................................200V
Operating Ratings (2)
Supply Voltage (VIN).......................................... 4.5V to 40V
Enable Voltage (VEN) .............................................. 0V to VIN
Dimming Voltage (VDIM).................................................................0V to VIN
Junction Temperature (TJ) ........................ 40C to 125C
Junction Thermal Resistance
10-pin 3x3 MLF (JA).......................................60.7C/W
10-pin 3x3 MLF (JC).......................................28.7C/W
Electrical Characteristics (4)
VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted.
Symbol Parameter Condition Min. Typ. Max. Units
Input Supply
VIN Input Voltage Range (VIN) 4.5 40 V
IS Supply Current DRV = Open 1.3 3 mA
ISD Shutdown Current VEN = 0V 2 µA
UVLO VIN UVLO Threshold VIN Rising 3.2 4 4.5 V
UVLOHYS V
IN UVLO Hysteresis 600 mV
VCC Supply
VCC V
CC Output Voltage VIN = 12V, ICC = 5mA 4.5 5 5.5 V
Current Sense
190 200 210 mV
VCS Average Current Sense
Threshold VCS =VINS VCS 188 200 212 mV
VCS Rising 50 ns
tCS Current Sense Response
Time VCS Falling 70 ns
ICS CS Input Current VIN = VCS 0.5 10 µA
VHYS Sense Voltage Hysteresis (5)
VIN =12V, VLED =3V,
L=47µH, FSW =250kHz,
VD = 0.7V, ILED = 1A
46 mV
Frequency
ITIMER CTIMER Pull-up Current 66 µA
VCTREF CTIMER Threshold 1.189 V
(4*ITIMER)/
VCTREF Frequency Coefficient (6) 1.776 × 10-4 2.22 × 10-4 2.664 × 10-4 A/V
Micrel, Inc. MIC3205
October 2012 4 M9999-102312-A
Electrical Characteristics (4) (Continued)
VIN = VEN = VDIM = 12V; CVCC = 4.7µF; TJ = 25C; bold values indicate 40C TJ 125C, unless noted.
Symbol Parameter Condition Min. Typ. Max. Units
Enable Input
ENHI EN Logic Level High 2.0 V
ENLO EN Logic Level Low 0.4 V
VEN = 12V 20 60 µA
IEN EN Bias Current VEN = 0V 1 µA
tSTART Start-Up Time From EN pin going high to DRV
going high 65 µs
Dimming Input
DIMHI DIM Logic Level High 2.0 V
DIMLO DIM Logic Level Low 0.4 V
VDIM = 12V 20 50 µA
IDIM DIM Bias Current VDIM = 0V 1 µA
tDIM DIM Delay Time From DIM pin going high to DRV
going high 450 ns
fDIM Maximum Dimming Frequency % of switching frequency 2 %
External FET Driver
Pull-Up, ISOURCE = 10mA 4
RON DRV On-Resistance Pull-Down, ISINK = -10mA 1.5
Rise Time, CLOAD = 1000pF 13 ns
tDRV DRV Transition Time Fall Time, CLOAD = 1000pF 7 ns
Thermal Protection
TLIM Overtemperature Shutdown TJ Rising 160 C
TLIMHYS Overtemperature Shutdown Hysteresis 20 C
Notes:
1. Exceeding the absolute maximum rating can damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5k in series with 100pF.
4. Specification for packaged product only.
5. See “Sense Voltage Hysteresis Range” in the “Application Information” section.
6. See “Frequency of Operation” in the “Application Information” section.
Micrel, Inc. MIC3205
October 2012 5 M9999-102312-A
Typical Characteristics
60
65
70
75
80
85
90
95
100
0 9 18 27 36 45
EFFICIENCY (%)
INPUT VOLTAGE (V)
Efficiency (ILED = 1A)
vs. Input Voltage
1 LED
L = 22µH
4 LED
L = 47µH
6 LED
L = 68µH 10 LED
L = 33µH
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 9 18 27 36 45
V
IN
SUPPLY CURRENT (mA)
INPUT VOLTAGE (V)
V
IN
Supply Current
vs. Input Voltage
I
LED
= 0A
T
A
= 25°C
0.0
0.2
0.4
0.6
0.8
1.0
0 9 18 27 36 45
V
IN
SHUTDOWN CURRENT (µA)
INPUT VOLTAGE (V)
V
IN
Shutdown Current
vs. Input Voltage
V
EN
= 0V
I
LED
= 0A
T
A
= 25°C
4.0
4.5
5.0
5.5
6.0
0 9 18 27 36 45
V
CC
OUTPUT VOLTAGE (V)
INPUT VOLTAGE (V)
V
CC
Output Voltage
vs. Input Voltage
T
A
= 25°C
I
LED
= 1A
0.0
0.5
1.0
1.5
2.0
0 9 18 27 36 45
NORMALIZED FREQUENCY
INPUT VOLTAGE (V)
Normalized Switching Frequency
vs. Input Voltage
1 LED
L = 22µH
I
LED
= 1A
R
CS
= 0.2
4 LED
L = 47µH
6 LED
L = 68µH
10 LED
L = 33µH
0.90
0.95
1.00
1.05
1.10
0 9 18 27 36 45
I
LED
OUTPUT CURRENT (A)
INPUT VOLTAGE (V)
I
LED
Output Current
vs. Input Voltage
T
A
= 25°C
R
CS
= 0.2
1 LED
L = 22µH
4 LED
L = 47µH
6 LED
L = 68µH
60
62
64
66
68
70
0 9 18 27 36 45
CTIMER CURRENT (µA)
INPUT VOLTAGE (V)
CTIMER Current
vs. Input Voltage
V
EN
= V
IN
T
A
= 25°C
0.0
0.3
0.6
0.9
1.2
1.5
0 9 18 27 36 45
ENABLE THRESHOLD (V)
INPUT VOLTAGE (V)
Enable Threshold
vs. Input Voltage
HYST
I
LED
= 1A
T
A
= 25°C
RISING
FALLING
0
20
40
60
80
100
0 9 18 27 36 45
ENABLE BIAS CURRENT (µA)
INPUT VOLTAGE (V)
Enable Bias Current
vs. Input Voltage
V
EN
= V
IN
T
A
= 25°C
I
LED
= 0A
Micrel, Inc. MIC3205
October 2012 6 M9999-102312-A
Typical Characteristics (Continued)
0
20
40
60
80
100
0 9 18 27 36 45
ENABLE BIAS CURRENT (µA)
ENABLE VOLTAGE (V)
Enable Bias Current
vs. Enable Voltage
V
EN
V
IN
T
A
= 25°C
I
LED
= 0A
V
IN
= 42V
0
40
80
120
160
200
0 9 18 27 36 45
THERMAL SHUTDOWN (°C)
INPUT VOLTAGE (V)
Thermal Shutdown
vs. Input Voltage
HYST
I
LED
= 1A
RISING
FALLING
1.0
1.2
1.4
1.6
1.8
2.0
-50 -25 0 25 50 75 100 125
SUPPLY CURRENT (mA)
TEMPERATURE (°C)
V
IN
Supply Current
vs. Temperature
V
IN
= 12V
I
LED
= 0A
0.0
0.4
0.8
1.2
1.6
2.0
-50-250 255075100125
SUPPLY CURRENT (µA)
TEMPERATURE (°C)
V
IN
Shutdown Current
vs. Temperature
V
IN
= 12V
I
LED
= 0A
V
EN
= 0V
0.98
0.99
1.00
1.01
1.02
1.03
-50 -25 0 25 50 75 100 125
ILED OUTPUT CURRENT (A)
TEMPERATURE (°C)
ILED Output Current
vs. Temperature
VIN = 12V
VLED = 3.5V
RCS = 0.2
430
450
470
490
510
530
-50-25 0 255075100125
FREQUENCY (kHz)
TEMPERATURE (°C)
Switching Frequency
vs. Temperature
V
IN
= 12V
V
LED
= 3.5V
L = 22µH
C
T
= 470pF
R
CS
= 0.2
4.0
4.5
5.0
5.5
6.0
-50 -25 0 25 50 75 100 125
VCC (V)
TEMPERATURE (°C)
VCC
vs. Temperature
VIN = 12V
ILED = 1A
0.0
0.4
0.8
1.2
1.6
-50 -25 0 25 50 75 100 125
ENABLE THRESHOLD (V)
TEMPERATURE (°C)
Enable Threshold
vs. Temperature
V
IN
= 12V
I
LED
= 1A
RISING
FALLING
HYST
10
15
20
25
30
-50 -25 0 25 50 75 100 125
EN BIAS CURRENT (µA)
TEMPERATURE (°C)
Enable Bias Current
vs. Temperature
V
IN
= 12V
I
LED
= 0A
V
EN
= 12V
Micrel, Inc. MIC3205
October 2012 7 M9999-102312-A
Typical Characteristics (Continued)
0
1
2
3
4
5
-50 -25 0 25 50 75 100 125
VIN UVLO THRESHOLD (V)
TEMPERATURE (°C)
VIN UVLO Threshold
vs. Temperature
RISING
FALLING
HYST
Micrel, Inc. MIC3205
Functional Characteristics
October 2012 8 M9999-102312-A
Micrel, Inc. MIC3205
Functional Characteristics (Continued)
October 2012 9 M9999-102312-A
Micrel, Inc. MIC3205
October 2012 10 M9999-102312-A
Functional Diagram
Figure 1. MIC3205 Block Diagram
Micrel, Inc. MIC3205
October 2012 11 M9999-102312-A
Functional Description
The MIC3205 is a hysteretic step-down driver that
regulates the LED current with a patent pending
frequency regulation scheme. This scheme maintains a
fixed operating frequency over a wide input voltage
range.
Theory of Operation
The device operates from a 4.5V to 40V input MOSFET
voltage. At turn-on, after the VIN input voltage crosses
4.5V, the DRV pin is pulled high to turn on an external
MOSFET. The inductor and series LED current builds up
linearly. This rising current results in a rising differential
voltage across the current sense resistor (RCS). When
this differential voltage reaches an upper threshold, the
DRV pin is pulled low, the MOSFET turns off, and the
Schottky diode takes over and returns the series LEDs
and inductor current to VIN. Then, the current through the
inductor and series LEDs starts to decrease. This
decreasing current results in a decreasing differential
voltage across RCS. When this differential voltage
reaches a lower threshold, the DRV pin is pulled high,
the MOSFET is turned on, and the cycle repeats. The
average of the CS pin voltage is 200mV below VIN
voltage. This is the average current sense threshold
(VCS). Thus, the CS pin voltage switches about VIN –
200mV with a peak-to-peak hysteresis that is the product
of the peak-to-peak inductor current times the current
sense resistor (RCS). The average LED current is set by
RCS, as explained in the “Application Information”
section.
MIC3205 dynamically adjusts hysteresis to
accommodate fixed-frequency operation. Average
frequency is programmed using an external capacitor
connected to the CTIMER pin, as explained in the
“Frequency of Operation” subsection in the “Application
Information” section. The internal frequency regulator
dynamically adjusts the inductor current hysteresis every
eight switching cycles to make the average switching
frequency a constant. If the instantaneous frequency is
higher than the programmed average value, the
hysteresis is increased to lower the frequency and vice
versa. In other hysteretic control systems, current sense
hysteresis is constant and frequency can change with
input voltage, inductor value, series LEDs voltage drop,
or LED current. However, with this patent pending
frequency regulation scheme, the MIC3205 changes
inductor current hysteresis and keeps the frequency
fixed even upon changing input voltage, inductor value,
series LEDs voltage drop, or LED current.
The MIC3205 has an on-board 5V regulator, which is for
internal use only. Connect a 4.7µF capacitor on VCC pin
to analog ground.
The MIC3205 has an EN pin that gives the flexibility to
enable and disable the output with logic high and low
signals. The maximum EN voltage is VIN.
Figure 2. Theory of Operation
LED Dimming
The MIC3205 LED driver can control the brightness of the
LED string through the use of pulse width modulated (PWM)
dimming. A DIM pin is provided, which can turn on and off
the LEDs if EN is in an active-high state. This DIM pin
controls the brightness of the LED by varying the duty cycle
of DIM pin from 1% to 99%.
An input signal from DC up to 20kHz can be applied to the
DIM pin (see “Typical Application”) to pulse the LED string
on and off. A logic signal can be applied on the DIM pin for
dimming, independent of input voltage (VIN). Using PWM
dimming signals above 120Hz is recommended to avoid any
recognizable flicker by the human eye. Maximum allowable
dimming frequency is 2% of operating frequency that is set
by the external capacitor on the CTIMER pin (see
“Frequency of Operation”). See “Functional Characteristics”
on page 9 for PWM dimming waveforms. Maximum DIM
voltage is VIN.
PWM dimming is the preferred way to dim an LED to prevent
color/wavelength shifting. Color/wavelength shifting occurs
with analog dimming. By using PWM dimming, the output
current level remains constant during each DIM pulse. The
hysteretic buck converter switches only when the DIM pin is
high. When the DIM pin is low, no LED current flows and the
DRV pin is low turning the MOSFET off.
Micrel, Inc. MIC3205
October 2012 12 M9999-102312-A
Application Information
The internal block diagram of the MIC3205 is shown in
Figure 1. The MIC3205 is composed of a current-sense
comparator, voltage reference, frequency regulator, 5V
regulator, and MOSFET driver. Hysteretic mode control,
also called bang-bang control, is a topology that does
not use an error amplifier, instead using an error
comparator.
The frequency regulator dynamically adjusts hysteresis
for the current sense comparator to regulate frequency.
The inductor current is sensed by an external sense
resistor (RCS) and controlled within a hysteretic window.
It is a simple control scheme with no oscillator and no
loop compensation. The control scheme does not need
loop compensation. This makes design easy, and avoids
instability problems.
Transient response to load and line variation is very fast
and depends only on propagation delay. This makes the
control scheme very popular for certain applications.
LED Current and RCS
The main feature in MIC3205 is that it controls the LED
current accurately within 5% of set current. Choosing a
high-side RCS resistor is helpful for setting constant LED
current regardless of wide input voltage range. The
following equation and Table 1 give the RCS value for
required LED current:
LED
CS I
200mV
R Eq. 1
RCS () ILED (A) I2R (W) Size (SMD)
1.33 0.15 0.03 0603
0.56 0.35 0.07 0805
0.4 0.5 0.1 0805
0.28 0.7 0.137 0805
0.2 1.0 0.2 1206
0.13 1.5 0.3 1206
0.1 2.0 0.4 2010
0.08 2.5 0.5 2010
0.068 3.0 0.6 2010
Table 1. RCS for LED Current
Frequency of Operation
The patent pending frequency regulation scheme allows
for operating frequency to be programmed by an
external capacitor from the CTIMER pin to AGND. The
frequency co-efficient (typically 2.22 × 10-4 A/F) divided
by the value of this external capacitor connected to the
CTIMER pin, gives the average frequency of operation, as
seen in the following equation:
T
-4
SW C
1022.2
F
Eq. 2
The actual average frequency can vary depending on the
variation of the frequency co-efficient and the parasitic board
capacitances in parallel to the external capacitor CT. As
shown in the Electrical Characteristics table, part to part
variation for the frequency co-efficient is ±20% over
temperature, from the target frequency co-efficient of
2.22 × 10-4.
Switching frequency selection is based on the trade-off
between efficiency and system size. Higher frequencies
result in smaller, but less efficient, systems and vice versa.
The operating frequency is independent of input voltage,
inductor value, series LEDs voltage drop, or LED current, as
long as 40mv VHYS 100mV is maintained as explained
in the next sections.
Sense Voltage Hysteresis Range
The frequency regulation scheme requires that the
hysteresis remain in a controlled window. Components and
operating conditions must be such that the hysteresis on the
CS pin is between 40mV and 100mV.
Hysteresis less than 40mV or more than 100mV can result in
loss of frequency regulation.
After average LED current (ILED) has been set by RCS and
operating frequency has been set by external capacitor CT,
the hysteresis VHYS is calculated as follows:
As seen in Figure 2, for the inductor,
CS
HYS
LR
V
I
Eq. 3
where:
IL = inductor ripple current
VHYS = hysteresis on CS pin
For rising inductor current (MOSFET is on):
L_RISE
L
rV
IL
t
Eq. 4
where:
VL_RISE = VIN ILED × RCS VLED
VLED is the total voltage drop of the LED string
VIN is the input voltage
RCS is the current sense resistor
ILED is the average LED current
Micrel, Inc. MIC3205
October 2012 13 M9999-102312-A
tr is the MOSFET ON-time
L is the inductor
For falling inductor current (MOSFET is off):
L_FALL
L
fV
IL
t
Eq. 5
where:
VL_FALL = VD + ILED × RCS VLED
VD is the freewheeling diode forward drop
tf is the MOSFET OFF-time
Operating frequency and time period are given by:
T
1
FSW Eq. 6
Eq. 7
fr ttT
Using Equations 3, 4, 5, 6, and 7:
SWDIN
CSLEDCSLEDDLEDCSLEDIN
HYS F L ) V V(
R) V RI (V) V- RI - (V
V
Eq. 8
The value of VHYS calculated in this way must be
between 40mV and 100mV to ensure frequency
regulation.
Inductor
According to the above equations, the inductor value can
be calculated once average LED current, operating
frequency and an appropriate hysteresis VHYS value
have been chosen.
Thus, inductor L is given by:
SWHYSDIN
CSLEDCSLEDDLEDCSLEDIN
F V ) V V(
R) V RI (V) V- RI - (V
L
Eq. 9
Table 2, Table 3, and Table 4 give reference inductor
values for an operating frequency of 400 kHz, for a given
LED current, freewheeling diode forward drop, and
number of LEDs. By selecting VHYS in the 55mV to
75mV range, we get the following inductor values:
RCS () ILED (A) VIN (V) L (µH) VHYS
(mV)
0.56 0.35 5 22 64.1
0.56 0.35 12 68 57.7
0.28 0.7 5 10 70.5
0.28 0.7 12 33 59.4
0.2 1.0 5 6.8 72.6
0.2 1.0 12 22 62.4
0.1 2.0 5 3.6 68.5
0.1 2.0 12 10 68.6
Table 2. Inductor for FSW = 400 kHz, VD = 0.4V, 1 LED
RCS () ILED (A) VIN (V) L (µH) VHYS
(mV)
0.56 0.35 24 150 55.8
0.56 0.35 36 220 56.8
0.28 0.7 24 68 61.6
0.28 0.7 36 100 62.5
0.2 1.0 24 47 62.4
0.2 1.0 36 68 64.3
0.1 2.0 24 22 66.6
0.1 2.0 36 33 66.2
Table 3. Inductor for FSW = 400 kHz, VD = 0.4V, 4 LED
RCS () ILED (A) VIN (V) L (µH) VHYS
(mV)
0.56 0.35 36 150 58.4
0.56 0.35 40 220 54.3
0.28 0.7 36 68 64.4
0.28 0.7 40 100 59.6
0.2 1.0 36 47 65.2
0.2 1.0 40 68 61.4
0.1 2.0 36 22 69.6
0.1 2.0 40 33 63.3
Table 4. Inductor for FSW = 400 kHz, VD = 0.4V, 8 LED
Given an inductor value, the size of the inductor can be
determined by its RMS and peak current rating.
Because LEDs are in series with the inductor,
LEDL II
Eq. 10
From Equations 1, 3, and 10:
200m
V
I
I HYS
L
L
Eq. 11
Micrel, Inc. MIC3205
October 2012 14 M9999-102312-A
With 40mv VHYS 100mV:
L
2
L
2
L)RMS(L II
12
1
II Eq. 12
)
400m
V
(1II HYS
LL(PK)
Eq. 13
where:
IL is the average inductor current
IL(PK) is the peak inductor current
Select an inductor with a saturation current rating at
least 30% higher than the peak current.
For space-sensitive applications, smaller inductors with
higher switching frequency could be used but regulator
efficiency will be reduced.
MOSFET
N-channel MOSFET selection depends on the maximum
input voltage, output LED current, and switching
frequency.
The selected N-channel MOSFET should have 30%
margin on maximum voltage rating for high reliability
requirements.
The MOSFET channel resistance (RDSON) is selected
such that it helps to get the required efficiency at the
required LED currents and meets the cost requirement.
Logic level MOSFETs are preferred as the drive voltage
is limited to 5V.
The MOSFET power loss has to be calculated for proper
operation. The power loss consists of conduction loss
and switching loss. The conduction loss can be found
by:
IN
LED
LEDRMS(FET)
DSON
2
RMS(FET)LOSS(CON)
V
V
D
DII
RIP
The switching loss occurs during the MOSFET turn-on
and turn-off transition and can be found by:
GATE
DRV
DRV
gd2gs
DRV
SWLEDIN
)TRAN(LOSS
R
V
=I
)Q+Q(×
I
F×I×V
=P
where:
RGATE is total MOSFET gate resistance; Qgs2 and Qgd can be
found in a MOSFET manufacturer data sheet.
A gate resistor can be connected between the MOSFET
gate and the DRV pin to slow down MOSFET switching
edges. A 2 resistor is usually sufficient.
The total power loss is:
)TRAN(LOSS)CON(LOSS)TOT(LOSS P+P=P
The MOSFET junction temperature is given by:
AJA)TOT(LOSSJ T+R×P=T
TJ must not exceed maximum junction temperature under
any conditions.
Freewheeling Diode
The freewheeling diode should have a reverse voltage rating
that is at least 20% higher than the maximum input supply
voltage. The forward voltage drop should be small to get the
lowest conduction dissipation for high efficiency. The forward
current rating should be at least equal to the LED current.
Schottky diodes with low forward voltage drop and fast
reverse recovery are ideal choices and give the highest
efficiency. The freewheeling diode average current (ID) is
given by:
LEDD I)D1(I
Diode power dissipation (PD) is given by:
DDD IVP
Typically, higher current rating diodes have a lower VD and
have better thermal performance, improving efficiency.
Input Capacitor
The ceramic input capacitor is selected by voltage rating and
ripple current rating. A 10µF ceramic capacitor is usually
sufficient. Select a voltage rating that is at least 30% larger
than the maximum input voltage.
LED Ripple Current
The LED current is the same as inductor current IL. A
ceramic capacitor should be placed across the series LEDs
to pass the ripple current. A 4.7µF capacitor is usually
sufficient for most applications. Voltage rating should be the
same as the input capacitor.
Micrel, Inc. MIC3205
October 2012 15 M9999-102312-A
PCB Layout Guidelines
NOTE: To minimize EMI and output noise, follow
these layout recommendations.
PCB layout is critical to achieve reliable, stable, and
efficient performance. A ground plane is required to
control EMI and minimize the inductance in power,
signal, and return paths.
Follow these guidelines to ensure proper operation of
the MIC3205.
IC
Use thick traces to route the input and output power
lines.
Keep signal and power grounds separate and
connect them at only one location.
Input Capacitor
Place the input capacitors on the same side of the
board and as close to the IC as possible.
Keep both the VIN and PGND traces as short as
possible.
If the application requires vias to the ground plane,
place them close to the input capacitor ground
terminal, but not between the input capacitors and
IC pins.
Use either X7R or X5R dielectric input capacitors.
Do not use Y5V or Z5U type capacitors.
Do not replace the ceramic input capacitor with any
other type of capacitor. Any type of capacitor can be
placed in parallel with the ceramic input capacitor.
If a tantalum input capacitor is placed in parallel with
the ceramic input capacitor, it must be recom-
mended for switching regulator applications and the
operating voltage must be derated by 50%.
In “Hot-Plug” applications, place a tantalum or
electrolytic bypass capacitor in parallel to the
ceramic capacitor to limit the overvoltage spike seen
on the input supply when power is suddenly applied.
In this case, an additional tantalum or electrolytic
bypass input capacitor of 22µF or higher is required
at the input power connection.
Inductor
Keep the inductor connection to the switch node
(MOSFET drain) short.
Do not route any digital lines underneath or close to
the inductor.
To minimize noise, place a ground plane underneath
the inductor.
LED Ripple Current Carrying Capacitor
Place this ceramic capacitor as close to the LEDs as
possible.
Use either X7R or X5R dielectric capacitors. Do not use
Y5V or Z5U type capacitors.
MOSFET
To avoid trace inductance, place the N-channel
MOSFET as close as possible to the MIC3205.
Provide sufficient copper area on MOSFET ground to
dissipate the heat.
Freewheeling Diode
Place the Schottky diode on the same side of the board
as the IC and input capacitor.
Keep the connection from the Schottky diode’s anode to
the switching node as short as possible.
Keep the diode’s cathode connection to the RCS as short
as possible.
RC Snubber
If an RC snubber is needed, place the RC snubber on
the same side of the board and as close to the Schottky
diode as possible. A 1.2 resistor in series with a 1nF
capacitor is usually a good choice.
RCS (Current-Sense Resistor)
VINS pin and CS pin must be as close as possible to
RCS.
Make a Kelvin connection to the VINS and CS pin,
respectively, for current sensing. For low values of VHYS
(around 40mV) the switching noise could cause faulty
switching on the DRV pin. If this occurs, place two 30
resistors and a 1nF capacitor, as shown in Figure 3, to
filter out switching noise for low values of VHYS.
Alternatively, as seen in Equation 8, a smaller inductor
value can be used to increase VHYS and make the
system more noise tolerant.
Micrel, Inc. MIC3205
October 2012 16 M9999-102312-A
For FSW = 400 kHz
CT = 550pF
The actual frequency may vary as explained in “Frequency
of Operation” in the “Application Information” section.
3. INDUCTOR SELECTION
From Equation 9:
SWHYSDIN
CSLEDCSLEDDLEDCSLEDIN
F V ) V V(
R) V RI (V) V- RI - (V
L
Given VSUPPLY = 24V rectified AC
The peak voltage = 2 x VSUPPLY
Thus for MIC3205, VIN 34V
VLED = 3.5 x 4 = 14, VD = 0.4V
Figure 3. Input Filter for Low Values of VHYS
Select VHYS = 60mV
Thus, L = 70µH
Trace Routing Recommendation
Keep the power traces as short and wide as possible.
There is one current flowing loop during the MOSFET
ON-time; the traces connect the input capacitor (CIN),
RCS, the LEDs, the inductor, the MOSFET, and back to
CIN. There is another current flowing loop during the
MOSFET OFF-time; the traces for this loop connect RCS,
the LED, the inductor, the freewheeling diode, and back
to RCS. These two loop areas should kept as small as
possible to minimize noise interference
Chose L = 68µH as closest available value.
As a side note, for this example, L = 68µH can be used even
if VSUPPLY = 24V DC. This is because VHYS calculates to
around 44mV (with VIN = VSUPPLY = 24V) which is acceptable.
From Equations 12 and 13:
IL(PK) = 1.15A
Thus, we choose L = 68µH with an RMS saturation current
of 1.5A or higher.
Keep all analog signal traces away from the switching
node and its connecting traces.
4. MOSFET SELECTION
For this example, VIN = 34V, a 50V rating or greater N-
channel MOSFET is required. A high current rating MOSFET
is a good choice because it has lower RDSON.
Design Example
A 60V, 12A MOSFET with 10m RDSON is a good choice.
SPECIFICATIONS:
5. CAPACITOR SELECTION
FSW = 400 kHz
Use a 10µF/50V X7R type ceramic capacitor for the input
capacitor.
VSUPPLY = 24V rectified AC
ILED = 1A
Use a 4.7µF/50V X5R type ceramic capacitor for the LED
ripple current carrying capacitor connected across the series
connection of 4 LEDs
Voltage drop per LED = 3.5V
Number of LEDs = 4
Schottky diode drop at 1A = 0.4V
6. FREEWHEELING DIODE SELECTION
1. CURRENT SENSE RESISTOR
From Equation 1:
LED
CS I
200mV
R
With VIN = 34V, choose a 2A, 60V Schottky diode with a
forward drop voltage of 0.4V at 1A forward current.
For ILED = 1A
RCS = 0.2
2. SWITCHING FREQUENCY
From Equation 2:
T
-4
SW C
1022.2
F
Micrel, Inc. MIC3205
October 2012 17 M9999-102312-A
Evaluation Board Schematic
Micrel, Inc. MIC3205
October 2012 18 M9999-102312-A
Bill of Materials
Item Part Number Manufacturer Description Qty.
12105C475KAZ2A AVX(1)
GRM32ER71H475KA88L Murata(2)
C1, C2,C3,C4,C11
CGA6P3X7R1H475K TDK(3)
4.7µF/50V, Ceramic Capacitor, X7R, Size 1210 5
GRM21BR71H105KA12L Murata
C5
CGA4J3X7R1H105K TDK
1µF/50V, Ceramic Capacitor, X7R, Size 0805 1
06035C471K4T2A AVX
GRM188R71H471KA01D Murata
C10
C1608X7R1H471K TDK
470pF/50V, Ceramic Capacitor, X7R, Size 0603 1
06036D475KAT2A AVX
GRM188R60J475KE19J Murata
C8
CGA3E1X5R0J475K TDK
4.7µF/6.3V, Ceramic Capacitor, X5R, Size 0603 1
06035C102KAT2A AVX
GRM188R71H102KA01D Murata
C7,C9
C1608X7R1H102K TDK
1nF/50V, Ceramic Capacitor, X7R, Size 0603 2
SK36-TP MCC(4)
SK36 Fairchild(5)
D1
SK36-7-F Diodes, Inc.(6)
60V, 3A, SMC, Schottky Diode 1
L1 SLF10145T-220M1R9-PF TDK 22µH, 2.1A, 0.0591, SMT, Power Inductor 1
M1 FDS5672 Fairchild MOSFET, N-CH, 60V, 12A, SO-8 1
RCS CSR1206FKR200
Stackpole
Electronics, Inc.(7) 0.2 Resistor, 1/2W, 1%, Size 1206 1
R5, R8 CRCW0603100KFKEA Vishay Dale(8) 100k Resistor, 1%, Size 0603 2
R2, R3 CRCW060330R0FKEA Vishay Dale 30 Resistor, 1%, Size 0603 2
R1, R9 CRCW06032R00FKEA Vishay Dale 2 Resistor, 1%, Size 0603 2
R4 CRCW060310K0FKEA Vishay Dale 10k Resistor, 1%, Size 0603 1
R6 CRCW060351R0FKEA Vishay Dale 51 Resistor, 1%, Size 0603 1
R7 CRCW06030000Z0EA Vishay Dale 0 Resistor, Size 0603 1
U1 MIC3205YML Micrel, Inc.(9) High-Brightness LED Driver Controller with
Fixed Frequency Hysteretic Control 1
Notes:
1. AVX: www.avx.com.
2. Murata: www.murata.com.
3. TDK: www.tdk.com.
4. MCC: www.mccsemi.com.
5. Fairchild: www.fairchildsemi.com.
6. Diodes Inc.: www.diodes.com.
7. Stackpole Electronics: www.seielect.com.
8. Vishay Dale: www.vishay.com.
9. Micrel, Inc.: www.micrel.com.
Micrel, Inc. MIC3205
October 2012 19 M9999-102312-A
PCB Layout Recommendations
Top Assembly
Top Layer
Micrel, Inc. MIC3205
October 2012 20 M9999-102312-A
PCB Layout Recommendations (Continued)
Bottom Layer
Micrel, Inc. MIC3205
October 2012 21 M9999-102312-A
Package Information
10-Pin 3mm x 3mm MLF (ML)
Micrel, Inc. MIC3205
October 2012 22 M9999-102312-A
Recommended Landing Pattern
10-Pin 3mm x 3mm MLF (ML) Land Pattern
Micrel, Inc. MIC3205
October 2012 23 M9999-102312-A
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TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This
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specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual
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whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties
relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems 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 injury to the user. A
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