LM3557
-+
R2
L
2.2 PH
Cin
4.7 PFCout
1 PF
NC
VIN
En
Gnd
Fb
Sw2
OvpSw1
D
VSUPPLY
Vout
LM3557
www.ti.com
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
LM3557 Step-Up Converter for White LED Applications
Check for Samples: LM3557
1FEATURES DESCRIPTION
The LM3557 is a complete solution for white LED
2• VIN Range: 2.7V–7.5V drive applications. With minimal external component
• Small External Components count, no DC current leakage paths to ground, cycle-
• 1.25 MHz Constant-Switching Frequency by-cycle current limit protection, and output over-
voltage protection circuitry, the LM3557 offer superior
• Output Over-Voltage Protection performance and cost savings over standard DC/DC
• Input Under-Voltage Protection boost component implementations.
• Cycle-By-Cycle Current Limit The LM3557 switches at a fixed-frequency of 1.25
• TRUE SHUTDOWN: No DC current paths to MHz, which allows for the use of small external
ground during shutdown components. Also, the LM3557 has a wide input
• Low Profile Package: <1 mm Height -8 Pin voltage range to take advantage of multi-cell input
applications. With small external components, high
WSON fixed frequency operation, and wide input voltage
• No External Compensation range, the LM3557 is the most optimal choice for
LED lighting applications.
APPLICATIONS
• White LED Display Lighting
• Cellular Phones
• PDAs
Typical Application Circuit
Figure 1. Backlight Configuration
1Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
2All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date. Copyright © 2004–2013, Texas Instruments Incorporated
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
1
4 5
2
6
7
8
3
LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
www.ti.com
Connection Diagram
Figure 2. 8-Lead Thin WSON Package
(Top View)
PIN DESCRIPTIONS
Name Pin No. Description
Sw1 1 Drain Connection of the Internal Power Field Effect Transistor (FET) Switch (Figure 3: N1)
VIN 2 Input Voltage Connection
NC 3 No Connection
En 4 Device Enable Connection
Ovp 5 Over-Voltage Protection Input Connection
Fb 6 Feedback Voltage Connection
Sw2 7 Drain Connection of an Internal Field Effect Transistor (FET) Switch (Figure 3: N2)
Gnd 8 Ground Connection
DAP DAP Die Attach Pad (DAP), must be soldered to the printed circuit board's ground plane for enhanced thermal dissipation
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1)(2)
VIN Pin −0.3V to +8V
En Pin −0.3V to +8V
Fb Pin −0.3V to +8V
Sw2 Pin −0.3V to +8V
Ovp Pin −0.3V to +30V
Sw1 Pin −0.3V to +40V
Continuous Power Dissipation Internally Limited
Maximum Junction Temperature
(TJ-MAX) +150°C
Storage Temperature Range −65°C to +150°C
ESD Rating (3)
Human Body Model 2 kV
Machine Model 150V
(1) Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical characteristic specifications do not
apply when operating the device outside of its rated operating conditions.
(2) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office / Distributors for
availability and specifications.
(3) The human body model is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin. The machine model is a 200 pF
capacitor discharged directly into each pin.
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Operating Conditions (1) (2)
Junction Temperature (TJ) Range −40°C to +125°C
Ambient Temperature (TA) Range −40°C to +85°C
Supply Voltage, VIN Pin 2.7V to 7.5V
En Pin 0V to VIN +0.4V
(1) Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical characteristic specifications do not
apply when operating the device outside of its rated operating conditions.
(2) All voltages are with respect to the potential at the GND pin.
THERMAL CHARACTERISTICS(1)(2)
over operating free-air temperature range (unless otherwise noted)
Junction-to-Ambient Thermal 55°C/W
Resistance (θJA), WSON Package
(1) The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal
resistance, θJA, and the ambient temperature, TA. See Thermal Properties for the thermal resistance. The maximum allowable power
dissipation at any ambient temperature is calculated using: PD(MAX) = (TJ(MAX) – TA)/θJA. Exceeding the maximum allowable power
dissipation will cause excessive die temperature.
(2) Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set
forth in the JEDEC standard JESD51-7. The test board is a 4 layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm with a 2 x 1 array
of thermal vias. The ground plane on the board is 50 mm x 50 mm. Thickness of copper layers are 36 µm/18 µm/18 µm/36 µm (1.5 oz/1
oz/1 oz/1.5 oz). Ambient temperature in simulation is 22°C, still air. Power dissipation is 1W. In applications where high maximum power
dissipation exists, special care must be paid to thermal dissipation issues. For more information on these topics, please refer to
Application Note 1187: Leadless Leadframe Package (LLP) and the Layout Guidelines section of this datasheet.
Electrical Characteristics (1) (2)
Limits in standard typeface are for TJ= 25°C. Limits in bold typeface apply over the full operating junction temperature range
(−40°C ≤TJ≤+125°C). Unless otherwise specified: VIN = 3.6V.
Parameter Test Conditions Min Typ Max Units
VIN Input Voltage 2.7 7.5 V
IQQuiescent Current VEN = 0V (Shutdown) 0.01 2µA
VEN = 1.8V; VOVP = 27V 0.55 0.8 mA
(Non-Switching)
En Device Enable Threshold Device On 0.9 0.3 V
Device Off
ICL Power Switch Current Limit (3) VIN = 3V 0.4 0.8 1.1 A
0.55 0.8 1.02
RDS(ON) Power Switch ON Resistance ISw1 = 175 mA 800 1000 mΩ
TC (RDS(ON)) RDS(ON) Temperature Coefficient 0.5 %/C
OVP Over-Voltage Protection (4) On Threshold 22 26 28.5 V
Off Threshold 21.5 25.5 28
UVP Under-Voltage Protection (4) On Threshold 2.2 V
Off Threshold 2.3
IOVP Over-Voltage Protection Pin Bias 410 µA
Current (5)
IEN Enable Pin Bias Current (5) VEN = 1.8V 0.8 3µA
FSSwitching Frequency VIN = 3V 0.9 1.25 1.6 MHz
VFb-Sw2 Feedback Pin Voltage (6) 0.459 0.51 0.561 V
IFb Feedback Pin Bias Current (5) 0.03 2µA
DMAX Maximum Duty Cycle VIN = 3V 85 90 %
(1) All voltages are with respect to the potential at the GND pin.
(2) Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the
most likely norm.
(3) The Power Switch Current Limit is tested in open loop configuration. For closed loop application current limit please see the Current
Limit vs Temperature performance graph.
(4) The on threshold indicates that the LM3557 is no longer switching or regulating LED current, while the off threshold indicates normal
operation.
(5) Current flows into the pin.
(6) Feedback pin voltage is with respect to the voltage at the Sw2 pin.
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 3
Product Folder Links: LM3557
+
-PWM
Control and FET
Driver Logic
1.2 MHz
Oscillator
+
-
+
-
7
5
3
4
6
1
8
2
Vin
NC
En
Sw1
Gnd
Ovp
Fb
Sw2
ERROR
AMPLIFIER
OVP Diodes
CURRENT SENSE
AMPLIFIER
UVP COMPARATOR
N2
N1
Thermal Shutdown
R
Fb
Reference
UVP
Reference
VREF +
-
OVP
Schmitt
Trigger
LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
www.ti.com
Electrical Characteristics (1) (2) (continued)
Limits in standard typeface are for TJ= 25°C. Limits in bold typeface apply over the full operating junction temperature range
(−40°C ≤TJ≤+125°C). Unless otherwise specified: VIN = 3.6V.
Parameter Test Conditions Min Typ Max Units
ILSw1 Sw1 Pin Leakage Current (5) VSw1 = 3.6V, Not Switching 0.002 2µA
ILSw2 Sw2 Pin Leakage Current (5) VSw2 = 3.6V, Not Switching 0.001 1µA
ILOVP Ovp Pin Leakage Current (5) VOvp = 3.6V, Not Switching 2 nA
RSw2 Sw2 Pin Switch Resistance ISw2 = 50 mA 8 10 Ω
TC(RSw2) RSw2 Temperature Coefficient 0.5 %/C
BLOCK DIAGRAM
Figure 3.
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LM3557
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SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
OPERATION
The LM3557 is a current-mode controlled constant-frequency step-up converter optimized for the facilitation of
white LED driving/current biasing.
The LM3557’s operation can be best understood by the following device functionality explanation. For the
following device functionality explanation, the block diagram in Figure 3 serves as a functional schematic
representation of the underlying circuit blocks that make up the LM3557. When the feedback voltage falls below,
or rises above, the internal reference voltage, the error amplifier outputs a signal that is translated into the correct
amount of stored energy within the inductor that is required to put the feedback voltage back into regulation when
the stored inductor energy is then transferred to the load. The aforementioned translation is a conversion of the
error amplifier’s output signal to the proper on-time duration of the N1 power field effect transistor (FET). This
conversion allows the inductor’s stored energy to increase, or decrease, to a sufficient level that when transferred
to the load will bring the feedback voltage back into regulation.
An increase in inductor current corresponds to an increase in the amount of stored energy within the inductor.
Conversely, a decrease in inductor current corresponds to a decrease in the amount of stored energy. The
inductor’s stored energy is released, or transferred, to the load when the N1 power FET is turned off. The
transferred inductor energy replenishes the output capacitor and keeps the white LED current regulated at the
designated magnitude that is based on the choice of the R2 resistor. When the N1 power FET is turned on, the
energy stored within the inductor begins to increase while the output capacitor discharges through the series
string of white LEDs, the R2 resistance, and N2 FET switch to ground. Therefore, each switching cycle consist of
some amount of energy being stored in the inductor that is then released, or transferred, to the load to keep the
voltage at the feedback pin in regulation at 510 mV above the Sw2 pin voltage.
Features:
CYCLE-BY-CYCLE CURRENT LIMIT
The current through the internal power FET (Figure 3: N1) is monitored to prevent peak inductor currents from
damaging the part. If during a cycle (cycle = 1/switching frequency) the peak inductor current exceeds the current
limit rating for the LM3557, the internal power FET would be forcibly turned off for the remaining duration of that
cycle.
OVER-VOLTAGE PROTECTION
When the output voltage exceeds the over-voltage protection (OVP) threshold, the LM3557’s internal power FET
will be forcibly turned off until the output voltage falls below the over-voltage protection threshold minus the 500
mV hysteresis of the internal OVP circuitry.
UNDER-VOLTAGE PROTECTION
When the input voltage falls below the under-voltage protection (UVP) threshold, the LM3557’s internal power
FET will be forcibly turned off until the input voltage is above the designated under-voltage protection threshold
plus the 100 mV hysteresis of the internal UVP circuitry.
TRUE SHUTDOWN
When the LM3557 is put into shutdown mode operation there are no DC current paths to ground. The internal
FET (Figure 3: N2) at the Sw2 pin turns off, leaving the white LED string open circuited.
THERMAL SHUTDOWN
When the internal semiconductor junction temperature reaches approximately 150°C, the LM3557’s internal
power FET (Figure 3: N1) will be forcibly turned off.
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-40 -25 -10 5 20 35 50 65 80 95 110
3.90
3.95
4.00
4.05
4.10
4.15
4.20
4.25
IOvp (PA)
TEMPERATURE (oC)
125
VIN = 3.6V
-25 -10 5 20 35 50 65 80 95 110125
TEMPERATURE (oC)
0.2
0.4
0.6
0.8
1
1.2
1.4
RDS(ON) (:)
-40
VIN = 3.6V
VIN = 4.5V
VIN = 2.7V
0 0.2 0.4 0.6 0.8 1 1.2 1.4 2 3
VEN (V)
0
0.5
1
1.5
2
2.5
3
IEN (PA)
25°C
-40°C
85°C
25°C
-40°C
85°C
-40 -25 -10 5 20 35 50 65 80 95 110
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
ICL (A)
TEMPERATURE (oC)
125
VIN = 3V
VIN = 3.6V
VIN = 4.5V
-40 -20 0 25 40 85
TEMPERATURE (°C)
0
0.5
1
1.5
2
2.5
3
IQ (SWITCHING) (mA)
ILED = 20 mA
VIN = 4.5V
VIN = 3.6V
VIN = 2.7V
VIN = 4.2V
-25 -10 5 20 35 50 65 80 95 110 125
TEMPERATURE (oC)
1.16
1.26
Fs (MHz)
-40
1.24
1.22
1.20
1.18
1.17
1.25
1.23
1.21
1.19
VIN = 4.5V
VIN = 3.6V
VIN = 2.7V
LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
www.ti.com
TYPICAL PERFORMANCE CHARACTERISTICS
( Circuit in Figure 1: L = DO1608C-223, D = SS16, and LED = LWT67C. Efficiency: η= POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN *
IIN]. TA= 25°C, unless otherwise stated).
IQ(SWITCHING) vs TEMPERATURE SWITCHING FREQUENCY vs TEMPERATURE
Figure 4. Figure 5.
En PIN CURRENT vs En PIN VOLTAGE CURRENT LIMIT vs TEMPERATURE
Figure 6. Figure 7.
OVP PIN CURRENT vs TEMPERATURE RDS(ON) (Figure 3: N1) vs TEMPERATURE
Figure 8. Figure 9.
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-40 25 85
TEMPERATURE (°C)
0
2
4
6
8
10
12
RSW2 (:)
VIN = 3.6V
VIN = 3V
VIN = 2.7V
VIN = 4.2V
-40 27 70 85
TEMPERATURE (°C)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
ON
OFF
VIN = 4.2V
VEN THRESHOLD (V)
LM3557
www.ti.com
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
TYPICAL PERFORMANCE CHARACTERISTICS (continued)
( Circuit in Figure 1: L = DO1608C-223, D = SS16, and LED = LWT67C. Efficiency: η= POUT/PIN = [(VOUT – VFb) * IOUT]/[VIN *
IIN]. TA= 25°C, unless otherwise stated).
RSw2(Figure 3: N2) vs TEMPERATURE ENABLE THRESHOLD vs TEMPERATURE
Figure 10. Figure 11.
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 7
Product Folder Links: LM3557
ILED R2
VFb-Sw2
=
LM3557
-+
R2
L
2.2 PH
Cin
4.7 PFCout
1 PF
NC
VIN
En
Gnd
Fb
Sw2
OvpSw1
D
R3
R4
VSUPPLY
Vout
LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
www.ti.com
APPLICATION INFORMATION
Figure 12. Programmable Output Voltage
WHITE LED CURRENT SETTING
For backlighting applications, the white LED current is programmed by the careful choice of the R2 resistor.
Backlight:
VEn≥0.9V (1)
where
• ILED white LED current
• VFb-Sw2 is the feedback voltage
• R2 is the resistor (2)
The feedback voltage is with respect to the voltage at the Sw2 pin, not ground. For example, if the voltage on the
Sw2 pin were 0.1V then the voltage at the Fb pin would be 0.61V (typical).
ADJUSTING LED CURRENT USING PWM SIGNAL
The LED current can be controlled using a PWM signal on the EN pin with frequencies in the range of 100Hz
(greater than visible frequency spectrum) to 1kHz. For controlling LED currents down to the µA levels, it is best
to use a PWM signal frequency between 200-500Hz. The LM3557 LED current can be controlled with PWM
signal frequencies above 1kHz but the controllable current decreases with higher frequency.
ADJUSTING OVER-VOLTAGE PROTECTION
If the over-voltage protection (OVP) threshold is too low for a particular application, a resistor divider circuit can
be used to adjust the OVP threshold of a given application. Instead of having the Ovp pin connected to the
output voltage, it can be adjusted through a resistor divider circuit to only experience a fraction of the output
voltage magnitude. The resistor divider circuit bias current should be at least 100 times greater than the Ovp pin
bias current. Using Figure 12, the following equation can be used to adjust the output voltage:
8Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated
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'iL = [VIN x D]
[L x Fs]
IL (avg) = [IOUT]
[(1-D) x Eff]
R = 'iL
2 x IL (avg)
IL (avg)
Time
Inductor Current
'iL
tON
TS
Vout [ R4 ]
[ R4 + R3 ]
=xVOvp
LM3557
www.ti.com
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
where
• VOVP is the OVP voltage threshold
• VOUT is the maximum output voltage (<35V)
• R3 is a resistor
• R4 is a resistor (3)
Figure 13. Inductor Current Waveform
CONTINUOUS AND DISCONTINUOUS MODES OF OPERATION
Since the LM3557 is a constant frequency pulse-width-modulated step-up regulator, care must be taken to make
sure the maximum duty cycle specification is not violated. The duty cycle equation depends on which mode of
operation the LM3557 is in. The two operational modes of the LM3557 are continuous conduction mode (CCM)
and discontinuous conduction mode (DCM). Continuous conduction mode refers to the mode of operation where
during the switching cycle, the inductor’s current never goes to and stays at zero for any significant amount of
time during the switching cycle. Discontinuous conduction mode refers to the mode of operation where during the
switching cycle, the inductor’s current goes to and stays at zero for a significant amount of time during the
switching cycle. Figure 13 illustrates the threshold between CCM and DCM operation. In Figure 13, the inductor
current is right on the CCM/DCM operational threshold. Using this as a reference, a factor can be introduced to
calculate when a particular application is in CCM or DCM operation. R is a CCM/DCM factor we can use to
compute which mode of operation a particular application is in. If R is ≥1, then the application is operating in
CCM. Conversely, if R is < 1, the application is operating in DCM. The R factor inequalities are a result of the
components that make up the R factor. From Figure 13, the R factor is equal to the average inductor current,
IL(avg), divided by half the inductor ripple current, ΔiL. Using Figure 13, the following equation can be used to
compute R factor:
(4)
(5)
(6)
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Product Folder Links: LM3557
D = [2 x IOUT x L x (VOUT - VIN) x Fs]
[(VIN)2 x Eff]
tON
TS
D =
D = [VOUT]
[VOUT - VIN]
tON
TS
D =
R = [2 x IOUT x L x Fs x (VOUT)2]
[(VIN)2 x Eff x (VOUT - VIN)]
LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
www.ti.com
where
• VIN is the input voltage
• VOUT is the output voltage
• Eff is the efficiency of the LM3557
• Fs is the switching frequency
• IOUT is the white LED current/load current
• L is the inductance magnitude/inductor value
• D is the duty cycle for CCM operation
•ΔiLis the inductor ripple current
• IL(avg) is the average inductor current (7)
For CCM operation, the duty cycle can be computed with:
(8)
where
• tON is the internal power FET on-time
• TSis the switching period of operation
• D is the duty cycle for CCM operation
• VIN is the input voltage
• VOUT is the output voltage (9)
For DCM operation, the duty cycle can be computed with:
(10)
where
• tON is the internal power FET on-time
• TSis the switching period of operation
• D is the duty cycle for CCM operation
• VIN is the input voltage
• VOUT is the output voltage
• Eff is the efficiency of the LM3557
• Fs is the switching frequency
• IOUT is the white LED current/load current
• L is the inductance magnitude/inductor value (11)
INDUCTOR SELECTION
In order to maintain inductance, an inductor used with the LM3557 should have a saturation current rating larger
than the peak inductor current of the particular application. Inductors with low DCR values contribute decreased
power losses and increased efficiency. The peak inductor current can be computed for both modes of operation:
CCM (continuous current mode) and DCM (discontinuous current mode).
The cycle-by-cycle peak inductor current for CCM operation can be computed with:
10 Submit Documentation Feedback Copyright © 2004–2013, Texas Instruments Incorporated
Product Folder Links: LM3557
[VIN * D]
[L * Fs]
IPeak
|
[IOUT]
[(1 - D) * Eff] +[VIN * D]
[2 * L * Fs]
IPeak
|
IPeak IL (avg) +
|
'iL
2
LM3557
www.ti.com
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
(12)
where
• VIN is the input voltage
• VOUT is the output voltage
• Eff is the efficiency of the LM3557
• Fs is the switching frequency
• IOUT is the white LED current/load current
• L is the inductance magnitude/inductor value
• D is the duty cycle for CCM operation
•ΔiLis the inductor ripple current
• IL(avg) is the average inductor current (13)
The cycle-by-cycle peak inductor current for DCM operation can be computed with:
where
• VIN is the input voltage
• VOUT is the output voltage
• Eff is the efficiency of the LM3557
• Fs is the switching frequency
• IOUT is the white LED current/load current
• L is the inductance magnitude/inductor value
• D is the duty cycle for CCM operation
•ΔiLis the inductor ripple current
• IL(avg) is the average inductor current (14)
Some recommended inductor manufacturers are:
Coilcraft [www.coilcraft.com]
Coiltronics [www.cooperet.com]
TDK [www.tdk.com]
CAPACITOR SELECTION
Multilayer ceramic capacitors are the best choice for use with the LM3557. Multilayer ceramic capacitors have
the lowest equivalent series resistance (ESR). Applied voltage or DC bias, temperature, dielectric material type
(X7R, X5R, Y5V, etc), and manufacturer component tolerance have an affect on the true or effective capacitance
of a ceramic capacitor. Be aware of how your application will affect a particular ceramic capacitor by analyzing
the aforementioned factors of your application. Before selecting a capacitor always consult the capacitor
manufacturer’s data curves to verify the effective or true capacitance of the capacitor in your application.
INPUT CAPACITOR SELECTION
The input capacitor serves as an energy reservoir for the inductor. In addition to acting as an energy reservoir for
the inductor the input capacitor is necessary for the reduction in input voltage ripple and noise experienced by
the LM3557. The reduction in input voltage ripple and noise helps ensure the LM3557’s proper operation, and
reduces the effect of the LM3557 on other devices sharing the same supply voltage. To ensure low input voltage
ripple, the input capacitor must have an extremely low ESR. As a result of the low input voltage ripple
requirement multilayer ceramic capacitors are the best choice. A minimum capacitance of 2.0 µF is required for
normal operation, consult the capacitor manufacturer’s data curves to verify whether the minimum capacitance
requirement is going to be achieved for a particular application.
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OUTPUT CAPACITOR SELECTION
The output capacitor serves as an energy reservoir for the white LED load when the internal power FET switch
(Figure 3: N1) is ON or conducting current. The requirements for the output capacitor must include worst case
operation such as when the load opens up and the LM3557 operates in over-voltage protection (OVP) mode
operation. A minimum capacitance of 0.5 µF is required to ensure normal operation. Consult the capacitor
manufacturer’s data curves to verify whether the minimum capacitance requirement is going to be achieved for a
particular application.
Some recommended capacitor manufacturers are:
TDK
[www.tdk.com]
Murata
[www.murata.com]
Vishay
[www.vishay.com]
DIODE SELECTION
To maintain high efficiency it is recommended that the average current rating (IFor IO) of the selected diode
should be larger than the peak inductor current (ILpeak). To maintain diode integrity the peak repetitive forward
current (IFRM) must be greater than or equal to the peak inductor current (ILpeak). Diodes with low forward voltage
ratings (VF) and low junction capacitance magnitudes (CJor CTor CD) are conducive to high efficiency. The
chosen diode must have a reverse breakdown voltage rating (VRand/or VRRM) that is larger than the output
voltage (VOUT). No matter what type of diode is chosen, Schottky or not, certain selection criteria must be
followed:
1. VRand VRRM > VOUT
2. IFor IO≥ILOAD or IOUT
3. IFRM ≥ILpeak
Some recommended diode manufacturers are as follows:
Vishay [www.vishay.com]
Diodes, Inc [www.diodes.com]
On Semiconductor [www.onsemi.com]
LAYOUT CONSIDERATIONS
All components, except for the white LEDs, must be placed as close as possible to the LM3557. The die attach
pad (DAP) must be soldered to the ground plane.
The input capacitor, Cin, must be placed close to the LM3557. Placing Cin close to the device will reduce the
metal trace resistance effect on input voltage ripple. The feedback current setting resistor R2 must be placed
close to the Fb and Sw2 pins. The output capacitor, Cout, must be placed close to the Ovp and Gnd pin
connections. Trace connections to the inductor should be short and wide to reduce power dissipation, increase
overall efficiency, and reduce EMI radiation. The diode, like the inductor, should have trace connections that are
short and wide to reduce power dissipation and increase overall efficiency. For more details regarding layout
guidelines for switching regulators refer to Applications Note AN-1149 (SNVA021).
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SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
REVISION HISTORY
Changes from Revision A (February 2013) to Revision B Page
• Changed layout of National Data Sheet to TI format .......................................................................................................... 12
Copyright © 2004–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LM3557
PACKAGE OPTION ADDENDUM
www.ti.com 20-Jan-2017
Addendum-Page 1
PACKAGING INFORMATION
Orderable Device Status
(1)
Package Type Package
Drawing Pins Package
Qty Eco Plan
(2)
Lead/Ball Finish
(6)
MSL Peak Temp
(3)
Op Temp (°C) Device Marking
(4/5)
Samples
LM3557SD-2/NOPB LIFEBUY WSON NGQ 8 1000 Green (RoHS
& no Sb/Br) CU SN Level-1-260C-UNLIM -40 to 85 L147B
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2) Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
(4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device.
(5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation
of the previous line and the two combined represent the entire Device Marking for that device.
(6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish
value exceeds the maximum column width.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
PACKAGE OPTION ADDENDUM
www.ti.com 20-Jan-2017
Addendum-Page 2
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device Package
Type Package
Drawing Pins SPQ Reel
Diameter
(mm)
Reel
Width
W1 (mm)
A0
(mm) B0
(mm) K0
(mm) P1
(mm) W
(mm) Pin1
Quadrant
LM3557SD-2/NOPB WSON NGQ 8 1000 178.0 12.4 3.3 3.3 1.0 8.0 12.0 Q1
PACKAGE MATERIALS INFORMATION
www.ti.com 2-Sep-2015
Pack Materials-Page 1
*All dimensions are nominal
Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm)
LM3557SD-2/NOPB WSON NGQ 8 1000 210.0 185.0 35.0
PACKAGE MATERIALS INFORMATION
www.ti.com 2-Sep-2015
Pack Materials-Page 2
MECHANICAL DATA
NGQ0008A
www.ti.com
SDA08A (Rev A)
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