LM3557 [TI]
Step-Up Converter for White LED Applications;
| 型号： | LM3557 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | Step-Up Converter for White LED Applications |
| 文件： | 总16页 (文件大小：396K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LM3557
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SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
LM3557 Step-Up Converter for White LED Applications
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1
FEATURES
DESCRIPTION
The LM3557 is a complete solution for white LED
drive applications. With minimal external component
count, no DC current leakage paths to ground, cycle-
by-cycle current limit protection, and output over-
voltage protection circuitry, the LM3557 offer superior
performance and cost savings over standard DC/DC
boost component implementations.
2
•
•
•
•
•
•
•
VIN Range: 2.7V–7.5V
Small External Components
1.25 MHz Constant-Switching Frequency
Output Over-Voltage Protection
Input Under-Voltage Protection
Cycle-By-Cycle Current Limit
The LM3557 switches at a fixed-frequency of 1.25
MHz, which allows for the use of small external
components. Also, the LM3557 has a wide input
voltage range to take advantage of multi-cell input
applications. With small external components, high
fixed frequency operation, and wide input voltage
range, the LM3557 is the most optimal choice for
LED lighting applications.
TRUE SHUTDOWN: No DC current paths to
ground during shutdown
•
•
Low Profile Package: <1 mm Height -8 Pin
WSON
No External Compensation
APPLICATIONS
•
•
•
White LED Display Lighting
Cellular Phones
PDAs
Typical Application Circuit
L
D
2.2 mH
Vout
Sw1 Ovp
V
V
IN
SUPPLY
Cin
4.7 mF
Cout
1 mF
LM3557
Fb
En
R2
NC
Gnd
Sw2
Figure 1. Backlight Configuration
1
Please 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.
All trademarks are the property of their respective owners.
2
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2004–2013, Texas Instruments Incorporated
LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
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Connection Diagram
1
2
3
4
8
7
6
5
Figure 2. 8-Lead Thin WSON Package
(Top View)
PIN DESCRIPTIONS
Name
Sw1
VIN
Pin No.
Description
Drain Connection of the Internal Power Field Effect Transistor (FET) Switch (Figure 3: N1)
Input Voltage Connection
1
2
NC
3
No Connection
En
4
Device Enable Connection
Ovp
Fb
5
Over-Voltage Protection Input Connection
Feedback Voltage Connection
6
Sw2
Gnd
DAP
7
Drain Connection of an Internal Field Effect Transistor (FET) Switch (Figure 3: N2)
Ground Connection
8
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.
(1)(2)
Absolute Maximum Ratings
VIN Pin
−0.3V to +8V
−0.3V to +8V
En Pin
Fb Pin
−0.3V to +8V
Sw2 Pin
−0.3V to +8V
Ovp Pin
−0.3V to +30V
−0.3V to +40V
Internally Limited
Sw1 Pin
Continuous Power Dissipation
Maximum Junction Temperature
(TJ-MAX
)
+150°C
Storage Temperature Range
−65°C to +150°C
(3)
ESD Rating
Human Body Model
Machine Model
2 kV
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 kΩ resistor into each pin. The machine model is a 200 pF
capacitor discharged directly into each pin.
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(1) (2)
Operating Conditions
Junction Temperature (TJ) Range
Ambient Temperature (TA) Range
Supply Voltage, VIN Pin
En Pin
−40°C to +125°C
−40°C to +85°C
2.7V to 7.5V
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.
(1) (2)
Electrical Characteristics
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
Input Voltage
Test Conditions
Min
2.7
Typ
Max
7.5
Units
VIN
IQ
V
Quiescent Current
VEN = 0V (Shutdown)
VEN = 1.8V; VOVP = 27V
(Non-Switching)
0.01
0.55
2
0.8
µA
mA
En
ICL
Device Enable Threshold
Power Switch Current Limit
Device On
Device Off
0.9
0.3
V
A
(3)
VIN = 3V
0.4
0.55
0.8
0.8
1.1
1.02
RDS(ON)
TC (RDS(ON)
OVP
Power Switch ON Resistance
RDS(ON) Temperature Coefficient
ISw1 = 175 mA
800
0.5
1000
mΩ
)
%/C
(4)
Over-Voltage Protection
On Threshold
Off Threshold
22
21.5
26
25.5
28.5
28
V
V
(4)
UVP
IOVP
Under-Voltage Protection
On Threshold
Off Threshold
2.2
2.3
Over-Voltage Protection Pin Bias
4
10
µA
(5)
Current
(5)
IEN
Enable Pin Bias Current
VEN = 1.8V
VIN = 3V
0.8
1.25
0.51
0.03
90
3
1.6
0.561
2
µA
MHz
V
FS
Switching Frequency
0.9
(6)
VFb-Sw2
IFb
Feedback Pin Voltage
0.459
(5)
Feedback Pin Bias Current
µA
%
DMAX
Maximum Duty Cycle
VIN = 3V
85
(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.
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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
VSw1 = 3.6V, Not Switching
VSw2 = 3.6V, Not Switching
VOvp = 3.6V, Not Switching
ISw2 = 50 mA
Min
Typ
0.002
0.001
2
Max
2
Units
µA
(5)
(5)
(5)
ILSw1
Sw1 Pin Leakage Current
Sw2 Pin Leakage Current
Ovp Pin Leakage Current
ILSw2
1
µA
ILOVP
nA
RSw2
Sw2 Pin Switch Resistance
RSw2 Temperature Coefficient
8
10
Ω
TC(RSw2
)
0.5
%/C
BLOCK DIAGRAM
Vin
Ovp
Sw1
2
5
1
OVP Diodes
Fb
6
OVP
Schmitt
Trigger
NC
3
ERROR
Thermal Shutdown
AMPLIFIER
PWM
Control and FET
Driver Logic
-
+
N1
Fb
Reference
UVP COMPARATOR
VREF
+
+
-
1.2 MHz
Oscillator
Sw2
-
7
UVP
Reference
4
+
-
R
N2
CURRENT SENSE
AMPLIFIER
En
8
Gnd
Figure 3.
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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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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
1.26
3
V
= 4.2V
IN
1.25
V
= 4.5V
IN
2.5
2
1.24
1.23
1.22
1.21
1.20
1.19
1.18
1.17
1.16
V
= 2.7V
= 4.5V
IN
V
= 3.6V
IN
V
IN
= 3.6V
V
= 2.7V
IN
V
IN
1.5
1
0.5
0
I
= 20 mA
LED
-40 -25 -10
5
20 35 50 65 80 95 110 125
-40
-20
0
25
40
85
TEMPERATURE (oC)
TEMPERATURE (°C)
Figure 4.
Figure 5.
En PIN CURRENT vs En PIN VOLTAGE
CURRENT LIMIT vs TEMPERATURE
3
0.95
0.9
0.85
0.8
2.5
V
= 4.5V
IN
25°C
2
V
= 3.6V
IN
-40°C
1.5
0.75
0.7
V
= 3V
IN
1
25°C
0.5
0.65
85°C
85°C
-40°C
0.6
0
-40 -25 -10
5 20 35 50 65 80 95 110 125
0
0.2 0.4 0.6 0.8
1
1.2 1.4
(V)
2
3
TEMPERATURE (oC)
V
EN
Figure 6.
Figure 7.
OVP PIN CURRENT vs TEMPERATURE
4.25
RDS(ON) (Figure 3: N1) vs TEMPERATURE
1.4
4.20
1.2
V
= 3.6V
V
= 3.6V
IN
IN
4.15
4.10
4.05
4.00
3.95
3.90
1
0.8
0.6
0.4
0.2
V
= 2.7V
IN
V
IN
= 4.5V
-40 -25 -10
5 20 35 50 65 80 95 110125
-40 -25 -10
5 20 35 50 65 80 95 110 125
TEMPERATURE (oC)
TEMPERATURE (oC)
Figure 8.
Figure 9.
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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
12
10
8
0.9
V
= 4.2V
IN
V
IN
= 2.7V
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
V
= 3.6V
V
IN
= 3V
IN
ON
OFF
6
V
= 4.2V
IN
4
2
0
-40
25
85
-40
27
70
85
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 10.
Figure 11.
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APPLICATION INFORMATION
L
D
2.2 mH
Vout
R3
R4
Sw1 Ovp
V
V
IN
SUPPLY
Cin
Cout
1 mF
4.7 mF
LM3557
Fb
En
R2
NC
Gnd
Sw2
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:
V
En≥ 0.9V
(1)
(2)
VFb-Sw2
R2
ILED
=
where
•
•
•
ILED white LED current
VFb-Sw2 is the feedback voltage
R2 is the resistor
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:
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[ R4 + R3 ]
[ R4 ]
VOvp
x
Vout =
where
•
•
•
•
VOVP is the OVP voltage threshold
VOUT is the maximum output voltage (<35V)
R3 is a resistor
R4 is a resistor
(3)
tON
I
L
(avg)
Di
L
Time
T
S
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:
2 x IL (avg)
R =
DiL
(4)
[IOUT
[(1-D) x Eff]
[VIN x D]
]
IL (avg) =
(5)
(6)
DiL =
[L x Fs]
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[2 x IOUT x L x Fs x (VOUT)2]
R =
[(VIN)2 x Eff x (VOUT - VIN)]
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
ΔiL is the inductor ripple current
IL(avg) is the average inductor current
(7)
(8)
For CCM operation, the duty cycle can be computed with:
tON
D =
TS
[VOUT - VIN]
D =
[VOUT
]
where
•
•
•
•
•
tON is the internal power FET on-time
TS is 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:
tON
TS
D =
(10)
[2 x IOUT x L x (VOUT - VIN) x Fs]
D =
[(VIN)2 x Eff]
where
•
•
•
•
•
•
•
•
•
tON is the internal power FET on-time
TS is 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:
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DiL
IPeak I (avg) +
ö
L
(12)
[IOUT
]
[VIN * D]
IPeak
+
ö [(1 - D) * Eff] [2 * L * Fs]
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
ΔiL is 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:
[VIN * D]
[L * Fs]
IPeak
ö
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
ΔiL is 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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LM3557
SNVS338B –NOVEMBER 2004–REVISED FEBRUARY 2013
www.ti.com
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 (IF or 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 (CJ or CT or CD) are conducive to high efficiency. The
chosen diode must have a reverse breakdown voltage rating (VR and/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. VR and VRRM > VOUT
2. IF or 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).
12
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Copyright © 2004–2013, Texas Instruments Incorporated
Product Folder Links: LM3557
LM3557
www.ti.com
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
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13
Product Folder Links: LM3557
PACKAGE OPTION ADDENDUM
www.ti.com
5-Nov-2017
PACKAGING INFORMATION
Orderable Device
Status Package Type Package Pins Package
Eco Plan
Lead/Ball Finish
MSL Peak Temp
Op Temp (°C)
Device Marking
Samples
Drawing
Qty
(1)
(2)
(6)
(3)
(4/5)
LM3557SD-2/NOPB
OBSOLETE
WSON
NGQ
8
TBD
Call TI
Call TI
-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) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based
flame retardants must also meet the <=1000ppm threshold requirement.
(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.
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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.
Addendum-Page 1
MECHANICAL DATA
NGQ0008A
SDA08A (Rev A)
www.ti.com
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