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PDF LM3557SDX-2 Data sheet ( Hoja de datos )
| Número de pieza | LM3557SDX-2 | |
| Descripción | Step-Up Converter for White LED Applications | |
| Fabricantes | National Semiconductor | |
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November 2004
LM3557
Step-Up Converter for White LED Applications
General 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.
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 advan-
tage of multi-cell input applications. With small external com-
ponents, high fixed frequency operation, and wide input
voltage range, the LM3557 is the most optimal choice for
LED lighting applications.
n 1.25 MHz Constant-Switching Frequency
n Output Over-Voltage Protection
n Input Under-Voltage Protection
n Cycle-By-Cycle Current Limit
n TRUE SHUTDOWN: No DC current paths to ground
during shutdown
n Low Profile Package: <1 mm Height -8 Pin LLP
n No External Compensation
Applications
n White LED Display Lighting
n Cellular Phones
n PDAs
Features
n VIN Range: 2.7V–7.5V
n Small External Components
Typical Application Circuit
FIGURE 1. Backlight Configuration
20131601
© 2004 National Semiconductor Corporation DS201316
www.national.com
1 page  
Operation (Continued)
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 2: 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 protec-
tion (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 hyster-
esis of the internal OVP circuitry.
UNDER-VOLTAGE PROTECTION
When the input voltage falls below the under-voltage protec-
tion (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 2: 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 2: N1) will be forcibly turned off.
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
20131604
5
20131605
www.national.com
5 Page 
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead Thin Leadless Leadframe Package
NS Package Number SDA08A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS
WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR
CORPORATION. As used herein:
1. Life support devices or systems are devices or systems
which, (a) are intended for surgical implant into the body, or
(b) support or sustain life, and whose failure to perform when
properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result
in a significant injury to the user.
2. A critical component is any component of a life support
device or system whose failure to perform can be reasonably
expected to cause the failure of the life support device or
system, or to affect its safety or effectiveness.
BANNED SUBSTANCE COMPLIANCE
National Semiconductor certifies that the products and packing materials meet the provisions of the Customer Products Stewardship
Specification (CSP-9-111C2) and the Banned Substances and Materials of Interest Specification (CSP-9-111S2) and contain no ‘‘Banned
Substances’’ as defined in CSP-9-111S2.
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Support Center
Email: [email protected]
Tel: 1-800-272-9959
www.national.com
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Fax: +49 (0) 180-530 85 86
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Tel: 81-3-5639-7560
11 Page | ||
| Páginas | Total 11 Páginas | |
| PDF Descargar | [ Datasheet LM3557SDX-2.PDF ] | |
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