LM3557 Datasheet, PDF (5/19 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

LM3557
www.ti.com
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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