LM3557 Datasheet, PDF (5/11 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

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 = P_OUT/P_IN = [(V_OUT – V_Fb) * I_OUT]/[V_IN * I_IN]. T_A= 25˚C, unless otherwise stated).
I_Q (SWITCHING) vs TEMPERATURE
SWITCHING FREQUENCY vs TEMPERATURE
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