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.

LWT67C. Efficiency:

η =P

I

SWITCHING FREQUENCY vs TEMPERATURE

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