Circuit Description

The LM3519 is a step-up converter for white LED applica-

tions that uses a unique and proprietary pulse frequency

modulation (PFM) architecture to optimize high efficiency at

high frequency operation. Unlike most PFM architecture

implementations, the LM3519’s unique architectural imple-

mentation results in non-pulse skipping variable frequency

operation. The regulator is forced to operate at the edge of

Continous Conduction Mode (CCM). The error amplifier will

set the end of the on-time (I

load (LEDs) current. During this operation, the inductor cur-

rent ramps up and reaches a peak current at end of the

on-time. At this point, the internal power switch is turned off

until the inductor current reaches zero, and the cycle repeats

again. The switching frequency is set based on the charge

(on-time) and discharge(off-time) of the inductor current. The

frequency can range between 2MHz to 8MHz over the op-

erating input range.

The LM3519 operation can be best understood through an

examination of the block diagram in Figure 2. When LED

current is out of regulation, the LED_rtn voltage falls below

or rises above the internal reference voltage (VREF). The

error amplifier will output a signal to increase or decrease the

proper on-time duration of N1 power FET. This correction

allows the inductor’s stored energy to increase or decrease

to a sufficient level that when transferred to the load will bring

the LED_rtn current back into regulation.

During steady-state operation for a typical switching cycle,

the oscillator sets the driver logic and turns on N1 power

device. N1 conducts current through the inductor and re-

verse biases the external diode. The LED current is supplied

by the output capacitor when N1 is conducting. Once N1

on-time period is concluded, the internal power device is

turned off and the external diode is forward baised. The

inductor current then flows through the diode to the LED load

to replenish the output capacitor and keep the LED current

regulated at the trimmed target.

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