White LED Step-Up Converter

LM1938

Aug. 2011 – Pre

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HTC

OPERATION

The LM1938 uses a constant frequency, current mode control scheme to provide excellent line and load

regulation. Operation can be best understood by referring to the Block Diagram in figure 2. At the start of each

oscillator cycle, the RS latch is set, which turns on the power switch Q1. A voltage proportional to the switch

current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM

comparator A2. When this voltage exceeds the level at the negative input of A2, the RS latch is reset turning off

the power switch. The level at the negative input of A2 is set by the error amplifier A1, and is simply an amplified

version of the difference between the feedback voltage and the reference voltage of 95mV. In this manner, the

error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output

increases, more current is delivered to the output; if it decreases, less current is delivered.

TYPICAL OPERATING CHARACTERISTICS

T.B.D