HM1538 datasheet(4/7 Pages) HMSEMI

4 / 7 page

The HM1538 uses a fixed frequency, peak current

mode boost regulator architecture to regulate

voltage at the feedback pin. The operation of the

HM1538 can be understood by referring to the block

diagram of Figure 3. At the start of each oscillator

cycle the MOSFET is turned on through the control

circuitry. To prevent sub-harmonic oscillations at

duty cycles greater than 50 per cent, a s tabilizing

ramp is added to the output of the current sense

amplifier and the result is fed into the negative input

of the PWM comparator. When this voltage equals

the output voltage of the error amplifier the power

MOSFET is turned off. The voltage at the output of

the error amplifier is an am plified version of the

difference between the 1.25V bandgap reference

voltage and the feedback voltage. In this way the

peak current level keeps the output in regulation. If

the feedback voltage starts to drop, the output of the

error amplifier increases. This results in more current

to flow through the power MOSFET, thus increasing

the power delivered to the output. The HM1538 has

internal soft start to limit the amount of input current

at startup and to also limit the amount of overshoot

on the output.

Figure 3. Functional Block Diagram

OPERATION

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HM1538 Datasheet(PDF) 4 Page - Shenzhen Huazhimei Semiconductor Co., Ltd