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MIC3838/3839

Micrel

MIC3838/3839

April 2005

Functional Description

The MIC3838/9 is a high-speed power supply controller with

push-pull output drive capability. MIC3838 has a higher VDD

turn-on threshold and more hysteresis between VDD turn-on

and turn-off than the MIC3839. The outputs of the controller

operate in a push-pull fashion with a guaranteed dead time

between them. A block diagram of the MIC3838/9 controller

is shown in Figure 1.

Functional Diagram

Oscillator

VDD OK

VDD

VDD Ð1V

Peak Current

Comparator

Overcurrent

Comparator

PWM

Comparator

PWM

Latch

Slope = 1V/ms

Soft Start

2.2V

2.0V

0.5V

0.8V

14V

VDD

OUTA

OUTB

GND

0.5V

1.2R

0.75V

Error

Amplifier

VREF

COMP

RAMP

ILIM

Voltage

Reference

3.65V

Figure 1. MIC3838 Block Diagram

MIC3838

330pF

VIN = 36V to 75V

50k

150k

13k

4.5k

VCOMP

0.5V

0.75V

Peak Current

Comparator

Over Current

Comparator

PWM

Comparator

Oscillator

VMAX at Ramp Input = 1.5V

VMAX at ILIM Input = 0.5V

Figure 2. Volt Second Clamp and Voltage Feed Forward Circuit

VDD and Turn-on Sequence

The oscillator and output gate drive signals are disabled

when VDD is lower than the turn on threshold. Circuitry in the

output drivers eliminates glitching or random pulsing during

the start-up sequence. The oscillator is enabled when VDD is

applied and reaches the turn-on threshold. The VDD com-

parator also turns off the internal soft-start discharge FET,

slowly bringing up the COMP pin voltage.

The VDD pin is internally clamped. As VDD approaches this

clamp voltage, the VDD current will increase over the normal

current draw of the IC. IDD currents greater than 20mA may

cause excessive power dissipation in the MIC3838/9.

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