MIC3808 Просмотр технического описания (PDF) - Micrel

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производитель

MIC3808

Push-Pull PWM Controller

Micrel 

MIC3808/3809

Functional Diagram

Overcurrent

Comparator

VDD OK

VDD —1V

Voltage

Reference

0.5V

FB

2

3.65V

0.75V

2.2V

2.0V

Error

Amplifier

S

Q

R

S1

S2 Q

R

Soft Start

COMP

CS

1

3

Peak Current

Comparator

0.5V

Oscillator

1.2R

0.8V

PWM

Comparator

VDD

R

Slope = 1V/ms

14V

PWM

Latch

S

Q

R

Q

T

/Q

4

RC

Figure 1. MIC3808 Block Diagram

Micrel

8

VDD

7

OUTA

6

OUTB

5

GND

Functional Description

The MIC3808/9 is a high-speed power supply controller with

push-pull output drive capability. MIC3808 has a higher VDD

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

and turn-off than the MIC3809. The outputs of the controller

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

between them. A block diagram of the MIC3808/9 controller

is shown in Figure 1.

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. Exceeding the VDD zener shunt

voltage may cause excessive power dissipation in the

MIC3808/9.

Soft-Start

The soft start feature helps reduce surge currents at the

power supply input source. An internal current source and

capacitor ramp up from 0V to near Vdd at a typical rate of

1V/ms. The softstart feature limits the output voltage of the

error amplifier at the COMP pin. As the softstart voltage rises,

it allows the COMP pin voltage to rise, which in turn allows the

duty cycle of the output drivers to increase. The internal

softstart voltage is discharged and remains discharged dur-

ing the following conditions:

1. The VDD voltage drops below the turn-off

threshold

2. The voltage on the CS pin exceeds the

overcurrent comparator threshold

Once the internal softstart discharge FET is turned on, it

cannot be turned off until the internal softstart voltage drops

down below 0.5V. This insures a clean restart.

Oscillator

The oscillator operates at twice the switching frequency of

either OUTA or OUTB. The oscillator generates a sawtooth

waveform on the RC pin. The rising edge of the waveform is

controlled by the external resistor/capacitor combination.

The fall time is set by the on-resistance of the discharge FET

(see Figure 2). The fall time sets the delay (dead time)

between the turn-off of one output driver and the turn-on of the

other driver. A toggle flip-flop insures that drive signals to

OUTA and OUTB are alternated and therefore insures a

maximum duty cycle of less than 50% for each output driver.

Graphs of component values vs. oscillator frequency and

dead time are shown in the typical characteristic section of

this specification.

MIC3808/3809

8

April 2005

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