ML4812IP datasheet(6/16 Pages) MICRO-LINEAR

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ML4812

OUTPUT DRIVER STAGE

The ML4812 output driver is a 1A peak output high speed

totem pole circuit designed to quickly drive capacitive

loads, such as power MOSFET gates. (Figure 3)

ERROR AMPLIFIER

The ML4812 error amplifier is a high open loop gain,

wide bandwidth, amplifier.(Figures 4-5)

GAIN MODULATOR

The ML4812 gain modulator is of the current-input type

to provide high immunity to the disturbances caused by

high power switching. The rectified line input sine wave is

converted to a current via a dropping resistor. In this way,

small amounts of ground noise produce an insignificant

effect on the reference to the PWM comparator. The

output of the gain modulator is a current of the form: IOUT

is proportional to ISINE × IEA, where ISINE is the current in

the dropping resistor, and IEA is a current proportional to

FUNCTIONAL DESCRIPTION (Continued)

Figure 5. Error Amplifier Open-Loop Gain and

Phase vs Frequency

Figure 7. Gain Modulator Linearity

EA–

–

0.5mA

EA OUT

Figure 4. Error Amplifier Configuration

the output of the error amplifier. When the error amplifier

is saturated high, the output of the gain modulator is

approximately equal to the ISINE input current. The gain

modulator output current is converted into the reference

voltage for the PWM comparator through a resistor to

ground on the gain modulator output. The gain modulator

output is clamped to 5V to provide current limiting.

Ramp compensation is accomplished by subtracting 1/2

of the current flowing out of RAMP COMP through a

buffer transistor driven by CT which is set by an external

resistor.

UNDER VOLTAGE LOCKOUT

On power-up the ML4812 remains in the UVLO

condition; output low and quiescent current low. The IC

becomes operational when VCC reaches 16V. When VCC

drops below 10V, the UVLO condition is imposed.

During the UVLO condition, the 5V VREF pin is “off”,

making it usable as a “flag” for starting up a downstream

PWM converter.

RAMP COMP

GM OUT

ISINE

ERROR CURRENT

IRAMP COMP

ISINE × ERROR CURRENT

– IRAMP COMP/2

Figure 6. Gain Modulator Block Diagram

500

400

300

200

100

SINE INPUT CURRENT (µA)

200

300

500

400

100

4.5

4.0

3.5

3.0

2.5

2.0

1.5

FREQUENCY (Hz)

100

10k

10M

100k

100

-20

-30

-60

-90

-120

-150

-180

GAIN

PHASE

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