2010.04 Ver. 1.21 Copyright Grenergy OPTO, Inc. www.grenergy-ic.com

8

GR8836

Application Information

Start-up Current

The typical start-up current is 8uA.

Very low

start-up current allows the PWM controller to

increase the value of start-up resistor and then

reduce the power dissipation on it.

Under-voltage Lockout (UVLO)

A hysteresis UVLO comparator is implemented in

GR8836, then the turn-on and turn-off thresholds

level are fixed at 15V and 9V respectively.

This

hysteresis shown in Fig.13 ensures that the start-up

capacitor will be adequate to supply the chip during

start-up.

Fig.13

Soft Start

During initial power on, the GR8836 provides soft

start function.

It effectively suppresses the start up

peak current to reduce the power MOSFET drain

voltage especially at high line.

Oscillator

The maximum duty-cycle of internal oscillator is

limited

about

75%

to

avoid

the

transformer

saturation.

The frequency of the oscillator is

decided by an external resistor connected from RT

pin to ground.

fosc = 6500KHz/R (Kohm)

Where R is the resistor connected at RT pin.

A

100Kohm resistor results in 65KHz switching

frequency.

The recommended range of oscillation

frequency is 50 KHz~130 KHz

.

Green-Mode Operation

When the load decreases to an extent, the

frequency of the controller will decrease so as to

reduce

the

system

power

consumption.

The

minimum frequency is about 20 KHz, which is

outside the audio range

.

Leading-edge Blanking (LEB)

Each time the power MOSFET is switched on, a

turn-on spike will inevitably occur at the sense

resistor. To avoid fault trigger, a 350ns leading-edge

blanking time is built in. Conventional RC filtering

can therefore be omitted. During this blanking

period, the current-limit comparator is disabled and

can not switch off the gate driver.

Internal Slope Compensation

A built-in slope compensation circuit is constructed

in GR8836. When the switch is on, a ramp voltage

is added to the sensed voltage across the CS pin,

which helps to stabilize the system and prevent

sub-harmonic oscillations

.

Over-load Protection (OLP)

The controller has over load protection function.

An internal circuit detects the load level, when the

load is larger than a threshold and the condition

lasts more than 60ms, the gate output will keep low

level.

Then VCC decreases below UVLO off level,

the controller resets again.

Fig.14 shows the

waveform of the OLP operation.