© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FSBH0F70A_F116, FSBH0170/0270/0370_F116 • Rev. 1.0.1

12

Functional Description

Startup Operation

The HV pin is connected to bulk voltage through an

voltage is applied to the power system, an internal HV

startup circuit provides a high current (around 3.5mA) to

lower power consumption, the HV startup circuit shuts

and provide operating current to controller.

Figure 26.

Startup Circuit

Slope Compensation

The FSBH-series is designed for flyback power

converters. The peak-current-mode control is used to

optimize system performance. Slope compensation is

added to reduce current loop gain and improve power

system stability. The FSBH-series has a built-in,

synchronized, positive slope for each switching cycle.

Soft-Start

The FSBH-series has an internal soft-start circuit that

reduces the SenseFET switching current during power

system startup. The characteristic curve of soft-start

level rises in six steps. By doing so, the power system

can smoothly build up the rated output voltage and

effectively reduce voltage stress on the PWM switch

and output diode.

Figure 27.

Soft-Start Function

Brown-In/Out Function

FSBH0x70 has a built-in internal brown-in/out protection

comparator monitoring voltage of VIN pin. Figure 28

shows a resistive divider with low-pass filtering for line-

voltage detection on the VIN pin.

Figure 28.

Brown-In/Out Function on VIN Pin

Once the VIN pin voltage is lower than 0.6V and lasts

for 500ms, the PWM gate is disabled to protect the

increases above 1.1V. Because the divider resistors of

the VIN pin are connected behind the bridge, the ratio

calculations for brownout in PFC and non-PFC system

are different, as shown in Figure 29. The formulas are

provided in the following equations:

Brownout with PFC:

_

2

20.6

C

AC OUT

AB

C

R

V

RR

R









(1)

Brownout with non-PFC:

_

20.6

C

AC OUT

AB

C

R

V

RR

R





(2)

Brown-in level is determined by:

_

1.1

2

A

BC

AC

IN

C

RR

R

V

R





(3)

Figure 29.