© 2009 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FSBH0F70A, FSBH0170/A, FSBH0270/A, FSBH0370 • Rev. 1.0.3

12

Functional Description

Startup Operation

The HV pin is connected to bulk voltage through an

voltage is applied to power system, an internal HV

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

better 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 slopes up like a six-step staircase. In doing so,

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

calculation 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

ABC

R

V

RR

R

π

⋅⋅

=

++

(1)

Brownout with non-PFC:

_

20.6

C

AC OUT

ABC

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.