Fremont Micro Devices

FT832DB

© 2012 Fremont Micro Devices Inc.

DS832DB-A0-page7

FUNCTIONAL DESCRIPTION

Operating Description

FT832DB are cost effective and high-performance AC-DC power supply controller for off-line low power

AC-DC applications including battery chargers and adaptors. Without secondary feedback circuit, the

constant voltage (CV) and constant current (CC) control can be achieved accurately.

Start up Control

Start-up current of FT832DB is very low so that a start-up resistor with high resistance and low-wattage

is allowed to supply the start-up power for the controller. The large value startup resistor can minimize

the power loss in application and starts up quickly. A 1.5Mo, 0.25W start-up resistor and a 10uF/25V

Vdd hold-up capacitor are sufficient for an AC-to DC power adapter.

Operating current

The operating current of FT832DB is as low as 1mA. Good efficiency is achieved with the low operating

current together with valley turn on of the external power NPN transistor. Low operating current also

reduces the Vcc hold-up capacitance requirement.

Constant voltage (CV) and constant current (CC) Operation

The FT832DB can accurately achieve CV/CC characteristic output without secondary side voltage and

current-feedback circuits. It operates in CV mode to regulate the output voltage by capturing the

auxiliary winding feedback voltage at FB pin. The auxiliary winding feedback voltage is proportional to

secondary winding, so it provides controller the feedback signal from secondary side and achieves

constant-voltage output. In CC mode, the controller detects the secondary discharger peak current and

the discharger time, which determines the off-time of the base driver to make the output average

current constant. In the CV or CC mode, the primary side peak current is constant if the Rcs is settled.

Leading edge blanking

Each time the power NPN transistor is switched on, a turn-on spike occurs at the sense resistor. To

avoid premature termination of the switching pulse, a 400ns leading edge blanking time is built in.

Conventional RC filtering can therefore be omitted. During this blanking period, the current limit

comparator is disabling and cannot switch off the base driver.

Under voltage lockout (UVLO)

FT832DB turn-on [Vcc(on)] and turn-off [Vcc(off)] are 15V and 5.4V. During start-up, the hold-up

capacitor must to be charged to 15V through the start-up resistor. The hold-up capacitor continues to

supply Vcc until power can be delivered from the auxiliary winding of the transformer. Vcc must not drop

below 5.4V during this start-up process. This UVLO hysteresis window ensures that hold-up capacitor is

sufficient to supply Vcc during start-up.

Protection control

With rich protection features of FT832DB, a good power supply system reliability is achieved. The

protection features including cycle by cycle current limiting, Vcc over voltage protection and clamp,