Fremont Micro Devices

Preliminary FT838MBD/838MDD

© 2013 Fremont Micro Devices Inc.

Confidential Rev0.80

DS838MBD/MDD-A0-Page9

FUNCTIONAL DESCRIPTION

Operating Description

FT838MBD/FT838MDD is a cost effective and high-performance AC-DC power supply controller for

off-line low power AC-DC applications including battery chargers, adaptors and LED lighting. The

constant voltage (CV) and constant current (CC) control are achieved accurately without the secondary

feedback circuit.

Start up Control

Start-up current of FT838MBD/838MDD 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

minimizes the power loss in operations and allows quick start up. With a special fast startup technology,

FT838MBD/FT838MDD starts up easily in capacitive loading applications.

Operating current

The operating current of FT838MBD/FT838MDD is as low as 500uA. Good efficiency is achieved with

the low operating current. Low operating current also reduces the Vcc hold-up capacitance

requirement.

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

The FT838MBD/FT838MDD 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.

Primary peak current modulation

Primary peak current is constant at constant current mode. And primary peak current is modulated at

constant voltage mode for the purposes of good dynamic load response and no audible noise over

entire operating range. According to the output loading current the current threshold voltage is

modulated from 0.55V to 0.27V. The current threshold voltage is 0.55V at rated load and 0.27V at light

load.

Leading edge blanking

Each time the power MOS 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)

FT838MBD/FT838MDD turn-on Vcc_on and turn-off Vcc_off are 16.2 V and 5.5 V, respectively. During

start-up, the hold-up capacitor must be charged to 16.2V through the start-up resistor. The hold-up