ACT412

Rev 1, 30-Oct-13

- 7 -

www.active-semi.com

Copyright © 2013 Active-Semi, Inc.

Active-Semi Proprietary―For Authorized Recipients and Customers

ActivePSR

ACT412 is a high performance peak current mode

low-voltage PWM controller IC. The controller

includes the most advance features that are

required in the adaptor applications up to 36 Watt.

Unique fast startup, frequency fold back, QR

switching technique, accurate OLP, low standby

mode operation, external compensation adjustment,

short winding protection, OCP, OTP, OVP and

UVLO are included in the controller.

Startup

Startup current of ACT412 is designed to be very

threshold level and device starts up quickly. A large

value startup resistor can therefore be used to

minimize the power loss yet reliable startup in

application. For a typical AC/DC adaptor with

universal input range design, two 1MΩ, 1/8 W

startup resistors could be used together with a VDD

capacitor(4.7uF) to provide a fast startup and yet

low power dissipation design solution.

During startup period, the IC begins to operate with

minimum Ippk to minimize the switching stresses

for the main switch, output diode and transformers.

And then, the IC operates at maximum power

‘soft-landing’ mode (decrease Ippk) to avoid output

overshoot.

Constant Voltage (CV) Mode Operation

In constant voltage operation, the ACT412 senses

the output voltage at FB pin through a resistor

divider network R5 and R6 in Figure 2. The signal

at FB pin is pre-amplified against the internal

reference voltage, and the secondary side output

voltage is extracted based on Active-Semi's

proprietary filter architecture.

This error signal is then amplified by the internal

error amplifier. When the secondary output voltage

is above regulation, the error amplifier output

voltage decreases to reduce the switch current.

When the secondary output voltage is below

regulation, the error amplifier output voltage

increases to ramp up the switch current to bring the

secondary output back to regulation. The output

regulation voltage is determined by the following

relationship:

is the rectifier diode forward drop voltage at

approximately 0.1A bias.

Constant Power (CP) Mode Operation

When the secondary output current reaches a level

set by the internal current limiting circuit, the

ACT412 enters current limit condition and causes

the secondary output voltage to drop. As the output

voltage decreases, so does the flyback voltage in a

proportional manner. An internal current shaping

circuitry adjusts the switching frequency and current

limit threshold slowly based on the flyback voltage

so that the transferred power is fixed to the output

voltage, resulting in a constant power at secondary

side output power profile. Through correctly setting

K1, K2, the energy transferred to the output during

each switching cycle is ½(LP × ILIM^2) , where LP

is the transformer primary inductance, ILIM is the

primary peak current, Formula can be present as

below:

power operation typically extends down to 20% of

nominal output voltage regulation.

Standby (No Load) Mode

In no load standby mode, the ACT412 oscillator

frequency is further reduced to a minimum

frequency while the current pulse is reduced to a

minimum level to minimize standby power. The

actual

minimum

switching

frequency

is

programmable with an output preload resistor.

Loop Compensation

The ACT412 allows external loop compensation by

connecting a capacitor to extend its applications,

range.

Primary Inductance Compensation

The

ACT412

integrates

a

built-in

primary

inductance

compensation

circuit

to

maintain

constant OLP despite variations in transformer

manufacturing. The compensated ranges is +/-7%.

Primary Inductor Current Limit

Compensation

The ACT412 integrates a primary inductor peak

FUNCTIONAL DESCRIPTION

(1)

D

A

S

2

FB

1

FB

OUTCV

V

N

N

)

R

R

1

(

V

20

.

2

V

-

×

+

×

=

SW

2

2

TH

_

CS

1

p

OUTCP

f

K

)

V

K

(

L

2

1

P

×

×

×

×

=

η

(2)