ACT512

Rev 4, 13-Feb-14

- 6 -

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Copyright © 2014 Active-Semi, Inc.

Active-Semi Proprietary―For Authorized Recipients and Customers

ActiveQR

ACT512 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 60 Watt.

Unique fast startup, frequency foldback, QR

switching technique, accurate peak current line

compensation, idle mode, short winding protection,

OCP, OTP, OVP and UVLO are included in the

controller.

Startup

Startup current of ACT512 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

ACT512

regulates its output voltage through secondary side

control circuit . The output voltage information is

sensed at FB pin through OPTO coupling. The error

signal at FB pin is amplified through TL431 and

OPTO circuit. 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:

feedback resistor of the TL431.

No Load Idle Mode

In no load standby mode, the feedback voltage falls

switching. After it stops, as a result of a feedback

reaction, the feedback voltage increases. When the

switching again. Feedback voltage drops again and

output voltage starts to bounds back and forward

with very small output ripple. ACT512 leaves idle

mode when load is added strong enough to pull

Figure 2:

Idle Mode

Primary Inductor Current Limit

Compensation

The ACT512 integrates a primary inductor peak

current limit compensation circuit to achieve

constant OLP over wide line and wide inductance.

Frequency Foldback

When the load drops to 75% of full load level,

ACT512 starts to reduce the switching frequency,

which is proportional to the load current ,to improve

the efficiency of the converter.

ACT512’s load adaptive switching frequency

enables applications to meet all latest green energy

standards. The actual minimum average switching

frequency

is

programmable

with

output

capacitance, feedback circuit and dummy load

(while still meeting standby power).

Valley Switching

ACT512 employed valley switching from no load to

heavy load to reduce switching loss and EMI. In

discontinuous mode operation, the resonant voltage

between inductance and parasitic capacitance on

MOSFET source pin is coupled by auxiliary winding

and reflected on VDET pin through feedback

network R5, R6. Internally, the VDET pin is

connected to an zero-crossing detector to generate

the switch turn on signal when the conditions are

met.

FUNCTIONAL DESCRIPTION

(1)

)

R

R

1

(

V

V

2

F

1

F

431

TL

_

REF

OUTCV

+

×

=

Ip

Vo

Vfb

t

Ip_FL

Ilim

Vfb_fl

Vfb_olp

2A

Vfbbm1

Io

12V

Vfbbm2

0A