ACT412
Rev 1, 30-Oct-13
FUNCTIONAL DESCRIPTION
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.
transformer secondary and auxiliary turns, and VD
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:
Startup
Startup current of ACT412 is designed to be very
low so that VDD could be charged to VDDON
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 1Mꢀ, 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
output to achieve fast rise time. After this, VOUT
reaches about 90% VOUT , the IC operates with a
‘soft-landing’ mode (decrease Ippk) to avoid output
overshoot.
1
2
POUTCP
=
× L p × ( K 1VCS
)
× η × K 2 fSW
(2)
_ TH
2
where fSW is the switching frequency. The constant
power operation typically extends down to 20% of
nominal output voltage regulation.
Standby (No Load) Mode
Constant Voltage (CV) Mode Operation
In no load standby mode, the ACT412 oscillator
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.
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,
especially with different VOUT in a wide output power
range.
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:
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%.
RFB1
RFB 2
NS
NA
VOUTCV = 2.20V × (1 +
) ×
-VD
(1)
Primary Inductor Current Limit
Compensation
The ACT412 integrates a primary inductor peak
where RFB1 (R5) and RFB2 (R6) are top and bottom
feedback resistor, NS and NA are numbers of
Innovative PowerTM
- 7 -
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