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APU3146

DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND AUTO-RESTART

Advanced Power Electronics Corp 

APU3146

Dual Soft-Start

The APU3146 has programmable soft-start to control the

output voltage rise and limit the inrush current during

start-up. It provides a separate Soft-Start function for each

outputs. This will enable to sequence the outputs by

controlling the rise time of each output through selection

of different value soft-start capacitors. The soft-start pins

will be connected together for applications where, both

outputs are required to ramp-up at the same time.

SS2 / SD 8

SS1 / SD 20

POR

25uA 25uA

64uA

Max

64uA

Error Amp1

To ensure correct start-up, the soft-start sequence ini-

tiates when the VCC, VCH1 and VCH2 rise above their

threshold (4.2V and 3.5V respectively) and generate the

Power On Reset (POR) signal. Soft-start function oper-

ates by sourcing an internal current to charge an exter-

nal capacitor to about 3V. Initially, the soft-start function

clamps the E/A’s output of the PWM converter. During

power up, the converter output starts at zero and thus

the voltage at Fb is about 0V. A current (64µA) injects

into the Fb pin and generates a voltage about 1.6V

(64µA×25K) across the negative input of E/A and (see

Figure6).

The magnitude of this current is inversely proportional to

the voltage at soft-start pin. The 25µA current source

starts to charge up the external capacitor. In the mean

time, the soft-start voltage ramps up, the current flowing

into Fb pin starts to decrease linearly and so does the

voltage at negative input of E/A.

When the soft-start capacitor is around 1V, the current

flowing into the Fb pin is approximately 32µA. The volt-

age at the positive input of the E/A is approximately:

32µA×25K = 0.8V

The E/A will start to operate and the output voltage starts

to increase. As the soft-start capacitor voltage contin-

ues to go up, the current flowing into the Fb pin will keep

decreasing. Because the voltage at pin of E/A is regu-

lated to reference voltage 0.8V, the voltage at the Fb is:

0.8V

Fb1 22

Comp1 21

VP2 26

Error Amp2

Fb2 6

Comp2 7

Figure 6 -Soft-start circuit for APU3146

Output of POR

Soft-Start 0V

Voltage

64uA

Current flowing

into Fb pin

Voltage at negative input ≅1.6V

of Error Amp

3V

≅2V

≅1V

0uA

0.8V

0.8V

VFB = 0.8-(25K×Injected Current)

The feedback voltage increases linearly as the injecting

current goes down. The injecting current drops to zero

when soft-start voltage is around 2V and the output volt-

age goes into steady state. Figure 7 shows the theoreti-

cal operational waveforms during soft-start.

0V

Voltage at Fb pin

Figure 7 - Theoretical operational waveforms

during soft-start.

The output start-up time is the time period when soft-

start capacitor voltage increases from 1V to 2V. The

start-up time will be dependent on the size of the exter-

nal soft-start capacitor. The start-up time can be esti-

mated by:

25µA×TSTART/CSS = 2V-1V

7/29

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