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APU3039

SYNCHRONOUS PWM CONTROLLER WITH OVER CURRENT PROTECTION

Advanced Power Electronics Corp 

Advanced Power

Electronics Corp.

APU3039

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.

Shutdown

The converter can be shutdown by pulling the soft-start

pin below 0.4V. The control MOSFET turns off and the

synchronous MOSFET turns on during shutdown.

As shown in Figure 4, the positive pin of feedback UVLO

comparator is always higher than 0.4V, therefore, feed-

back UVLO is not functional during soft-start.

Output of UVLO

POR

3V

Soft-Start 0V

Voltage

Current flowing 64uA

into Fb pin

≅2V

≅1V

0uA

Over-Current Protection

Over-current protection is achieved with a cycle by cycle

scheme and it is performed by sensing current through

the RDS(ON) of low side MOSFET. As shown in Figure 5,

an external resistor (RSET) is connected between OCSet

pin and the drain of low side MOSFET (Q2) and sets the

current limit set point. The internal current source devel-

ops a voltage across RSET. When the low side switch is

turned on, the inductor current flows through the Q2 and

results a voltage which is given by:

VOCSET = IOCSET x RSET-RDS(ON) x iL

---(1)

Voltage at negative input ≅1.6V

of Error Amp and Feedback

UVLO comparator

0.8V

0.8V

Voltage at Fb pin 0V

Figure 4 - Theoretical operational waveforms

during soft-start.

IOCSET

APU3039

Osc

OCSet RSET

Q1

L1

Q2

VOUT

Figure 5 - Diagram of the over current sensing.

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 external

soft-start capacitor. The start-up time can be estimated

by:

20µA x TSTART/CSS = 2V-1V

For a given start up time, the soft-start capacitor can be

estimated as:

CSS ≅ 20µA x TSTART/1V

Internal Regulator

The regulator powers directly from Vcc and generates a

regulated voltage (6V @ 40mA). The output is protected

for short circuit. This voltage can be used for charge

pump circuitry as shown in Figure 1.

Supply Voltage Under-Voltage Lockout

The under-voltage lockout circuit assures that the

MOSFET driver outputs remain in the off state whenever

the supply voltage drops below set parameters. Lockout

occurs if Vc or Vcc fall below 3.4V and 4.4V respec-

tively. Normal operation resumes once Vc and Vcc rise

above the set values.

When voltage VOCSET is below zero, the current sensing

comparator flips and disables the oscillator. The high

side MOSFET is turned off and the low side MOSFET is

turned on until the inductor current reduces to below

current set value. The critical inductor current can be

calculated by setting:

VOCSET = IOCSET x RSET - RDS(ON) x IL = 0

ISET

=

IL(CRITICAL)

=

RSET x IOCSET

RDS(ON)

---(2)

If the over-current condition is temporary and goes away

quickly, the APU3039 will resume its normal operation.

If output is shorted or over-current condition persists,

the output voltage will keep going down until it is below

0.4V. Then the output under-voltage lock out comparator

goes high and turns off both MOSFETs. The operation

waveforms are shown in Figure 6.

6

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