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APU3039

SYNCHRONOUS PWM CONTROLLER WITH OVER CURRENT PROTECTION

Advanced Power Electronics Corp 

Advanced Power

Electronics Corp.

APU3039

APPLICATION INFORMATION

Design Example:

The following example is a typical application for APU3039,

the schematic is Figure 17 on page 16.

VIN = 18V

VOUT = 3.3V

IOUT = 8A

∆VOUT = 100mV (output voltage ripple ≅ 3% of VOUT)

fS = 200KHz

Output Voltage Programming

Output voltage is programmed by reference voltage and

external voltage divider. The Fb pin is the inverting input

of the error amplifier, which is referenced to the voltage

on non-inverting pin of error amplifier. For this applica-

tion, this pin (VP) is connected to reference voltage (VREF).

The output voltage is defined by using the following equa-

( ) tion:

VOUT = VP x

1

+

R6

R5

---(7)

VP = VREF = 0.8V

Css ≅ 20 x tSTART (µF)

---(8)

Where tSTART is the desired start-up time (ms)

For a start-up time of 5ms, the soft-start capacitor will

be 0.1µF. Choose a ceramic capacitor at 0.1µF.

Boost Supply Vc

To drive the high side switch, it is necessary to supply a

gate voltage at least 4V grater than the bus voltage. This

is achieved by using a charge pump configuration as

shown in Figure 9. This method is simple and inexpen-

sive. The operation of the circuit is as follows: when the

lower MOSFET is turned on, the capacitor (C1) is pulled

down to ground and charges, up to VOUT2 value, through

the diode (D1). The bus voltage will be added to this

voltage when upper MOSFET turns on in next cycle,

and providing supply voltage (Vc) through diode (D2). Vc

is approximately:

Vc ≅ VOUT2 + VBUS - (VD1 + VD2)

When an external resistor divider is connected to the

output as shown in Figure 8.

APU3039

VREF

Fb

VP

VOUT

R6

R5

Capacitors in the range of 0.1µF and 1µF are generally

adequate for most applications. The diode must be a

fast recovery device to minimize the amount of charge

fed back from the charge pump capacitor into VOUT2. The

diodes need to be able to block the full power rail volt-

age, which is seen when the high side MOSFET is

switched on. For low voltage application, schottky di-

odes can be used to minimize forward drop across the

diodes at start up.

Figure 8 - Typical application of the APU3039 for

programming the output voltage.

Equation (7) can be rewritten as:

( ) R6 = R5 x

VOUT

VP

-

1

Choose R5 = 1K

This will result to R6 = 3.16K

VOUT2

Regulator

APU3039

D1

C3

D2

Vc

C2

VBUS

C1

Q1

L2

HDrv

Q2

If the high value feedback resistors are used, the input

bias current of the Fb pin could cause a slight increase

in output voltage. The output voltage set point can be

more accurate by using precision resistor.

Soft-Start Programming

The soft-start timing can be programmed by selecting

the soft-start capacitance value. The start-up time of the

converter can be calculated by using:

Figure 9 - Charge pump circuit.

Input Capacitor Selection

The input filter capacitor should be based on how much

ripple the supply can tolerate on the DC input line. The

ripple current generated during the on time of upper

MOSFET should be provided by input capacitor. The RMS

value of this ripple is expressed by:

8

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