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APU3046

DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND LDO CONTROLLER

Advanced Power Electronics Corp 

APU3046

APPLICATION INFORMATION

Design Example:

The following example is a typical application for APU3046

in current sharing mode. The schematic is Figure 13 on

page 15.

For Switcher:

VIN1(MASTER) = 5V

VIN2(SLAVE) = 12V

VOUT1 = 1.5V

IOUT = 16A

DVOUT = 75mV

fS = 200KHz

For Linear Regulator:

VIN3 = 3.3V

VOUT2 = 2.5V

IOUT2 = 2A

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:

tSTART = 75 3 Css (ms)

---(2)

Where:

CSS is the soft-start capacitor (mF)

For a start-up time of 7.5ms, the soft-start capacitor will

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

PWM Section

Output Voltage Programming

Output voltage is programmed by reference voltage and

external voltage divider. The Fb1 pin is the inverting input

of the error amplifier, which is internally referenced to

1.25V. The divider is ratioed to provide 1.25V at the Fb1

pin when the output is at its desired value. The output

voltage is defined by using the following equation:

( ) VOUT1 = VREF 3

1

+

R6

R5

---(1)

When an external resistor divider is connected to the

output as shown in Figure 3.

APU3046

Fb1

VOUT1

R6

R5

Figure 3 - Typical application of the APU3046 for

programming the output voltage.

Equation (1) can be rewritten as:

( ) R6 = R5 3 VOUT1 - 1

VREF

This will result to:

VOUT1 = 1.5V, VREF = 1.25V, R5 = 1K, R6 = 200V

Boost Supply

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

gate voltage at least 4V greater than the bus voltage.

This is achieved by using a charge pump configuration

as shown in Figure 1. The capacitor is charged up to

approximately twice the bus voltage. A capacitor in the

range of 0.1mF to 1mF is generally adequate for most

applications.

Sense Resistor Selection

These resistors will determine the current sharing

between two channels. The relationship between the

Master and Slave output currents is expressed by:

RSEN1 3 IMASTER = RSEN2 3 ISLAVE

---(3)

For an equal current sharing, RSEN1=RSEN1

Choose RSEN1=RSEN2=5mV

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 control

MOSFET should be provided by input capacitor. The RMS

value of this ripple is expressed by:

IRMS = IOUT D3(1-D)

---(4)

Where:

D is the Duty Cycle, simply D=VOUT/VIN.

IRMS is the RMS value of the input capacitor current.

IOUT is the output current for each channel.

For VIN1=5V, IOUT1=8A and D1=0.3

Results to: IRMS1=3.6A

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.

And for VIN2=12V, IOUT2=8A and D2=0.125

Results to: IRMS2=2.6A

6/18

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