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APU3046

DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND LDO CONTROLLER

Advanced Power Electronics Corp 

APU3046

4) Place third pole at the half of the switching frequency.

FP3 =

fS

2

C12 =

1

2p 3 R7 3 FP3

C12 > 50pF

If not, change R7 selection.

The transfer function of power stage is expressed by:

G(s) =

IL2(s)

Ve(s)

=

VIN - VOUT

sL2 3 VOSC

---(17)

Where:

VIN = Input Voltage

VOUT = Output Voltage

L2 = Output Inductor

VOSC = Oscillator Peak Voltage

5) Place R7 in (16) and calculate C10:

C10 [

2p 3 Lo 3 FO 3 Co

R7

3 VOSC

VIN

6) Place second pole at ESR zero.

FP2 = FESR

R8

=

2p

3

1

C10

3

FP2

1

Check if R8 > gm

If R8 is too small, increase R7 and start from step 2.

7) Place second zero around the resonant frequency.

FZ2 = FLC

R6 =

1

2p 3 C10 3 FZ2

-

R8

8) Use equation (1) to calculate R5:

R5 =

VREF

VOUT - VREF

3 R6

These design rules will give a crossover frequency ap-

proximately one-tenth of the switching frequency. The

higher the band width, the potentially faster the load tran-

sient speed. The gain margin will be large enough to

provide high DC-regulation accuracy (typically -5dB to -

12dB). The phase margin should be greater than 458 for

overall stability.

As shown the transfer function is a function of inductor

current.

The transfer function for the compensation network is

given by equation (18), when using a series RC circuit

as shown in Figure 8:

( ) ( ) D(s) = Ve(s) =

RS2 3 IL2(s)

gm3

RS1

RS2

3

1 + sC2R2

sC2

---(18)

IL2

L2

Fb2

RS2

Vp2

E/A2

RS1

L1

Comp2

Ve

R2

C2

IL1

Figure 8 - The PI compensation network

for slave channel.

The loop gain function is:

H(s)=[G(s) 3 D(s) 3 RS2]

( ) ( ) ( ) H(s)=RS23

gm3

RS1

RS2

3

1+sR2C2

sC2

3

VIN-VOUT

sL23VOSC

The slave error amplifier is a differential-input transcon-

ductance amplifier as well, the main goal for the slave

feed back loop is to control the inductor current to match

the masters inductor current as well provides highest

bandwidth and adequate phase margin for overall stabil-

ity.

Select a zero crossover frequency (FO2) one-tenth of the

switching frequency:

FO2 =

fS

10

FO2 = 20KHz

11/18

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