APU3039M View Datasheet(PDF) - Advanced Power Electronics Corp

Part Name

Description

Manufacturer

APU3039M

SYNCHRONOUS PWM CONTROLLER WITH OVER CURRENT PROTECTION

Advanced Power Electronics Corp 

Advanced Power

Electronics Corp.

APU3039

The APU3039’s error amplifier is a differential-input

transconductance amplifier. The output is available for

DC gain control or AC phase compensation.

The E/A can be compensated with or without the use of

local feedback. When operated without local feedback,

the transconductance properties of the E/A become evi-

dent and can be used to cancel one of the output filter

poles. This will be accomplished with a series RC circuit

from Comp pin to ground as shown in Figure 12.

Note that this method requires that the output capacitor

should have enough ESR to satisfy stability requirements.

In general, the output capacitor’s ESR generates a zero

typically at 5KHz to 50KHz which is essential for an

acceptable phase margin.

The ESR zero of the output capacitor expressed as fol-

lows:

FESR

=

2π

x

1

ESR

x

Co

---(14)

VOUT

R6 Fb

R5

Vp=VREF

E/A

Comp

Ve

C9

R4

Gain(dB)

H(s) dB

First select the desired zero-crossover frequency (Fo):

Fo > FESR and FO [ (1/5 ~ 1/10) x fS

Use the following equation to calculate R4:

R4

=

VOSC

VIN

x

FoxFESR

FLC2

x

R5 + R6

R5

x

1

gm

---(18)

Where:

VIN = Maximum Input Voltage

VOSC = Oscillator Ramp Voltage

Fo = Crossover Frequency

FESR = Zero Frequency of the Output Capacitor

FLC = Resonant Frequency of the Output Filter

R5 and R6 = Resistor Dividers for Output Voltage

Programming

gm = Error Amplifier Transconductance

For:

VIN = 18V

VOSC = 1.25V

Fo = 20KHz

FESR = 12KHz

FLC = 2.8KHz

R5 = 1K

R6 = 3.16K

gm = 700µmho

This results to R4=12.08K

Choose R4=14K

To cancel one of the LC filter poles, place the zero be-

fore the LC filter resonant frequency pole:

FZ ≅ 75%FLC

FZ ≅ 0.75X

2π

For:

Lo = 4.7µH

Co = 660µF

1

LO x CO

---(19)

FZ = 2.1KHz

R4 = 14K

FZ Frequency

Figure 12 - Compensation network without local

feedback and its asymptotic gain plot.

The transfer function (Ve / VOUT) is given by:

( ) H(s) =

gm x

R5

R6 + R5

x

1 + sR4C9

sC9

---(15)

The (s) indicates that the transfer function varies as a

function of frequency. This configuration introduces a gain

and zero, expressed by:

|H(s=jx2πxFO)|

=

gm

x

R5

R6xR5

x

R4

---(16)

FZ

=

1

2πxR4xC9

---(17)

|H(s)| is the gain at zero cross frequency.

Using equations (17) and (19) to calculate C9, we get:

C9 ≅ 5300pF; Choose C9 =5600pF

One more capacitor is sometimes added in parallel with

C9 and R4. This introduces one more pole which is mainly

used to suppress the switching noise. The additional

pole is given by:

1

FP

=

2

π

x

R

4

x

C9xCPOLE

C9 + CPOLE

The pole sets to one half of switching frequency which

results in the capacitor CPOLE:

CPOLE =

1

π x R4 x fS -

1

C9

for FP <<

fS

2

1

≅ πx R4xfS

11

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