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APW7067N

Synchronous Buck PWM and Linear Controller

Anpec Electronics 

APW7067N

Application Information (Cont.)

PWM Compensation (Cont.)

5. Set the second pole F at the half of the switching

P2

frequency and also set the second zero F at the output LC

Z2

filter double pole FLC. The compensation gain should not

exceed the error amplifier open loop gain, check the

compensation gain at FP2 with the capabilities of the

error amplifier.

F = 0.5 X F

P2

S

FZ2 = FLC

Combine the two equations will get the following

component calculations:

R3 =

R1

FS − 1

2 × FLC

C3 =

1

π × R3 × FS

FZ1 FZ2

FP1

FP2

20log

(R2/R1)

Compensation Gain

20log

(VIN/ΔVOSC)

FLC

FESR

PWM & Filter Gain

Converter Gain

Frequency(Hz)

Figure 10. Converter Gain and Frequency

Output Inductor Selection

The inductor value determines the inductor ripple current

and affects the load transient response. Higher inductor

value reduces the inductor’s ripple current and induces

lower output ripple voltage. The ripple current and ripple

voltage can be approximated by:

IRIPPLE

=

VIN1 − VOUT1 ×

FS × L

VOUT1

VIN1

∆VOUT1 = IRIPPLE × ESR

where Fs is the switching frequency of the regulator.

Although increase of the inductor value and frequency

reduces the ripple current and voltage, a tradeoff will

exist between the inductor’s ripple current and the

regulator load transient response time.

A smaller inductor will give the regulator a faster load

transient response at the expense of higher ripple current.

Increasing the switching frequency (FS) also reduces the

ripple current and voltage, but it will increase the

switching loss of the MOSFET and the power dissipation

of the converter. The maximum ripple current occurs at

the maximum input voltage. A good starting point is to

choose the ripple current to be approximately 30% of

the maximum output current. Once the inductance value

has been chosen, select an inductor that is capable of

carrying the required peak current without going into

saturation. In some types of inductors, especially core

that is made of ferrite, the ripple current will increase

abruptly when it saturates. This will result in a larger out-

put ripple voltage.

Output Capacitor Selection

Higher capacitor value and lower ESR reduce the output

ripple and the load transient drop. Therefore, selecting

high performance low ESR capacitors is intended for

switching regulator applications. In some applications,

multiple capacitors have to be parallel to achieve the

desired ESR value. A small decoupling capacitor in

parallel for bypassing the noise is also recommended,

and the voltage rating of the output capacitors also must

be considered. If tantalum capacitors are used, make

sure they are surge tested by the manufactures. If in doubt,

consult the capacitors manufacturer.

Input Capacitor Selection

The input capacitor is chosen based on the voltage

rating and the RMS current rating. For reliable operation,

select the capacitor voltage rating to be at least 1.3 times

higher than the maximum input voltage. The maximum

RMS current rating requirement is approximately I /2,

OUT1

where IOUT1 is the load current. During power up, the input

capacitors have to handle large amount of surge current.

If tantalum capacitors are used, make sure they are surge

Copyright © ANPEC Electronics Corp.

16

Rev. A.3 - Mar., 2008

www.anpec.com.tw

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