APW7073KE-TY Просмотр технического описания (PDF) - Anpec Electronics

Номер в каталоге

Компоненты Описание

производитель

APW7073KE-TY

Synchronous Buck PWM Controller

Anpec Electronics 

APW7073

Application Information (Cont.)

MOSFET Selection (Cont.)

conduction loss and transition loss. For the upper

and lower MOSFET, the losses are approximately

given by the following:

PUPPER = IOUT (1+ TC)(RDS(ON))D + (0.5)( IOUT)(VIN)( tSW)FS

P = I (1+ TC)(R )(1-D)

LOWER OUT

DS(ON)

Where IOUT is the load current

TC is the temperature dependency of R

DS(ON)

F is the switching frequency

S

t is the switching interval

SW

D is the duty cycle

Note that both MOSFETs have conduction loss while

the upper MOSFET include an additional transition

loss. The switching internal, tSW, is a function of the

reverse transfer capacitance C . The (1+TC) term is

RSS

to factor in the temperature dependency of the R

DS(ON)

and can be extracted from the “R vs Temperature”

DS(ON)

curve of the power MOSFET.

PWM Compensation

The output LC filter of a step down converter introduces

a double pole, which contributes with -40dB/decade

gain slope and 180 degrees phase shift in the control

loop. A compensation network among COMP, FB and

VOUT should be added. The compensation network is

shown in Fig. 8. The output LC filter consists of the

output inductor and output capacitors. The transfer

function of the LC filter is given by:

GAIN LC

=

1 + s × ESR × C OUT

s 2 × L × C OUT + s × ESR × C OUT

+1

The poles and zero of this transfer functions are:

FLC = 2 × π ×

1

L × C OUT

FESR

=

1

2 × π × ESR

× C OUT

The FLC is the double poles of the LC filter, and FESR is

the zero introduced by the ESR of the output capacitor.

PHASE

L

OUTPUT

COUT

ESR

Figure 5. The Output LC Filter

FLC

-40dB/dec

FESR

-20dB/dec

Frequency(Hz)

Figure 6. The LC Filter GAIN and Frequency

The PWM modulator is shown in Figure 7. The input

is the output of the error amplifier and the output is the

PHASE node. The transfer function of the PWM

modulator is given by:

GAIN PWM

=

VIN

∆ V OSC

VIN

ΔVOSC

OSC

PWM

Comparator

Driver

Output of

Error Amplifier

PHASE

Driver

Figure 7. The PWM Modulator

The compensation network is shown in Figure 8. It

provides a close loop transfer function with the highest

zero crossover frequency and sufficient phase margin.

The transfer function of error amplifier is given by:

Copyright © ANPEC Electronics Corp.

16

Rev. A.1 - Apr., 2006

www.anpec.com.tw

Share Link: