16

 2001 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2450

The control model of SC2450 can be depicted in Fig. 1. This

model can also be used in Spice kind of simulator to generate

loop gain Bode plots.

The bandgap reference is 1 V and

trimmed to +/-1% accuracy. The desired output voltage can

be achieved by setting the resistive divider network, R1 and

R2.

The error amplifier is transconductance type with fixed gain of:

+

-

Vbg

C

R

+

-

Vin

Co

Rc

Ro

L

R2

R1

+

-

Error-Amp

0

0

0

0

0

Gpwm

Verror

Duty

. . . .

G error .

.

V

A

0.002 ⋅

The task here is to properly choose the compensation network

for a nicely shaped loop-gain Bode plot. The following design

procedures are recommended to accomplish the goal:

(1) Calculate the corner frequency of the output filter:

(2) Calculate the ESR zero frequency of the output filter ca-

pacitor:

(3) Check that the ESR zero frequency is not too high.

If this condition is not met, the compensation structure may

not provide loop stability. The solution is to add some electro-

lytic capacitors to the output capacitor bank to correct the

output filter corner frequency and the ESR zero frequency. In

some cases, the filter inductance may also need to be ad-

justed to shift the filter corner frequency. It is not recommended

to use only high frequency multi-layer ceramic capacitors for

output filter.

G pwm

1

V ramp

Ts

() T o

1s R

. C

.

sR

. C

.

.

1s R c

.

C o

.

1s R c C o

.

L

R o

.

s

2

L

. C

o

.

1

R c

R o

.

.

. . . .

T o G error Vin G pwm R

.

R 2

R 1 R 2

.

.

.

. esr

1

2 π

. R c

.

C o

.

. ers

. sw

5

<

. o

1

2 π

.

L C o

.

.

R

1

G pwm V in

.

G error

.

F esr

F o

2

.

F x_over

F esr

.

V o

V bg

.

.x_over

. sw

5

. o . esr

<

. x_over

<

Control Loop Design

Fig. 1. SC2450 control model.

The compensation network includes a resistor and a capacitor

in series, which terminates from the output of the error ampli-

fier to the ground.

This device uses voltage mode control with input voltage feed

forward. The peak-to-peak ramp voltage is proportional to the

input voltage, which results in an excellent performance to re-

ject input voltage variation. The PWM gain is inversion of the

ramp amplitude, and this gain is given by:

where the ramp amplitude (peak-to-peak) is 3 volts when input

voltage is 24 volts.

The total control loop-gain can then be derived as follows:

where

(4) Choose the loop gain cross over frequency (0 dB frequency).

It is recommended that the crossover frequency is always less

than one fifth of the switching frequency or the output ripple

frequency in bi-phase mode operation:

If the transient specification is not stringent, it is better to choose

a crossover frequency that is less than one tenth of the switch-

ing frequency for good noise immunity.

The resistor in the

compensation network can then be calculated as:

when: