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POWER MANAGEMENT

SC2608A

Applications Information (Cont.)

The control model of SC2608A is depicted in Fig. 4.

This model can also be used to generate loop gain Bode

plots. The bandgap reference is 0.8V 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:

The compensation network includes a resistor and a ca-

pacitor in series, which terminates the output of the

error amplifier to the ground.

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 accom-

plish the goal:

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

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

capacitor:

(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 electrolytic 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 adjusted to shift the filter corner

frequency. It is not recommended to use only high fre-

quency multi-layer ceramic capacitors for output filter.

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

quency). It is recommended that the crossover frequency

is always less than one fifth of the switching frequency :

If the transient specification is not stringent, it is better

to choose a crossover frequency that is less than one

tenth of the switching frequency for good noise immunity.

The resistor in the compensation network can then be

calculated as:

when

Fig. 4. SC2608A small signal model.

VIN

C

G_PWM

Rc

R

E/A

VBG

0.8V

Ro

L

R1

Ci

R2

Co

()

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

+

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

+

+

•

•

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

•

•

=

o

c

o

o

o

c

o

c

c

o

bg

in

pwm

m

R

R

LC

s

R

L

C

R

s

C

sR

s

H

V

V

V

G

G

s

T

1

1

1

2

()

i

c

sC

sC

R

s

H

+

+

=

1

1

1

o

o

LC

F

π

2

1

=

o

c

esr

C

R

F

π

2

1

=

5

SW

esr

F

F

<

SW

OVER

X

F

F

•

=

5

1

_

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

•

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

•

•

•

=

bg

o

esr

OVER

X

o

esr

m

in

pwm

V

V

F

F

F

F

G

V

G

R

_

2

1

<

5

<

V

A

G

m

007

.

0

=

The total control loop-gain can then be derived as

follows:

where the ramp amplitude is fixed at 1 volts.

ramp

pwm

V

G

1

=

Compensation Network Design

Compensation Network Design

Compensation Network Design

Compensation Network Design

Compensation Network Design

A note to the user is needed: The device cannot restart

until both COMP and SENSE are low, to prevent start up

into a charged output. In the event of an overcurrent

condition, the output is quickly discharged by the load,

therefore bringing SENSE below the 300mV threshold. If

the COMP pin is pulled low by an external device (such as

an open-drain logic gate used for system shutdown), and

SENSE is high(above 300mV) while COMP is low, then the

SC2608A turns on the low side FET to discharge the output

before changing to shutdown or soft-start mode. The low

side FET turns off when SENSE drops below 300mV and

the converter remains in the tri-state condition until COMP

is released. Although this shutdown technique can be used

successfully on the SC2608A, the system designer using

COMP for external shutdown will need to consider the load

on the low side FET when discharging the output capacitor

bank. For large capacitor bank, this peak current can be

quite large as it is limited only by the R

FET. Fortunately the duration of this event is quite short,

and has been shown in the lab to have no detrimental effect

on the performance of the external FETs.

Disabling the output by pulling down COMP/SS pin is only

recommended when the output capacitor bank is not too

large.

The PWM gain is inversion of the ramp amplitude, and

this gain is given by: