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POWER MANAGEMENT

SC2608

Applications Information (Cont.)

The control model of SC2608 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 net-

work for a nicely shaped loop-gain Bode plot. The follow-

ing 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

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 cor-

rect 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 frequency 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 calcu-

lated as:

when

Fig. 4. SC2608 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)

is high while COMP is low, then the SC2608 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 SC2608,

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

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: