11

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2602L

The control model of SC2602L control loop small signal

can be depicted in Fig. 2. This model can also be used in

SPICE kind of simulator 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

capacitor in series, which terminates from the output of

the error amplifier to the ground.

This device uses voltage mode control with input volt-

age feed forward. The peak-to-peak ramp voltage is

proportional to the input voltage, which results in an ex-

cellent performance to reject 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 1.0 volts

when input voltage is 12 volts.

The total control loop-gain can then be derived as

follows:

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

.

.

G pwm

1

V ramp

⋅

⋅

R

⋅











⋅

:=

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 ac-

complish 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.

5

<

1

2

π

⋅

⋅

⋅

:=

1

2

π

⋅

⋅

:=

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

frequency). It is recommended that the crossover fre-

quency 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 cal-

culated as:

when

5

≤

R

1

⋅

⋅











2

⋅











⋅











⋅

:=

<

<

Fig. 2. SC2602L small signal model.

G m

1.8 mA

.

V

C

Vbg

1.25Vdc

L

Rc

R

Co

Gpwm

EA

Ro

R2

Vin

R1

0.5Vdc

0.8Vdc