6

Oct10-07 RevJ

SP6652 1A, High Efficiency, Current Mode PWM Buck Regulator

© 2007 Sipex Corporation

when the output voltage is low. The inductor

current tends to rise until the energy loss

from the discharge resistances are equal to

the energy gained during the charge phase.

For this reason, the clock frequency is cut in

half when the feedback pin is below 0.3V, ef-

fectively reducing the minimum duty cycle in

is normal (see Typical Operating Character-

Voltage Loop and Compensation

in PWM Mode

The voltage loop section of the circuit con-

sists of the error amplifier and the translator

circuits (see functional diagram). The input

of the voltage loop is the 0.8V reference volt-

age minus the divided down output voltage

at the feedback pin. The output of the error

amplifier is translated from a ground referred

signal (the COMP node) to a power input

voltage referred signal. The output of the

voltage loop is fed to the positive terminal

of the Current Loop comparator, and repre-

sents the peak inductor current necessary

to close the loop.

The total power supply loop is compensated

with a series RC network connected from

the COMP pin to ground. Compensation is

simple due to current-mode control. The

modulator has two dominant poles: one at a

low frequency, and one above the crossover

frequency of the loop, as seen in the graph

below, Linearized Modulator Frequency

Response vs. Inductor Value.

The low frequency pole for L1= 5µH is

4kHz, the second pole is 500kHz, and the

gain-bandwidth is 20kHz. The total loop

crossoverfrequencyischosentobe200kHz,

sets the second modulator pole at 2.5 times

the crossover frequency. Therefore the gain

of the error amplifier can be 200kHz/20kHz

= 10 at the first modulator pole of 4kHz. The

error amp transconductance is 1mA/V, so

pensation network at 10/1mA/V = 10kΩ.

The zero frequency is placed at the first

pole to provide at total system response of

-20dB/decade (the zero from the error amp

cancels the first modulator pole, leaving the

L1VAL

2u

3u

4u

5u

6u

7u

8u

9u

10u

Mod_pole1

Mod_pole2

Gbw_modfb

0

4K

8K

12K

16K

20K

1

0

0.4M

0.8M

1.2M

1.6M

2.0M

2

0

10K

20K

30K

40K

50K

3

>>

1

2

3

2µH to 10µH

Linearized Modulator Frequency Response vs. Inductor

DetaileD Description