12

FN9052.2

November 15, 2004

.

Compensation Break Frequency Equations

Figure 9 shows an asymptotic plot of the DC-DC converter’s

gain vs frequency. The actual modulator gain has a high gain

peak due to the high Q factor of the output filter and is not

shown in Figure 9. Using the above guidelines should give a

compensation gain similar to the curve plotted. The open loop

error amplifier gain bounds the compensation gain. Check the

amplifier. The closed loop gain is constructed on the graph of

Figure 9 by adding the modulator gain (in dB) to the

compensation gain (in dB). This is equivalent to multiplying

the modulator transfer function to the compensation transfer

function and plotting the gain.

The compensation gain uses external impedance networks

loop. A stable control loop has a gain crossing with

-20dB/decade slope and a phase margin greater than 45

degrees. Include worst case component variations when

determining phase margin.

Component Selection Guidelines

Output Capacitor Selection

An output capacitor is required to filter the output and supply

the load transient current. The filtering requirements are a

function of the switching frequency and the ripple current.

The load transient requirements are a function of the slew

rate (di/dt) and the magnitude of the transient load current.

These requirements are generally met with a mix of

capacitors and careful layout.

Modern digital ICs can produce high transient load slew

rates. High-frequency capacitors initially supply the transient

and slow the current load rate seen by the bulk capacitors.

The bulk filter capacitor values are generally determined by

the ESR (effective series resistance) and voltage rating

requirements rather than actual capacitance requirements.

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance

components. Consult with the manufacturer of the load on

specific decoupling requirements.

Use only specialized low-ESR capacitors intended for

switching-regulator applications for the bulk capacitors. The

bulk capacitor’s ESR will determine the output ripple voltage

and the initial voltage drop after a high slew-rate transient. An

aluminum electrolytic capacitor’s ESR value is related to the

case size with lower ESR available in larger case sizes.

However, the equivalent series inductance (ESL) of these

capacitors increases with case size and can reduce the

usefulness of the capacitor to high slew-rate transient loading.

Unfortunately, ESL is not a specified parameter. Work with

your capacitor supplier and measure the capacitor’s

impedance with frequency to select a suitable component. In

most cases, multiple electrolytic capacitors of small case size

perform better than a single large case capacitor.

FIGURE 8. VOLTAGE-MODE BUCK CONVERTER

COMPENSATION DESIGN

REFERENCE

ESR

ERROR

AMP

PWM

DRIVER

(PARASITIC)

+

-

REFERENCE

COMP

FB

ISL6530

COMPARATOR

DRIVER

DETAILED COMPENSATION COMPONENTS

PHASE

+

-

+

-

OSC

F

Z2

1

2

π x R

1

R

3

+

() x C

3

-------------------------------------------------------

=

F

P1

1

2

π x R

C

C

1

C

2

+

----------------------







---------------------------------------------------------

=

F

P2

1

2

π x R

------------------------------------

=

F

Z1

1

2

π R

2

×

C

2

×

----------------------------------

=

FIGURE 9. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN

100

80

60

40

20

0

-20

-40

-60

10M

1M

100K

10K

1K

100

10

OPEN LOOP

ERROR AMP GAIN

COMPENSATION

FREQUENCY (Hz)

GAIN

MODULATOR

GAIN

LOOP GAIN

20

V

IN

V

OSC

----------------







log

20

R2

R1

--------





log

ISL6530