6

ML4866

FREQUENCY COMPENSATION

Frequency compensation of the ML4866 is required when

the converter is operating in PWM mode. Two simple

methods are provided to ensure the converter is frequency

stable. Both these methods will work only if the inductor

current is selected to be in CCM at the maximum load

current (see Inductor Selection). The first, called dominant

pole compensation, is used when non-varying loads are

expected. This method requires a single capacitor

connected from the error amplifier output (COMP Pin) to

ground.

For loads which change suddenly, the transient response

(or bandwidth) of the circuit must be increased to prevent

the output voltage from going outside of the regulation

band. The method used to accomplish this is called

zero/pole compensation and requires a series resistor

capacitor network from COMP to ground.

To determine which method works best for a given

application, apply the components found from the

zero/pole compensation method to an actual circuit

and examine the output voltage variation. If the voltage

variation is acceptable, connect the simpler, single

capacitor and re-check the output voltage for acceptable

load transient response.

NON-VARYING LOAD CURRENT

For the best possible response to load transients using only

a single capacitor, dominant pole compensation is

implemented with a single capacitor value of:

C

g

f

COMP

m

COMP

=

™

2

(6)

increased but at the risk of increased output voltage

variations with transient loads.

VARYING LOAD CURRENT

To minimize output voltage variations due to rapidly

changing load currents, use the series RC zero

compensation method to find the compensation network

component values that will improve the output voltage

response to load transients.

The unity gain bandwidth of the converter is extended to

15kHz using an RC network determined by:

R

G

g

where G

f

f

COMP

m

O

COMP

>=

,

(7)

C

R

COMP

COMP

=

™

1

50

p

(8)

390k

Either method of compensation for CCM mode with result

in continued stability as the ML4866 changes to DCM

mode at lighter load currents. Figure 3 shows a typical

application circuit for the ML4866.

VOLTAGE

ESR @

CAPACITANCE

RATING

SIZE

100kHz

4.7µF

16V

3216

0.490

W

10µF

6.3V

3216

0.368

W

22µF

16V

7343

0.149

W

33µF

6.3V

6032

0.291

W

47µF

10V

7343

0.144

W

100µF

6.3V

7343

0.088

W

Table 2. ESR Values for Low Cost Tantalum Capacitors

Figure 3. Typical Application Circuit

1

2

3

4

8

7

6

5

VOUT

COMP

VREF

BURST

GND

VL

SHDN

VIN

ML4866

15nF

100nF

15nF

390k

Ω

VIN

3.5V to 6.5V

33µF

100µF

100nF

VOUT

3.3V

100µH