ML4902

7

DESIGN CONSIDERATIONS

This section is a quick-check guide for getting ML4902

circuits up and running, with a special emphasis on

Pentium Pro and Pentium II applications. Unless otherwise

noted, all component designators refer to the circuit

shown in Figure 1.

COMPENSATION

The R and C values connected to the COMP pin for loop

compensation are 330k

Ω and 33pF, respectively. These

values yield stable operation and rapid transient response

10,000µF), and will generally not need to be altered. If

changes do need to be made, note that the drive

capability of the transconductance error amplifier is

crossover frequency is approximately 10 MHz.

INPUT AND OUTPUT CAPACITORS

The input and output capacitors used in conjunction with

the ML4902, especially in Pentium Pro and Pentium II

applications, must be able to meet several criteria:

1. The input capacitors must be able to handle a

relatively high ripple current

2. The output capacitors must have a low Equivalent

Series Resistance (ESR) and Equivalent Series

Inductance (ESL)

3. The output capacitors must be able to hold up the

output during the time that the current through the

buck inductor is slewing to meet a transient load step.

The circuit’s input bypass capacitance should be able to

converter sees load peaks only occasionally, and for less

than 30 seconds at a time during those intervals, then the

only be sized to accommodate the average output load.

Note that tantalum input capacitors have much less

thermal mass than aluminum electrolytics, so this

relaxation of ripple current requirements may not apply to

them.

During a 30A/µs load transient, it is not practical for a

buck converter to slew the its current fast enough to

regulate the instantaneous output voltage required by this

application. During the first few microseconds following

such a “load step,” the output capacitance of the

converter must act as a passive energy source. In

delivering its energy to the load, the output capacitance

must not introduce any considerable impedance, or its

purpose will be defeated. A total voltage aberration

during load transients of ±5% is allowed for the Pentium

Pro and Pentium II. The voltage transient due to ESL and

ESR is:

DD

V

ESR

V

ESL di

dt

OUT

=´

+

´

FHG IKJ

L

NM

O

QP

bg

(1)

For example, assume that the output voltage of the

ML4902 is set to 2.8V. To allow no more than 3% of

capacitance, and 2% by its ESL (56mV), the output ESR

should not exceed:

ESR MAX)

mV

A

m

(

==

84

14

6 W

(2)

Similarly, the output ESL should be less than or equal to:

ESL MAX)

s

A

mV

nH

(.

=´

=

1

30

56

18

m

(3)

Achieving these low values of ESL and ESR is not trivial;

doing so typically requires using multiple high-quality

capacitors in parallel, often with dedicated power and

ground planes to minimize interconnection impedance.

The output capacitance should have a value of > 2200µF

to hold the output voltage relatively constant (< 50mV of

sag) until the current in the buck inductor can catch up

with the change in output current. To meet the ESR and

ESL requirements, the actual output capacitance will

usually be significantly greater than this theoretical

minimum. These capacitors can be of all one type, or a

tantalum devices.

Figures 2(a) and 2(b) show oscilloscope photographs of the

transient response of the circuit shown in Figure 1.

OVERCURRENT PROTECTION

Overcurrent protection for the ML4902 application circuit

can be accomplished either by using a low value sense

resistor placed between the current recirculating rectifier

and ground, or by directly monitoring the voltage drop

across a synchronous rectifier MOSFET (Q3||Q4) during

its conduction period. Using a current sense resistor has

the advantages of accuracy over the entire operating

temperature range, and of allowing the use of a Schottky

diode in place of a synchronous rectifier if the efficiency

loss is acceptable. The disadvantages to using a sense

resistor are higher cost and increased power dissipation.

Sensing across the synchronous rectifier has the

advantages of lower cost and of enhanced protection

against overtemperature conditions (the current limit point

is linearly reduced as the MOSFET temperature rises).

If a current sensing resistor is employed (see Figure 3), the

resistor monitors the inductor current during the buck

converter’s off period. This is the interval during which

current will recirculate through the synchronous rectifier,

or the Schottky diode if no synchronous rectifier is used.

Given a –87mV minimum trip point for the overcurrent

comparator, the value required for the sense resistor can

be found by:

R

mV

I

SENSE

OUT MAX

=

−

×

||

(.

)

()

87

105

(4)