ML4900

6

DESIGN CONSIDERATIONS

This section is a quick-check guide for getting ML4900

circuits up and running, with a special emphasis on

Pentium Pro applications. 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

W 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 typically 10µA,

approximately 10 MHz.

INPUT AND OUTPUT CAPACITORS

The input and output capacitors used in conjunction with

the ML4900, especially in Pentium Pro VRM 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

aluminum electrolytic or OS-Con input capacitors need

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 possible for a buck

converter to slew the output current fast enough to

regulate the voltage in this application. During this

interval, the output capacitance of the converter must act

as passive energy storage. 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 (see Intel AP-523). The voltage transient due to

ESL and ESR is:

(1)

For example, assume that a 3.3V output has 3% of the

output's

DV contributed by ESR (100mV)and 2% by the

ESL (66mV). To meet this requirement, the output ESR

should not exceed:

ESR MAX)

mV

A

m

(

.

.

==

100

137

73

W

(2)

With the effects of ESL limited to 2% of 3.3V, the

maximum ESL is:

(3)

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

doing so typically requires using several high-quality

capacitors in parallel.

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

combination of aluminum electrolytic, OS-Con, and

tantalum devices.

OVERCURRENT PROTECTION

Current sense resistor R1 is used to monitor the inductor

current during the off period, i.e., while current is flowing

through the synchronous rectifier (or Schottky diode, if no

synchronous rectifier MOSFET is used). The internal

current sense comparator has been designed to provide in

excess of 14A of output current when used with a 5m

W

resistor. R1 must be a low inductance part such as Dale/

Vishay’s type WSL-2512-.005±2.5%. This is a 5m

W surface

mount part rated at 1 Watt. Using a PCB trace as a current

sense element is not recommended due to the high

temperature coefficient of copper, and due to etching and

plating tolerances which can occur from board to board.

The R and C values connected to the PROTECT pin for

setting the current limit delay and the off-time of the

hiccup mode are 100k

W and 1µF, respectively. These

values will protect most MOSFETs from overheating

during a short circuit condition. If it is necessary to change

the ratio of ON and OFF times during overcurrent

conditions, this can be done by selecting a different value

for C13. Larger values of C13 will increase the delay

between retry attempts (the length of the “hiccup”).

The voltage across current sense resistor R1 must be

Kelvin-sensed. This ensures that the ML4900 monitors only

the voltage across this resistor and not the voltage drops or

inductive transients in the PCB traces which carry current

into and out of this resistor. The two pins of the ML4900

which must be Kelvin-connected to the sense resistor are

ML4900 between GND (pin 8) and PWR GND (pin 12).

This is to facilitate the requisite Kelvin-sensing of the

voltage across R1. Because of this, there must be a good

electrical connection between the ML4900 PWR GND