ML4902CT datasheet(9/12 Pages) MICRO-LINEAR

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ML4902

DESIGN CONSIDERATIONS (Continued)

The power handling requirement for RSENSE is given by:

DOUT MAX

OUT

SENSE

=×

−

KJ ×

()

(5)

For example, for a 14A output, RSENSE should be:

SENSE =

−

=≅

105 14

592

ΩΩ

The maximum dissipation in RSENSE for a 5.0V input

occurs at 1.80V out, where PDISS is ≅ 0.94W.

RSENSE must be a low inductance part, such as Dale/

Vishay’s type WSL-2512 series (WSL–2512–.006±1%).

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.

If a current sense resistor is not employed for overcurrent

protection, the voltage drop across (Q3||Q4)’s channel

during its conducting interval (the synchronous

rectification interval) is used to monitor the inductor

current. Ignoring the AC component of the current in the

buck inductor, the voltage across (Q3||Q4) will be:

SENSE

DS ON QQ

=´

3 4

afbg

(6)

RDS(ON) is typically specified at a MOSFET junction

temperature (Tj) of 25ºC, but its value at other junction

temperatures can either be found graphically in the

MOSFET data sheet, or can be estimated by:

DS ON T

DS ON

()( )

()( º )

.º

225

1007

2 25

=´

af (7)

With a nominal threshold of -97mV for the ISENSE

comparator, the current limit threshold is then:

LIMIT

DS ON T

−97

()( )

(8)

For Pentium Pro and Pentium II applications, the

continuous current may be as high as 14A, so the current

limit threshold should be set for a minimum value of 16A

at the (Q3||Q4)’s highest anticipated Tj. If necessary, the

voltage across the channel of (Q3||Q4) may be divided

using two moderately-valued resistors (use R5 = 100

Ω)

and presented after that division to the ML4902.

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 1M

Ω and 220nF, respectively. These

values will protect the external power components and

the power source from overheating during an overcurrent

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 C12. Larger values of

C12 will increase the delay between retry attempts (the

length of the “hiccup”), and smaller values will reduce

the delay.

HIGHER CURRENT LEVELS

Next generation processor chips will require currents of up

to 20A. Additionally, it is often desirable in larger

systems to distribute all power from one 5V buss,

regulating it down to other voltages as needed at the

points of use. These applications are readily met by the

ML4902. For instance, the circuit shown in Figure 1 will

deliver an output current of 20A with only three changes:

• As IOUT increases, the ripple current through the input

capacitor bank will also increase. Add at least one

1500µF, 6.3V input capacitor in parallel with the three

shown (C1 - C3).

• Synchronous rectifier transistors Q3 and Q4 will see a

significantly greater RMS drain current at 20A output

than at 14A. Therefore, the use of lower RDS(ON) parts

such as Siliconix’ Si4420DY is required.

• The value of R1 may require adjustment, depending

upon factors such as the specific MOSFET type chosen

for Q3 and Q4, and the required operating ambient

temperature.

In dealing with circuits handling greater than 50W, it is

always important to pay attention to thermal issues.

When the circuit of Figure 1 is modified for >20A

applicatons, a key consideration is that it be provided

with adequate heatsinking. Ideally, the system should

provide 100 linear feet per minute (LFM) of airflow as

specified in Intel’s standards relating to VRMs. Micro

Linear does not recommend using the sense resistor

method of overcurrent protection at high output current

levels, as this does not provide the inherent thermal

foldback of IOUT(MAX) which is obtained by directly

sensing the VDS(ON) of the rectifier MOSFETs.

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