ML4901

7

PROTECT

VDD

P DRV

N DRV

PWR GND

COMP

ISENSE

VFB

ML4901

R1

6m

Ω

1W

4X

1800

µF

10V

L2

2.5.

µH

R3

330k

Ω

C9

33pF

C8

220nF

16V

D0

D1

D2

D3

SHDN

PWR GOOD

VREF

GND

3X

1800

µF

10V

C11

22

µF

25V

C10

220nF

16V

R4

1k

Ω

C12

220nF

16V

C13

1

µF

16V

R5

100k

Ω

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

D1

BAW56

OUTEN

UP#

VID0

VID1

VID2

VID3

PWRGD

C4

C5

C6

C7

C3

C1

C2

Q1

Q2

C14

1nF

Because of this, there must be a good electrical

connection between the ML4901 PWR GND and GND

pins. At the same time, PWR GND must have a low

impedance connection to the ground plane used on the

board, as high instantaneous currents will flow in PWR

GND when N DRV L and N DRV H switch the capacitive

loads of the output MOSFET gates. A layout technique

which satisfies these requirements is to return PWR GND

to the grounded end of R1 using a high current Kelvin

connection. Figure 2 shows one successful

implementation of these PCB layout requirements.

comparator. It is often helpful to shield the trace running

The compensation components R3 and C9 are high-

impedance nodes connected to the output of the voltage

loop error amplifier. These components should be kept in

close proximity to the ML4901. C9 should be returned to

GND, not to PWR GND or the ground plane of the PC

board. It may be helpful to shield the trace running from

R3 to COMP with a “guard trace” to circuit ground.

Ensure that its ground connection is to GND, not PWR

GND or the ground plane of the PCB.

returned to PWR GND or to the PC board ground plane.

They should not be returned to GND due to high transient

currents which could interfere with the current sensing

function.

If a given design uses power MOSFETs in an SO-8

package style, keep in mind that their thermal dissipation

capability is largely dictated by the copper area available

to their drains. A good layout will maximize this area.

Figure 2. Kelvin Sense Connections

SENSE

RESISTOR

TO

ISENSE

TO

GND

TO

PWR GND

POWER GROUND RETURN

(GROUND PLANE)

TO

SYNCHRONOUS

RECTIFIER

MOSFET

SOURCE

Figure 1. Pentium Pro VRM Circuit