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ML4903

High Current Synchronous Buck Controller

Micro Linear Corporation 

ML4903

DESIGN CONSIDERATIONS (Continued)

DRV and P DRV switch the capacitive loads of the output

MOSFET gates. At the same time, GND must not see the

resulting switching spikes.

If a current sensing resistor is used, the voltage across the

resistor must be Kelvin-sensed. This ensures that the

ML4903 monitors only the voltage across the resistor, and

ignores the voltage drops and inductive transients in the

PCB traces which carry current into and out of this

resistor. The two pins of the ML4903 which must be

Kelvin-connected to the sense resistor are ISENSE and

GND. PWR GND should then return to the to the

grounded end of RSENSE as well, using a high current

Kelvin connection. This causes any noise across the

resistor to appear primarily as a common-mode signal on

ISENSE, GND, and PWR GND. Figure 4 shows a

recommended implementation of these PCB layout

requirements.

When directly monitoring the voltage across the channel

of the synchronous rectifier, the voltage across that

MOSFET should be sensed as closely as possible to its

drain. If a resistor divider is used to reduce the voltage at

the ISENSE pin for a given current through (Q3||Q4)’s

channel resistance, then the lower end of the divider

should be returned to the immediate vicinity of its source.

This ensures that the ML4903 monitors only the voltage

across the synchronous rectifier, and not the voltage drops

or inductive transients in the PCB traces which carry

current into and out of it. If a PC board with a dedicated

ground plane is used (recommended), the best return

points for GND and PGND are directly into the ground

plane. If the board does not have a dedicated ground

plane, GND must be returned to a point near the IC which

is relatively free from switching transients. Such a point

may need to be empirically determined but will usually

be near the ground connection of the output capacitor

bank.

MISCELLANEOUS POINTS

ISENSE is the input to a medium-speed, high-sensitivity

comparator (roughly comparable to an LM339-type

comparator in terms of speed of response). Because of the

leading-edge blanking on this comparator, it has a

substantial ability to reject switching noise. Still, proper

circuit function requires that the comparator not see

significant noise at the time during which the synchronous

rectifier MOSFET is on.

The compensation components R3 and C13 are high-

impedance nodes connected to the output of the voltage

loop error amplifier. These components should be kept in

close proximity to the ML4903. C13 should be returned to

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

board.

Keep the VREF bypass capacitor C14 close to the ML4903.

Ensure that its ground connection is to GND, not to PWR

GND.

The 12V VDD input is both the converter's bulk power

input, and the supply from which the internal circuitry of

the ML4903 operates. VDD also provides the gate drive

for P DRV and N DRV. The VDD pin of the ML4903 should

be decoupled from the raw 12V system power to prevent

noise on the 12V supply from affecting the circuit's

switching action. A 10W resistor between 12VIN and VDD

TO

ISENSE

TO

PWR GND

TO

GND

TO

SYNCHRONOUS

RECTIFIER

MOSFET

SOURCE

SENSE

RESISTOR

POWER GROUND RETURN

(GROUND PLANE)

Figure 4. Kelvin Sense Connections

10

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