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ML4903

High Current Synchronous Buck Controller

Micro Linear Corporation 

DESIGN CONSIDERATIONS (Continued)

of the ML4903, and the VDD bypass capacitors C10 and

C15 serve this purpose. The VDD bypass capacitors should

be 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.

VCC is the input to the 5V undervoltage lockout

comparator circuitry. The 5V UVLO function makes the

start-up of the ML4903 independent of power sequencing.

It also provides load protection in case VCC should go

below acceptable levels. To reject logic switching noise

on the 5V input, an RC filter should be used between the

15kVWs,oaunrcdeCa1n1d=V2C2C0. nTyf.pical values for this filter are R2 =

ML4903

Optional capacitor C16 may be needed in some layouts

to filter out “glitches” which could occur on the PWR

GOOD signal. In conjunction with the resistive pullup for

the PWR GOOD line, its value should yield an RC

product of approximately 5µs.

Power MOSFETs in 8-pin SOIC packages are among the

best for this application, especially for the P-channel

devices. Using P-channel MOSFETs minimizes

component count while ensuring full enhancement of

both the P-channel and N-channel MOSFETs. If 8-pin

SOIC MOSFETs are chosen, keep in mind that the

thermal dissipation capability of these parts is largely

dictated by the copper area available to their drains. A

good layout will maximize this area.

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