ML4903
DESIGN CONSIDERATIONS
(Continued)
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.
of the ML4903, and the V
DD
bypass capacitors C10 and
C15 serve this purpose. The V
DD
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.
V
CC
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 V
CC
should go
below acceptable levels. To reject logic switching noise
on the 5V input, an RC filter should be used between the
5V source and V
CC
. Typical values for this filter are R2 =
1k
W
, and C11 = 220nf.
11
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
