ML4903
DESIGN CONSIDERATIONS
(Continued)
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
I
SENSE
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 V
REF
bypass capacitor C14 close to the ML4903.
Ensure that its ground connection is to GND, not to PWR
GND.
The 12V V
DD
input is both the converter's bulk power
input, and the supply from which the internal circuitry of
the ML4903 operates. V
DD
also provides the gate drive
for P DRV and N DRV. The V
DD
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 10
W
resistor between 12V
IN
and V
DD
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 I
SENSE
and
GND. PWR GND should then return to the to the
grounded end of R
SENSE
as well, using a high current
Kelvin connection. This causes any noise across the
resistor to appear primarily as a common-mode signal on
I
SENSE
, 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 I
SENSE
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
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
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
