MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER
MP4350 Rev. 1.0 www.MonolithicPower.com 12
5/26/2008 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
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Recommended VIN (max)
vs Switching Frequency
30
25
20
15
10
5
1500 2000 2500 3000 3500 4000
V
OUT
=3.3V
f
s
(KHz)
V
IN (MAX)
(V)
V
OUT
=2.5V
Figure 2—Recommend Max V
IN
vs. f
s
Since the internal bootstrap circuitry has higher
impedance, which may not be adequate to
charge the bootstrap capacitor during each
(1-D)×Ts charging period, an external bootstrap
charging diode is strongly recommended if the
switching frequency is above 2MHz (see
External Bootstrap Diode section for detailed
implementation information).
With higher switching frequencies, the inductive
reactance (X
L
) of capacitor comes to dominate,
so that the ESL of input/output capacitor
determines the input/output ripple voltage at
higher switching frequency. As a result of that,
high frequency ceramic capacitor is strongly
recommended as input decoupling capacitor
and output filtering capacitor for such high
frequency operation.
Layout becomes more important when the
device switches at higher frequency. It is
essential to place the input decoupling
capacitor, catch diode and the MP4350 (Vin pin,
SW pin and PGND) as close as possible, with
traces that are very short and fairly wide. This
can help to greatly reduce the voltage spike on
SW node, and lower the EMI noise level as well.
Try to run the feedback trace as far from the
inductor and noisy power traces as possible. It
is often a good idea to run the feedback trace
on the side of the PCB opposite of the inductor
with a ground plane separating the two. The
compensation components should be placed
closed to the MP4350.
Do not place the compensation components
close to or under high dv/dt SW node, or inside
the high di/dt power loop. If you have to do so,
the proper ground plane must be in place to
isolate those. Switching loss is expected to be
increased at high switching frequency. To help
to improve the thermal conduction, a grid of
thermal vias can be created right under the
exposed pad. It is recommended that they be
small (15mil barrel diameter) so that the hole is
essentially filled up during the plating process,
thus aiding conduction to the other side. Too
large a hole can cause ‘solder wicking’
problems during the reflow soldering process.
The pitch (distance between the centers) of
several such thermal vias in an area is typically
40mil. Please refer to the layout example on
EV4460 datasheet.
External Bootstrap Diode
It is recommended that an external bootstrap
diode be added when the input voltage is no
greater than 5V or the 5V rail is available in the
system. This helps improve the efficiency of the
regulator. The bootstrap diode can be a low
cost one such as IN4148 or BAT54.
MP4350
SW
BS
5V
Figure 3—External Bootstrap Diode
This diode is also recommended for high duty
cycle operation (when V
OUT
/V
IN
>65%) or low
V
IN
(<5Vin) applications.
At no load or light load, the converter may
operate in pulse skipping mode in order to
maintain the output voltage in regulation. Thus
there is less time to refresh the BS voltage. In
order to have enough gate voltage under such
operating conditions, the difference of V
IN
–V
OUT
should be greater than 3V. For example, if the
V
OUT
is set to 3.3V, the V
IN
needs to be higher
than 3.3V+3V=6.3V to maintain enough BS
voltage at no load or light load. To meet this
requirement, EN pin can be used to program
the input UVLO voltage to Vout+3V.
