MPQ4459 – INDUSTRIAL GRADE, 1.5A, 4MHz, 36V STEP-DOWN CONVERTER
MPQ4459 Rev. 1.01 www.MonolithicPower.com 15
1/24/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
Since the internal bootstrap circuitry has higher
impedance, which may not be adequate to
charge the bootstrap capacitor during each
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 (XL) of a capacitor dominates, such
that the ESL of the input/output capacitor
determines the input/output ripple voltage at
higher switching frequencies. As a result, high
frequency ceramic capacitors are strongly
recommended as input decoupling capacitors
and output filtering capacitors.
Layout becomes more important when the
device switches at higher frequency. It is
essential to place the input decoupling
capacitor, catch diode and the MPQ4459 as
close together as possible, with traces that are
very short and fairly wide. This can help to
greatly reduce the voltage spikes on SW and
also lower the EMI noise level.
Try to run the feedback trace as far from the
inductor and noisy power traces as possible. It
is 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
close to the MPQ4459. Do not place the
compensation components close to or under
the 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 these
nodes. Switching losses are expected to
increase at high switching frequencies. To help
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.
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.
MPQ4459
SW
BS
0.1μ F
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
(<5V
IN
) 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
output voltage is set to 3.3V, the input voltage
needs to be higher than 3.3V+3V=6.3V to
maintain enough BS voltage at no load or light
loads. To meet this requirement, the EN pin can
be used to program the input UVLO voltage to
V
OUT
+3V.
