MP3421—5.5A, 600KHZ HIGH EFFICIENCY, SYNCHRONOUS, STEP-UP CONVERTER WITH OUTPUT DISCONNECT IN 2X2MM QFN PACKAGE
MP3421 Rev. 1.0 www.MonolithicPower.com 11
11/13/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MP3421 is a 600kHz, synchronous step-up
converter with true output disconnect offered
in a compact QFN 2X2-14 lead package. It
features fixed-frequency current mode PWM
controls for excellent line and load regulation.
Internal soft-start and loop compensation
simplifies the design process and minimizes
external components. The internal, R
DS(ON)
low
MOSFETs, combined with frequency-stretching
operation, allows the device to maintain high
efficiency over a wide-load current range.
Start-Up
When the IC is enabled and IN voltage exceeds
V
uvlo_in-R
, the MP3421 starts up in the linear
charge period. During this period, the PMOS
rectifier turns on until the output capacitor is
charged to 1.7V. The PMOS current is limited
to 0.2A when V
OUT
is 0V to avoid an inrush
current. While the output ramps, the PMOS
current limit increases and ramps to 0.7A at
1.7V output. This circuit helps to limit the
output current under short-circuit conditions.
Once the output is charged to 1.7V, the linear
charge period elapses and the MP3421 starts
switching in normal closed-loop operation. In
normal operation, with Vo lower than V
in
+0.3V,
the MP3421 operates in step-down mode with
4A typical peak-current limit. Also it works in
boost mode when Vo is higher than V
in
+0.3V
with more than 5.5A current limit (see Table 1).
Table 1: Work Mode during Startup
V
OUT
<1.7V
Linear Charge
Mode
V
OUT
≥1.7V & V
OUT
< V
IN
+ 0.3V
Down Mode
V
OUT
≥1.7V & V
OUT
≥ V
IN
+ 0.3V
Boost Mode
In down mode (if HS-FET is on), the HS-FET
gate is pulled to VIN and works with high
impedance. In down mode, power loss is high
and regulation is poor. Down mode is designed
to work in startup and SCP conditions; do not
set MP3421 in down mode for normal operation
(unless the thermal temperature and regulation
do not affect system performance).
Once the output voltage exceeds the input
voltage, the MP3421 powers its internal circuits
from V
OUT
instead of V
in.
Soft-Start (SS)
The MP3421 provides soft-start by charging an
internal capacitor with a current source. During
the linear-charge period, the soft-start voltage
rises, following the FB voltage. Once the linear
charge period elapses and the voltage on the
capacitor is charged, the reference voltage
slowly ramps. The reference soft-start time is
typically 2ms from 0V to 0.807V.
The soft-start capacitor is discharged
completely at output in the event of a
commanded shutdown, thermal shutdown, or
short circuit.
Enable (EN)
Operation is enabled when EN is switched high;
it enters shutdown mode when EN is switched
low. In shutdown mode, the regulator stops
switching and all internal control circuitry is
turned off; the load is isolated from the input.
Power-Save Mode (PSM)
The MP3421 automatically enters power-save
mode (PSM) when the load decreases; it
resumes PWM mode when the load increases.
When the device goes into PSM, it lowers the
switching frequency. This prevents switching
and driver losses and switches to pulse-skip
mode if the load continues to decrease.
Error Amplifier (EA)
The error amplifier (EA) is an internally-
compensated amplifier. The EA compares the
internal 0.807V reference voltage against V
FB
to
generate an error signal. The output voltage of
the MP3421 is adjusted by an external resistor
divider. A voltage divider from V
OUT
to ground
programs the output voltage via the FB from
2.5V to 5.5V using the equation:
V
OUT
=0.807V x (1+R1/R2)
Set the value of R1 and R2 to achieve low-
quiescent current. R1 values larger than 600k
