MP3422—6.5A, 600KHZ HIGH EFFICIENCY, SYNCHRONOUS, STEP-UP CONVERTER WITH OUTPUT DISCONNECT IN 2X2MM QFN PACKAGE
MP3422 Rev.1.01 www.MonolithicPower.com 11
8/6/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MP3422 is a 600kHz, synchronous step-up
converter with true output disconnect. It is
packaged in a QFN 2X2-14 lead package. The
device 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
low RDSon MOSFETs, combined with
frequency stretching operation, enables the
device to maintain high efficiency over a wide
load current range.
Start-Up
When the IC is enabled and the voltage on the
IN pin exceeds V
uvlo_in-R
, the MP3422 starts up
in the linear charge period. During this linear
charge period, the PMOS rectifier turns on until
the output capacitor is charged to 1.7V. The
PMOS current is limited to 0.2A when Vout is
0V to avoid inrush current. While the output
ramps up, the PMOS current limit also
increases and ramps to 0.7A at 1.7V output.
This circuit also 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 MP3422 starts switching in
normal closed loop operation. In normal
operation, with Vo lower than Vin+0.3V the
MP3422 operates in step down mode with 4A
typical peak current limit, and works in boost
mode when Vo is higher than Vin+0.3V with
more than 6.5A current limit.
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, gate of HS-FET is pulled to VIN,
and it works with high impedance when HS-
FET is on, the power-loss is high and regulation
is bad in down mode. Down mode is designed
for work in startup and SCP condition, it is not
suggested to set MP3422 in down mode in
normal work, unless the thermal and regulation
will not affect the system performance.
Once the output voltage exceeds the input
voltage, the MP3422 powers its internal circuits
from Vout instead of Vin.
Soft-Start
The MP3422 provides soft-start by charging an
internal capacitor with a current source. This
soft start voltage continues to rise, following the
FB voltage, during the linear charge period.
Once the linear charge period elapses, and the
voltage on this capacitor is charged, the
reference voltage is slowly ramped up. The
reference soft start time is typically 2ms from
0V to 0.807V.
The soft start capacitor is discharged
completely in the event of a commanded
shutdown, thermal shutdown or short circuit at
the output.
Device Enable
Operation is enabled when the EN pin is
switched high and placed into shutdown mode
when low. In shutdown mode, the regulator
stops switching and all internal control circuitry
is off. The load is isolated from the input.
Power Save Mode
The MP3422 will automatically enter power
save mode (PSM) when the load decreases
and resume PWM mode when the load
increases. When the device goes into PSM, it
lowers the switching frequency saving switching
and driver losses, and switches to pulse skip
mode if the load continues to decrease.
Error Amplifier
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 MP3422 is adjusted by an external resistor
divider. A voltage divider from V
OUT
to ground
programs the output voltage via the FB pin 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
are recommended for good stability and
transient balance.
