MP2144 – 2A, 5.5V, 1.2MHz. 40μA I
Q
, SYNCHRONOUS STEP-DOWN SWITCHER
MP2144 Rev. 1.03 www.MonolithicPower.com 10
12/20/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
OPERATION
The MP2144 uses constant on-time control with
input voltage feed-forward to stabilize the
switching frequency over its full input range. At
light load, the MP2144 employs proprietary
control over the low-side MOSFET (LS-FET)
and inductor current to eliminate ringing on
switching node and improve efficiency.
Constant-On–Time Control
When compared to fixed-frequency PWM control,
constant-on–time control offers advantages
including simpler control loop and faster transient
response. By using input voltage feed-forward,
the MP2144 maintains a nearly constant
switching frequency across the entire input and
output voltage range. The on-time of the
switching pulse can be estimated as:
OUT
ON
IN
V
t0.833s
V
=⋅μ
To prevent inductor current runaway during the
load transient, the MP2144 has a fixed minimum
off time of 50ns. However, this minimum off time
limit does not affect the operation of the MP2144
in steady state in any way.
Light-Load Operation
Under light-load conditions, the MP2144 uses a
proprietary control scheme to save power and
improve efficiency: it gradually ramps down the
LS-FET current to its minimum instead of
turning off the LS-FET immediately when the
inductor current starts to reverse. The gradual
current drop avoids ringing at the switching
node that always occurs in discontinuous
conduction mode (DCM) operation.
Enable
When the input voltage exceeds the under-
voltage lockout (UVLO) threshold—typically
2.2V—the MP2144 can be enabled by pulling
the EN pin higher than 1.2V. Leaving EN pin
floating or grounded will disable the MP2144.
There is an internal 1M resistor from the EN
pin to ground.
Soft-Start/Stop
MP2144 has a built-in soft-start that ramps up
the output voltage at a constant slew rate that
avoids overshooting at startup. The soft-start
time is typically about 1ms. When disabled, the
MP2144 ramps down the internal reference
voltage to allow the load to linearly discharge
the output.
Power GOOD Indictor
MP2144 has an open drain with a 500k pull-
up resistor pin for power good (PG) indication.
When the FB pin is within ±10% of the
regulatory voltage (0.6V), the PG pin is pulled
up to VIN by the internal resistor. If the FB pin
voltage is outside the ±10% window, the PG pin
is pulled to ground by an internal MOSFET. The
MOSFET has a maximum R
dson
of less than
100.
Current limit
The MP2144 has a 3.3A minimum current limit
for the high side switch (HS-FET). When the
HS-FET hits its current limit, MP2144 enters
hiccup mode until the current drops to prevent
the inductor current from rising and possibly
damaging the components.
Short Circuit and Recovery
The MP2144 also enters short-circuit protection
(SCP) mode when it hits the current limit, and
tries to recover from the short circuit by entering
hiccup mode. In SCP, the MP2144 disables the
output power stage, discharges a soft-start
capacitor, and then enacts a soft-start
procedure. If the short-circuit condition still
holds after soft-start ends, the MP2144 repeats
this operation until the short circuit ceases and
output rises back to regulation level.
