MP2143 – 3A, 5.5V, 1.2MHz, 40μA I
Q
, COT SYNCHRONOUS STEP-DOWN SWITCHER
MP2143 Rev. 1.07 www.MonolithicPower.com 11
7/4/2016 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2016 MPS. All Rights Reserved.
OPERATION
The MP2143 uses constant on-time control with
input-voltage feed-forward to stabilize the
switching frequency over its full input range. At
light load, the MP2143 employs a proprietary
control over the low-side MOSFET (LS-FET)
and inductor current to eliminate ringing on
switching node and to improve efficiency.
Constant On-Time Control
When compared to fixed-frequency PWM control,
constant on-time control offers a simpler control
loop and faster transient response. By using
input-voltage feed-forward, the MP2143 maintains
a nearly constant switching frequency across the
entire input and output voltage range. The
switching pulse ON time can be estimated as:
OUT
ON
IN
V
t0.833s
V
To prevent inductor current runaway during the
load transient, the MP2143 has a fixed minimum
OFF time of 50ns. However, this minimum OFF
time limit does not affect the operation of the
MP2143 in steady state in any way.
Light Load Operation
In light load condition, the MP2143 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.
Considering the internal circuit propagation time,
the typical delay is 50ns. It means the inductor
current still fall after the ZCD is trigger in this
delay. If the inductor current falling slew rate is
fast (VOUT voltage is high or close to Vin), the
low side MOSFET is turned off and inductor
current may be negative. This phenomena will
cause MP2143 can not enter DCM operation. If
the DCM mode is required, the off time of low
side MOSFET in CCM should be longer than
100ns. For example, Vin is 3.6V and Vo is 3.3V,
the off time in CCM is 50ns. It is difficult to enter
DCM at light load. And using smaller inductor
can improve it and make it enter DCM easily.
Enable
When the input voltage exceeds the under-
voltage lockout (UVLO) threshold—typically
2.2V—the MP2143 is enabled by pulling the EN
pin above 1.2V. Leaving the EN pin floating or
grounded will disable the MP2143. There is an
internal 1M resistor from the EN pin to ground.
Soft-Start/Stop
MP2143 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
MP2143 ramps down the internal reference
voltage to allow the load to linearly discharge
the output.
Power GOOD Indicator
MP2143 has an open drain with 500k pull-up
resistor pin for power good (PG) indication.
When the FB pin is within ±10% of regulation
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 MP2143 has a 4.8A current limit for the
high side switch (HS-FET). When the HS-FET
hits its current limit, the MP2143 enters hiccup
mode until the current drops to prevent the
inductor current from building and possibly
damaging the components.
Short Circuit and Recovery
The MP2143 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 MP2143 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 MP2143 repeats
this operation until the short circuit ceases and
output rises back to regulation level.
