MP2158 – 1A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN SWITCHER
OPERATION
MP2158 uses constant on-time control with
input voltage feed forward to stabilize the
switching frequency over full input range. At
light load, MP2158 employs a proprietary
control of low side switch and inductor current
to eliminate ringing on switching node and
improve efficiency.
Constant On-time Control
Compare to fixed frequency PWM control,
constant on-time control offers the advantage of
simpler control loop and faster transient response.
By using input voltage feed forward, MP2158
maintains a nearly constant switching frequency
across input and output voltage range. The on-
time of the switching pulse can be estimated as:
T
ON
½
V
OUT
0.667
s
V
IN
an internal 1Meg Ohm resistor from EN pin to
ground.
Soft Start
MP2158 has built-in soft start that ramps up the
output voltage in a controlled slew rate,
avoiding overshoot at startup. The soft start
time is about 1.5ms typical.
Power GOOD Indictor
MP2158 has an open drain with 550kΩ pull-up
resistor pin for power good indicator PGOOD.
When FB pin is within +/-10% of regulation
voltage, i.e. 0.6V, PGOOD pin is pulled up to IN
by the internal resistor. If FB pin voltage is out
of the +/-10% window, PGOOD pin is pulled
down to ground by an internal MOS FET. The
MOS FET has a maximum R
dson
of less than
150Ω.
Current limit
MP2158 has a typical 2A current limit for the
high side switch. When the high side switch hits
current limit, MP2158 will touch the hiccup
threshold until the current lower down. This will
prevent inductor current from continuing to build
up which will result in damage of the
components.
Short Circuit and Recovery
MP2158 enters short circuit protection mode
also when the current limit is hit, and tries to
recover from short circuit with hiccup mode. In
short circuit protection, MP2158 will disable
output power stage, discharge soft-start cap
and then automatically try to soft-start again. If
the short circuit condition still holds after soft-
start ends, MP2158 repeats this operation cycle
till short circuit disappears and output rises
back to regulation level.
To prevent inductor current run away during load
transient, MP2158 fixes the minimum off time to
be 60ns. However, this minimum off time limit will
not affect operation of MP2158 in steady state in
any way.
Light Load Operation
In light load condition, MP2158 uses a
proprietary control scheme to save power and
improve efficiency. The MP2158 will turn off the
low side switch when inductor current starts to
reverse. Then MP2158 works in discontinuous
conduction mode (DCM) operation.
Enable
When input voltage is greater than the under-
voltage lockout threshold (UVLO), typically
2.3V, MP2158 can be enabled by pulling EN pin
to higher than 1.2V. Leaving EN pin float or pull
down to ground will disable MP2158. There is
MP2158 Rev. 1.0
10/10/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
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