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MP2187 Scheda tecnica(PDF) 12 Page - Monolithic Power Systems

Il numero della parte MP2187
Spiegazioni elettronici  Dual 3A, 5.5V, 1.2MHz, COT Synchronous Step-Down Switcher
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Produttore elettronici  MPS [Monolithic Power Systems]
Homepage  http://www.monolithicpower.com
Logo MPS - Monolithic Power Systems

MP2187 Scheda tecnica(HTML) 12 Page - Monolithic Power Systems

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MP2187 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER
MP2187 Rev. 1.0
www.MonolithicPower.com
12
12/12/2014
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MP2187 uses constant on-time control with
input voltage feed-forward to stabilize the
switching frequency over its full input range. At
light load, the MP2187 employs proprietary
control over the low-side MOSFET (LS-FET)
and inductor current to eliminate ringing on
switching node and improve efficiency. The
output voltages are fixed. The output voltages
of two channels are 1.1V and 1.8V respectively.
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 MP2187 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
t0.833 s
V
=⋅
μ
To prevent inductor current runaway during the
load transient, the MP2187 has a fixed minimum
off time of 50ns. However, this minimum off time
limit does not affect the operation of the MP2187
in steady state in any way.
Light-Load Operation
Under light-load conditions, the MP2187 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.
There is a zero current cross detect circuit
(ZCD) to judge if the inductor current starts to
reverse. When the inductor current touch ZCD
threshold, the low side switch will start to be
turned off.
The DCM mode happens only after low side
switch turned off by ZCD circuit. Considering
the ZCD circuit propagation time, the typical
delay is 30ns. It means the inductor current still
fall after the ZCD is trigger during this delay. If
the inductor current falling slew rate is fast
(Output voltage is high or close to input
Voltage), the low side MOSFET is turned off at
the moment inductor current may be negative.
This phenomena will cause PM2187 cannot
enter DCM operation. If the DCM mode is
required, the off time of low side MOSFET in
CCM should be longer than 60ns. It means the
maximum duty is 92% to guarantee DCM mode
at light load. For example, Vin is 3.4V and Vo is
3.3V, the off time in CCM is 24.5ns. 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 MP2187 can be enabled by pulling
the EN pin higher than 1.2V. Leaving EN pin
floating or grounded will disable the MP2187.
There is an internal 1MΩ resistor from the EN
pin to ground.
Soft-Start/Stop
MP2187 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 0.6ms. When disabled,
the MP2187 ramps down the internal reference
voltage to allow the load to linearly discharge
the output.
Power GOOD Indictor
MP2187 has an open drain with a 500kΩ pull-
up resistor pin for power good (PG) indication,
and the power good indication is only for
channel 1. 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 Rdson
of less than 100Ω.


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