MP2187 – DUAL 3A, 5.5V, 1.2MHz, SYNCHRONOUS STEP-DOWN SWITCHER

MP2187 Rev. 1.0

www.MonolithicPower.com

12

12/12/2014

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© 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

of less than 100Ω.