MP2131 – 4A, 5.5V, 1.2MHZ SYNCHRONOUS STEP-DOWN SWITCHER

MP2131 Rev.1.0

www.MonolithicPower.com

11

5/6/2015

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© 2015 MPS. All Rights Reserved.

OPERATION

The MP2131 uses constant-on-time (COT)

control with input voltage feed forward to

stabilize the switching frequency over a full

input range. At light load, the MP2131 employs

a power-save mode that forces the part to

operate in discontinuous conduction mode

(DCM) to improve efficiency.

Constant-On-time (COT) Control

Compared to fixed frequency PWM control,

constant-on-time (COT) control offers the

advantage of a simpler control loop and faster

transient response. By using input voltage feed

forward, the MP2131 maintains a nearly

constant switching frequency across the input

and output voltage range. The on time of the

switching pulse can be estimated using

Equation (1):

OUT

ON

IN

V

T0.833 s

V

=⋅

μ

(1)

To prevent inductor current runaway during

load transient, the MP2131 fixes the minimum

off time at 100 ns. However, this minimum off

time limit will not affect operation in steady state.

AAM Operation at Light Operation

The MP2131 has advanced asynchronous

modulation (AAM) power-save mode with a

current zero-crossing detector (ZCD) circuit for

light load.

side MOSFET (HS-FET) on time is less than

the AAM blanking time (600 ns, typically), AAM

is disabled, and the HS-FET on time is

determined by the loop. Otherwise, the HS-FET

AAM current. Simplified AAM control is shown

in Figure 2.

HS_ driver

VREF

VFB

COT

Generator

EN

IAAM

IL_ sense

R

SQ

FBCOMP

AAMCOMP

Figure 2—Simplified AAM control logic

The AAM threshold decreases as Ton gradually

increases (see Figure 3).

Figure 3—AAM vs. on time

The MP2131 has a current zero-crossing

detector (ZCD) circuit to monitor if the inductor

current begins to reverse. When the inductor

current reaches the ZCD threshold, the low-side

MOSFET (LS-FET) is turned off.

The combination of AAM mode and the ZCD

circuit allow the MP2131 to always operate in

Enable (EN)

When the input voltage is greater than the

under-voltage

lockout

threshold

(2.48

V,

typically), the MP2131 is enabled by pulling EN

higher than 1.2 V. Floating EN or pulling it down

to ground disables the MP2131. There is an

internal 1 megohm resistor from EN to ground.

Soft Start/Stop

The MP2131 has a built-in soft start that ramps

up the output voltage in a controlled slew rate,

avoiding overshoot at start-up. The soft-start

time is about 0.5 ms typically. When EN shuts

down, ZCD is disabled, and the MP2131 ramps

down the internal reference, thus allowing the

load to discharge the output linearly. The soft-

stop time is typically 0.8 ms.

Power Good Indicator (PG)

The MP2131 has an open drain with a 400 kΩ

pull-up resistor pin for the power good indicator

(PG). When FB is within +/-10% of the

regulation voltage (i.e., 0.6 V), PG is pulled up

to VIN by the internal resistor. If the FB voltage

is out of the +/-10% window, PG is pulled down