MP2321 Rev. 1.0

www.MonolithicPower.com

12

10/26/2016

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© 2016 MPS. All Rights Reserved.

OPERATION

PWM Operation

The MP2321 is a fully-integrated, synchronous,

rectified, step-down, switch-mode converter.

The MP2321 uses constant-on-time (COT)

control to provide fast transient response and

easy loop compensation. Figure 2 shows the

simplified ramp compensation block.

At the beginning of each cycle, the high-side

MOSFET (HS-FET) turns on whenever the

insufficient output voltage. The input voltage

and the frequency-set resistor determine the

on period elapses, the HS-FET enters the off

state. By cycling HS-FET between the on and

off states, the converter regulates the output

voltage. The integrated low-side MOSFET (LS-

FET) turns on when the HS-FET is off to

minimize conduction loss. Shoot-through occurs

when the HS-FET and LS-FET are both turned

on at the same time, causing a dead short

between the input and GND. Shoot-through

reduces efficiency dramatically. The MP2321

prevents shoot-through by generating a dead-

time (DT) internally between HS-FET off and

LS-FET on, and LS-FET off and HS-FET on.

The MP2321 enters either heavy-load operation

or light-load operation depending on the output

current.

Figure 2: Simplified Ramp Compensation Block

MODE Selection

Connect a resistor (R6) from FREQ/MODE to

VIN to set the switching frequency and operate

the MP2321 in forced pulse-width modulation

(PWM) mode (see Figure 3). Connect a resistor

(R7) from FREQ/MODE to GND to set the

switching frequency and operate the MP2321 in

auto PWM or pulse-frequency modulation (PFM)

mode.

Figure 3: Mode Selection

Switching Frequency

The MP2321 uses constant-on-time (COT)

control. There is no dedicated oscillator in the

IC. The input voltage is forward fed into the on-

time one-shot timer through the frequency

the switching frequency is fairly constant over

the input voltage range. The approximate

typical switching frequency can be determined

with Equation (1):

6

SW

IN

on

OUT

10

F(KHz)

V (V)

T(ns)

V

(V)





(1)

and auto PFM/PWM mode. Approximate the

Equation (2):

FREQ

ON _ PWM

DELAY _ PWM

IN

14.5 R

(k )

TT

(ns)

V(V) 0.4









(2)

PFM/PWM mode with Equation (3):

FREQ

ON _ PFM

DELAY _ PFM

IN

13 R

(k )

TT

(ns)

V(V) 0.4









(3)

comparator delay. The typical values are

approximately 15ns and 10ns, respectively.

When

the

MP2321

enters

continuous

conduction mode (CCM), the duty ratio changes

slightly from light load to full load due to the

power loss. The frequency changes slightly

from light load to full load, even in CCM.

Because of the minimum on time and minimum

off time, the switching frequency is limited.