MP2148 Rev. 1.0

www.MonolithicPower.com

11

6/15/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP21600 uses constant-on-time (COT)

control with

input

voltage feed-forward to

stabilize the switching frequency over the full

input range. The MP21600 achieves 0.6A of

continuous output current from a 2.3V to 5.5V

input voltage range with excellent load and line

regulation. The output voltage can be regulated

as low as 0.6V.

Constant-On-Time (COT) Control

Compared

to

fixed-frequency

pulse-width

modulation (PWM) control, constant-on-time

(COT) control offers a simpler control loop and

a faster transient response. By using input-

voltage feed-forward, the MP21600 maintains a

nearly constant switching frequency across the

input and output voltage ranges. The switching

pulse on time can be estimated with Equation

(1):

OUT

ON

IN

V

T

0.416 s

V







(1)

To prevent inductor current runaway during the

load transient, the MP21600 has a fixed

minimum off time of 60ns.

Sleep Mode Operation

The MP21600 features sleep mode to achieve

high efficiency at extremely light loads. In sleep

mode, most of the circuit blocks are turned off,

except for the error amplifier (EA) and PWM

comparator. Therefore, the operation current is

reduced to a minimal value (see Figure 2).

Bias

&

Voltage

Reference

0.6V

FB

+

+

-

FBCOMP

Soft Start

E.A.

+

-

+

Ramp

Generator

SW

Lo-Iq

Lo-Iq

Figure 2: Operation Blocks in Sleep Mode

When the load becomes lighter, the ripple of the

output voltage is larger and drives the error

amplifier output (EAO) lower. When the EAO

reaches an internal low threshold, it is clamped

at that level, and the MP21600 enters sleep

mode. During sleep mode, the valley of the FB

voltage is regulated to the internal reference

voltage, making the average output voltage

slightly higher than the output voltage at

discontinuous

conduction

mode

(DCM)

or

continuous conduction mode (CCM). The on-

time pulse at sleep mode is around 40% larger

than that in DCM or CCM. Figure 3 shows the

average

FB

voltage

relationship

with

the

internal reference in sleep mode.

FB

Vref

Average FB

Figure 3: FB Average Voltage in Sleep Mode

When MP21600 is in sleep mode, the average

output voltage is higher than the internal

reference voltage. The EAO is kept low and

clamped in sleep mode. When the loading

increases,

the

PWM

switching

period

decreases to keep the output voltage regulated,

and the output voltage ripple is decreased

relatively. When the EAO is greater than the

internal low threshold, the MP21600 exits sleep

mode and enters DCM or CCM depending on

the load. In DCM or CCM, the EA regulates the

average output voltage to the internal reference

(see Figure 4).

FB+ Ramp

EA out

Average FB= REF

Figure 4: DCM Control

There is always a loading hysteresis when

entering sleep mode and exiting sleep mode

due to the EA clamping response time.

AAM Operation in Light-Load Operation

The MP21600 uses an advanced asynchronous

modulation (AAM) power-save mode together

with zero-current cross detection (ZCD) circuit

for light loads.

The MP21600 uses AAM power-save mode for

light loads (see Figure 5). The AAM current

decided by the on-time generator and AAM

comparator. At light load condition, the SW on

pulse time is the longer one. If the AAM

comparator pulse is longer than the on-timer

generator, the operation mode follows the

diagram shown in Figure 6.