MP2147 – 5.5V, 4A, 1.2MHz, COT, BUCK SWITCHER

MP2147 Rev. 1.1

www.MonolithicPower.com

13

6/8/2016

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2016 MPS. All Rights Reserved.

OPERATION

The MP2147 uses constant on-time control with

input voltage feed-forward to stabilize the

switching frequency over its full input Voltage

range. During light loads, the MP2147 employs

a proprietary control over the low-side MOSFET

(LS-FET) and inductor current to improve

efficiency.

Constant-On-Time Control

When compared to fixed-frequency PWM

control,

constant-on-time

control

offers

a

simpler control loop and faster transient

response. The MP2147’s input-voltage feed-

forward 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

t0.83 s

V





To prevent inductor current runaway during the

load transient, the MP2147 has a fixed

minimum off time of 60ns. However, this

minimum off time limit does not affect the

operation of the MP2147 in steady state in any

way.

Sleep Mode Operation

MP2147 features sleep mode to get high

efficiency at extreme light load. In sleep-mode,

most of the circuit blocks are turned off, except

the error amplifier and PWM comparator, thus

the operation current is reduced to a minimal

value, as Figure 2.

When the loading gets lighter, the ripple of the

output voltage is bigger and it drives the error

amplifier output (EAO) lower. When EAO hits

an internal low threshold, it will be clamped at

that level, MP2147 enters sleep mode. During

sleep mode, the valley of the FB pin voltage is

regulated to the internal reference voltage, thus,

the average output voltage is slightly higher

than the output voltage at DCM or CCM mode.

The on-time pulse at sleep mode is around 40%

larger than that on DCM or CCM mode. Figure

3

shows

the

average

FB

pin

voltage

relationship with the internal reference at sleep

mode.

Figure 2: Operation Blocks at Sleep Mode

Figure 3: FB Average Voltage at Sleep Mode

When MP2147 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 in order to keep the output voltage

regulated and the output voltage ripple is

decrease relatively. Once EAO is more than

internal low threshold, MP2147 will be out of

sleep mode and enter DCM or CCM mode

depending on the loading. In DCM or CCM

mode, the EA regulates the average output

voltage to the internal reference which is shown

in Figure 4.

Figure 4: DCM Mode Control

There is always a loading hysteresis of entering

sleep mode and leaving sleep mode due to the

error amplifier clamping response time.

Light-Load Operation

During light loads, the MP2147 uses a

proprietary control scheme to save power and

improve efficiency: There is a zero current

cross circuit to detect if the inductor current

starts to reverse. LS-FET turns off immediately

when the inductor current starts to reverse and

trigger the ZCD in discontinuous conduction

mode (DCM) operation.