MP2158 – 1A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN SWITCHER

MP2158 Rev. 1.0

www.MonolithicPower.com

9

10/10/2012

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

© 2012 MPS. All Rights Reserved.

OPERATION

MP2158 uses constant on-time control with

input voltage feed forward to stabilize the

switching frequency over full input range. At

light load, MP2158 employs a proprietary

control of low side switch and inductor current

to eliminate ringing on switching node and

improve efficiency.

Constant On-time Control

Compare to fixed frequency PWM control,

constant on-time control offers the advantage of

simpler control loop and faster transient response.

By using input voltage feed forward, MP2158

maintains a nearly constant switching frequency

across input and output voltage range. The on-

time of the switching pulse can be estimated as:

OUT

ON

IN

V

T0.667 s

V





To prevent inductor current run away during load

transient, MP2158 fixes the minimum off time to

be 60ns. However, this minimum off time limit will

not affect operation of MP2158 in steady state in

any way.

Light Load Operation

In light load condition, MP2158 uses a

proprietary control scheme to save power and

improve efficiency. The MP2158 will turn off the

low side switch when inductor current starts to

reverse. Then MP2158 works in discontinuous

conduction mode (DCM) operation.

Enable

When input voltage is greater than the under-

voltage lockout threshold (UVLO), typically

2.3V, MP2158 can be enabled by pulling EN pin

to higher than 1.2V. Leaving EN pin float or pull

down to ground will disable MP2158. There is

an internal 1Meg Ohm resistor from EN pin to

ground.

Soft Start

MP2158 has built-in soft start that ramps up the

output voltage in a controlled slew rate,

avoiding overshoot at startup. The soft start

time is about 1.5ms typical.

Power GOOD Indictor

MP2158 has an open drain with 550kΩ pull-up

resistor pin for power good indicator PGOOD.

When FB pin is within +/-10% of regulation

voltage, i.e. 0.6V, PGOOD pin is pulled up to IN

by the internal resistor. If FB pin voltage is out

of the +/-10% window, PGOOD pin is pulled

down to ground by an internal MOS FET. The

150Ω.

Current limit

MP2158 has a typical 2A current limit for the

high side switch. When the high side switch hits

current limit, MP2158 will touch the hiccup

threshold until the current lower down. This will

prevent inductor current from continuing to build

up which will result in damage of the

components.

Short Circuit and Recovery

MP2158 enters short circuit protection mode

also when the current limit is hit, and tries to

recover from short circuit with hiccup mode. In

short circuit protection, MP2158 will disable

output power stage, discharge soft-start cap

and then automatically try to soft-start again. If

the short circuit condition still holds after soft-

start ends, MP2158 repeats this operation cycle

till short circuit disappears and output rises

back to regulation level.