MP2161A – 2A, 6V, 1.5MHz SYNCHRONOUS STEP-DOWN CONVERTER

MP2161A Rev. 1.02

www.MonolihicPower.com

11

6/30/2016

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

© 2016 MPS. All Rights Reserved.

OPERATION

The MP2161A uses constant-on-time (COT)

control with input voltage feed forward to stabilize

the switching frequency over the full input range.

At light load, the MP2161A employs a proprietary

control of the low-side switch and inductor

current to eliminate ringing on the switching node

and 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 MP2161A maintains a nearly

constant switching frequency across the input

and output voltage range. The on time of the

switching pulse is estimated with Equation (1):

OUT

ON

IN

V

T0.667 s

V





(1)

To prevent inductor current runaway during load

transient, the MP2161A fixes the minimum off

time at 60ns. However, this minimum off time

limit will not affect operation of the MP2161A in

steady state.

Light-Load Operation

In a light-load condition, the MP2161A uses a

proprietary control scheme to save power and

improve efficiency. The MP2161A turns off the

low-side switch when the inductor current starts

to reverse. Then the MP2161A works in

discontinuous conduction mode (DCM) operation.

There is a zero current cross circuit to detect if

the

inductor

current

starts

to

reverse.

Considering the internal circuit propagation time,

the typical delay is 50ns. This means that with

this delay the inductor current will still fall after

the ZCD is triggered. If the inductor current falling

slew rate is fast (Vo voltage is high or close to

Vin), the low-side MOSFET (LS-FET) is turned

off, and the inductor current may be negative.

This phenomenon prevents the MP2161A from

entering DCM operation, even with no load. If

DCM mode is required, the off time of the LS-

FET in CCM should be longer than 100ns (2

times the propagation delay). For example, if Vin

is 3.6V and Vo is 3.3V, the off time in CCM is

55ns. It is difficult to enter DCM at light load.

Using a smaller inductor can improve this

condition and make it easier to enter DCM.

Enable (EN)

When the input voltage is greater than the under-

voltage lockout threshold (UVLO), typically 2.3V,

the MP2161A is enabled by pulling EN higher

than 1.2V. Floating or pulling EN down to ground

disables the MP2161A. There is an internal 1MΩ

resistor from EN to ground.

Soft Start (SS)

MP2161A has 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

1.15ms typically.

Power GOOD Indicator

MP2161A has an open drain with a 550kΩ pull-

up resistor pin for a power good indicator (PG).

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

voltage (e.g., 0.6V), PG is pulled up to VIN by the

internal resistor. If the FB voltage is out of the +/-

10% window, PG is pulled down to ground by an

internal MOSFET. The MOSFET has a maximum

Current Limit

MP2161A has a typical 3.2A current limit for the

high-side switch. When the high-side switch hits

the current limit, the MP2161A reaches the

hiccup threshold until the current decreases. This

prevents the inductor current from continuing to

build up, which results in damage to the

components.

Short Circuit and Recovery

The MP2161A enters short-circuit protection

mode when the current limit is reached. It tries to

recover from the short circuit with hiccup mode.

In short-circuit protection, the MP2161A disables

the output power stage, discharges a soft-start

capacitor, and then automatically tries to soft

start again. If the short-circuit condition remains

after the soft start ends, the MP2161A repeats

this operation cycle until the short circuit

disappears, and the output rises back to

regulation level.