MP3418 – 400mA, 1.2MHz, SYNCHRONOUS, STEP-UP CONVERTER WITH OUTPUT DISCONNECT

MP3418 Rev 1.0

www.MonolithicPower.com

9

2/7/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP3418 is a 1.2MHz, synchronous, step-

up converter housed in a compact TSOT23

package with true output disconnect. Able to

operate at low-input voltages of less than 0.8V,

the device features fixed-frequency current-

mode PWM control for exceptional line and load

regulation.

Internal

soft-start

and

loop

compensation simplify the design process and

minimize the external components. The internal

Save Mode operation enable the device to

maintain high efficiency over a wide current-

load range.

Synchronous Rectifier

The MP3418 integrates an N-channel and a P-

channel MOSFET to realize a synchronous

rectifier. Replacing the traditional Schottky

efficiency.

In

a

conventional

synchronous

rectifier, the PMOS body diode is forward-

The MP3418 allows for true output disconnect

by eliminating the body diode, and prevents

battery depletion when the converter shuts

down. To prevent excessive inductor current,

the PMOS synchronous rectifier only functions

Start-Up

When EN is on, the MP3418 starts up with a

linear charge period. During this linear charge

period, the rectifier PMOS turns on until the

PMOS current is limited during this period to

around 200mA to avoid inrush current. This

circuit also helps to limit the output current

under short circuit conditions.

After the linear charging period, the device

part then works in free-running mode; in this

mode, the device works in open loop first at a

variable frequency then at a fixed 500kHz

switching frequency, the duty cycle depends on

the input-output ratio, and the switching current

is limited to 400mA to avoid the start-up current

inrush.

The internal soft-start (SS) does not take

charge and continues to rise following the FB

voltage during the linear charging and free-

running

period.

Once

the

output

voltage

reaches 1.7V, the normal closed-loop operation

SS. It then works either in boost mode or down

the operation modes during start up.

Table 1: Operation Mode during Start-Up

Sequence

Linear Charge

Free Running

Down mode

Boost Mode

This allows the battery voltage to drop to as low

as 0.6V without affecting the circuit operation.

The battery supplies sufficient energy to the

output and becomes the only limiting factor in

the application.

Device Enable

The device operates when EN is high (>0.7V). It

enters shutdown mode when EN is set to low

(<0.1V). In shutdown mode, the regulator stops

switching and halts all internal control circuitry

and eliminate body-diode conduction of the

internal PMOS rectifier. This isolates the load

from the input, and means that the output

voltage can drop below the input voltage during

shutdown.

Power-Save Mode

The MP3418 automatically enters power-save

mode (PSM) when the load decreases, and

resumes PWM mode when the load increases.

In PSM, the converter only operates when the

output voltage falls below the set threshold, and

ramps up by several pulses with the same

switching frequency as PWM mode. It resumes

power-save mode once the output voltage

exceeds the high threshold. PSM requires

fewer circuit resources than PWM mode. All

unused circuitry is disabled, so as to reduce

quiescent

power

dissipation

as

well

as

switching power loss. The OUT pin voltage-