MP4559 – 1.5A, 2MHz, 55V STEP-DOWN CONVERTER

MP4559 Rev. 1.01

www.MonolithicPower.com

8

12/23/2013

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© 2013 MPS. All Rights Reserved.

OPERATION

The MP4559 is a programmable frequency,

non-synchronous, step-down switching regulator

with an integrated high-side high voltage power

MOSFET. It provides a single highly efficient

solution with current mode control for fast loop

response and easy compensation. It features a

wide input voltage range, internal soft-start

control and precision current limiting. Its very low

operational quiescent current makes it suitable

for battery powered applications.

PWM Control Mode

At moderate to high output current, the MP4559

operates in a fixed frequency, peak current

control mode to regulate the output voltage. A

PWM cycle is initiated by the internal clock. The

power MOSFET is turned on and remains on

until its current reaches the value set by the

COMP voltage. When the power switch is off, it

remains off for at least 100ns before the next

cycle starts. If, in one PWM period, the current in

the power MOSFET does not reach the COMP

set current value, the power MOSFET remains

on, saving a turn-off operation.

Pulse Skipping Mode

Under

light

load

condition

the

switching

frequency stretches down to zero to reduce the

switching loss and driving loss.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal reference (REF) and outputs a

current proportional to the difference between the

two. This output current is then used to charge

the external compensation network to form the

COMP voltage, which is used to control the

power MOSFET current.

During operation, the minimum COMP voltage is

clamped to 0.9V and its maximum is clamped to

2.0V. COMP is internally pulled down to GND in

shutdown mode. COMP should not be pulled up

beyond 2.6V.

Internal Regulator

Most of the internal circuitries are powered from

the 2.6V internal regulator. This regulator takes

the VIN input and operates in the full VIN range.

When VIN is greater than 3.0V, the output of the

regulator is in full regulation. When VIN is lower

than 3.0V, the output decreases.

Enable Control

The MP4559 has a dedicated enable control pin

(EN). With high enough input voltage, the chip

can be enabled and disabled by EN which has

positive logic. Its falling threshold is a precision

1.2V, and its rising threshold is 1.8V (600mV

higher).

When floating, EN is pulled up to about 3.0V by

an internal 1µA current source so it is enabled.

To pull it down, 1µA current capability is needed.

When EN is pulled down below 1.2V, the chip is

put into the lowest shutdown current mode.

When EN is higher than zero but lower than its

rising threshold, the chip is still in shutdown

mode but the shutdown current increases slightly.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage. The UVLO rising threshold is

about 3.0V while its falling threshold is a

consistent 2.6V.

Internal Soft-Start

The soft-start is implemented to prevent the

converter output voltage from overshooting

during startup and short circuit recovery. When

the chip starts, the internal circuitry generates a

soft-start voltage (SS) ramping up from 0V to

2.6V. When it is lower than the internal reference

(REF), SS overrides REF so the error amplifier

uses SS as the reference. When SS is higher

than REF, REF regains control.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip

from

operating

at

exceedingly

high

temperatures. When the silicon die temperature

is higher than its upper threshold, it shuts down

the whole chip. When the temperature is lower

than its lower threshold, the chip is enabled again.