MP4460 – 2.5A, 4MHz, 36V STEP-DOWN CONVERTER

MP4460 Rev. 1.02

www.MonolithicPower.com

8

12/24/2013

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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 MP4460 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 1.5V (300mV

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. 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.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered

by an external bootstrap capacitor. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.2V with a threshold

of 150mV.

The bootstrap capacitor is charged and

regulated to about 5V by the dedicated internal

bootstrap regulator. When the voltage between

the BST and SW nodes is lower than its

regulation, a PMOS pass transistor connected

from VIN to BST is turned on. The charging

current path is from VIN, BST and then to SW.

External circuit should provide enough voltage

headroom to facilitate the charging.

As long as VIN is sufficiently higher than SW,

the bootstrap capacitor can be charged. When

the power MOSFET is ON, VIN is about equal

to SW so the bootstrap capacitor cannot be

charged. When the external diode is on, the

difference between VIN and SW is largest, thus

making it the best period to charge. When there

is no current in the inductor, SW equals the

bootstrap capacitor.