MP4558

– 1A, 2MHz, 55V STEP-DOWN CONVERTER

MP4558 Rev. 1.01

www.MonolithicPower.com

10

10/28/2013

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© 2013 MPS. All Rights Reserved.

Internal Regulator

The 2.6V internal regulator powers most of the

internal circuits. This regulator takes the VIN

input and operates in the full VIN range. When

output drops to 0V.

Enable Control

The MP4558 has a dedicated enable control pin

(EN): An input voltage that exceeds an upper

threshold enables the chip, while a voltage the

drops below a lower threshold disables the chip.

Its falling threshold is precisely 1.2V, and its

rising threshold is 300mV higher, or 1.5V.

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

an internal 1µA current source to enable the chip.

Pulling it down requires a 1µA current.

When EN drops below 1.2V, the chip enters the

lowest

shutdown

current

mode.

When

EN

exceeds

0V

but

remains

below

its

rising

threshold, the chip is still in shutdown mode but

with a slightly higher shutdown current.

Under-Voltage Lockout

Under-voltage lockout (UVLO) protects the chip

from operating at an insufficient supply voltage.

The UVLO rising threshold is about 3.0V while its

falling threshold is a consistent 2.6V.

Internal Soft-Start

Soft-Start prevents the converter output voltage

from overshooting during start-up and short-

circuit recovery. When the chip starts, the internal

circuitry generates a soft-start (SS) voltage that

ramps up from 0V to 2.6V. When this voltage is

less than the internal reference (REF), SS

overrides REF so the error amplifier uses SS as

the reference. When SS exceeds REF, REF

regains control.

Thermal Shutdown

Thermal

shutdown

prevents

the

chip

from

operating at exceedingly high temperatures.

When the silicon die temperature exceeds its

upper threshold, it shuts down the whole chip.

When the temperature falls below its lower

threshold, the chip is enabled again.

Floating Driver and Bootstrap Charging

An external bootstrap capacitor powers the

floating power MOSFET driver. This floating

driver has its own UVLO protection with a rising

threshold of 2.2V and a hysteresis of 150mV.

The driver

’s UVLO is connected to the SS: If the

bootstrap voltage hits its UVLO, the soft-start

circuit resets. To prevent noise, there is 20µs

delay before the reset action. When the device

exits the bootstrap UVLO condition, the reset

turns off and then soft-start process resumes.

The

dedicated

internal

bootstrap

regulator

charges and regulates the bootstrap capacitor to

about 5V. When the voltage between the BST

and SW nodes falls below regulation, a PMOS

pass transistor connected from VIN to BST turns

on. The charging current path goes from VIN, to

BST and then to SW. The external circuit must

provide enough voltage headroom to facilitate

charging.

bootstrap capacitor will charge. When the power

bootstrap capacitor cannot charge. When the

external diode is on, the difference between VIN

and SW is at its largest, thus making it the best

period to charge. When there is no current in the

the bootstrap capacitor.

Under higher duty-cycle operation conditions, the

time period available for bootstrap charging is

smaller so the bootstrap capacitor may not

sufficiently charge.

In case the internal circuit does not have

sufficient voltage and the bootstrap capacitor is

not charged, extra external circuitry can ensure

the bootstrap voltage is in the normal operational

region. Refer to the External Bootstrap Diode in

Application section.

The DC quiescent current of the floating driver is

about 20µA. Make sure the bleeding current at

the SW node is higher than this value, such that:

A

20

)

2

R

1

R

(

V

I

O

O

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

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