MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER

MP4350 Rev. 1.0

www.MonolithicPower.com

7

5/26/2008

MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.

© 2008 MPS. All Rights Reserved.

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.

At higher duty cycle operation condition, the

time period available to the bootstrap charging

is less so the bootstrap capacitor may not be

sufficiently charged.

In case the internal circuit does not have

sufficient voltage and the bootstrap capacitor is

not charged, extra external circuitry can be

used to ensure the bootstrap voltage is in the

normal operational region. Refer to 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

µ

>

+

+

Current Comparator and Current Limit

The power MOSFET current is accurately

sensed via a current sense MOSFET. It is then

fed to the high speed current comparator for the

current mode control purpose. The current

comparator takes this sensed current as one of

its inputs. When the power MOSFET is turned

on, the comparator is first blanked till the end of

the turn-on transition to avoid noise issues. The

comparator then compares the power switch

current with the COMP voltage. When the

sensed current is higher than the COMP

voltage, the comparator output is low, turning

off the power MOSFET. The cycle-by-cycle

maximum

current

of

the

internal

power

MOSFET is internally limited.

Startup and Shutdown

If both VIN and EN are higher than their

appropriate thresholds, the chip starts. The

reference block starts first, generating stable

reference voltage and currents, and then the

internal regulator is enabled. The regulator

provides stable supply for the remaining

circuitries.

While the internal supply rail is up, an internal

timer holds the power MOSFET OFF for about

50µs to blank the startup glitches. When the

internal soft-start block is enabled, it first holds

its SS output low to ensure the remaining

circuitries are ready and then slowly ramps up.

Three events can shut down the chip: EN low,

VIN low and thermal shutdown. In the shutdown

procedure, the power MOSFET is turned off

first to avoid any fault triggering. The COMP

voltage and the internal supply rail are then

pulled down.

Programmable Oscillator

The MP4350 oscillating frequency is set by an

from:

1

.

1

s

freq

)

KHz

(

f

180000

)

K

(

R

=

Ω