MPQ4560

– 2A, 2MHz, 55V STEP-DOWN CONVERTER

MPQ4560 Rev. 1.1

www.MonolithicPower.com

11

5/24/2016

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~5V. When the voltage between the BST and SW

nodes is less than its regulation, a PMOS pass

transistor from VIN to BST turns ON. The

charging current path is from VIN, BST and then

to SW. An external circuit must provide enough

voltage headroom to facilitate charging.

bootstrap capacitor can charge. When the power

capacitor cannot charge. When the external

is at its largest, thus making it the best period to

charge. When there is no current in the inductor,

can charge the bootstrap capacitor.

At higher duty cycles, the time period available

for bootstrap charging is shorter so the bootstrap

capacitor may not sufficiently charge. If the

internal circuit does not have sufficient voltage

and the bootstrap capacitor is not charged, extra

external

circuitry

can

ensure the

bootstrap

voltage is within the normal operational region.

The DC quiescent current of the floating driver is

about 20µA. Make sure the bleeding current at

the SW node exceeds this value, such that:

A

20

)

2

R

1

R

(

V

I

O

O









Current Comparator and Current Limit

A current-sense MOSFET accurately senses the

power MOSFET

’s current. The result goes to the

high-speed current comparator for current-mode

control.: When the power MOSFET turns 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

turning

OFF

the

power

MOSFET.

The

cycle-by-cycle maximum current of the internal

power MOSFET is internally limited.

Short Circuit Protection

When the output is shorted to the ground, the

switching frequency folds back and the current

limit falls to lower the short-circuit current. When

current limit reaches 100%.

the short circuit causes the output voltage to

frequency and current limit return to normal

values.

Startup and Shutdown

thresholds, the chip starts. The reference block

initiates to generate a stable reference voltage

and currents, and then the internal regulator is

enabled. The regulator provides a 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 noise. 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.

LOW

and

thermal

shutdown.

During

shutdown, the power MOSFET turns OFF first to

internal supply rail drop.

Programmable Oscillator

ground sets the MPQ4560 oscillating frequency.

FREQ

S

100000

R

(k

Ω) =

- 5

f (kHz)