MPQ4559

– 1.5A, 2MHz, 55V STEP-DOWN CONVERTER AEC-Q100 QUALIFIED

MPQ4559 Rev. 1.01

www.MonolithicPower.com

9

5/24/2016

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© 2016 MPS. All Rights Reserved.

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 hysteresis of

150mV. The driver

’s UVLO is soft-start related. In

case the bootstrap voltage hits its UVLO, the

soft-start circuit is reset. To prevent noise, there

is 20µs delay before the reset action. When

bootstrap UVLO is gone, the reset is off and then

soft-start process resumes.

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 output

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









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.

Short Circuit Protection

When the output is shorted to the ground, the

switching frequency is folded back and the

current limit is reduced to lower the short circuit

current. When the voltage of FB is at zero, the

current limit is reduced to about 50% of its full

current limit. When FB voltage is higher than

0.4V, current limit reaches 100%.

In short circuit FB voltage is low, the SS is pulled

down by FB and SS is about 100mV above FB.

In case the short circuit is removed, the output

voltage will recover at the SS pace. When FB is

high enough, the frequency and current limit

return to normal values.

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.