MP2452―1A, 36V, 1MHz, STEP-DOWN CONVERTER

MP2452 Rev. 1.0

www.MonolithicPower.com

10

9/27/2010

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SS is also associated with FB. Though SS can

be much lower than FB, it can only be slightly

higher than FB. If somehow FB is brought down,

SS follows to track FB. This function is designed

to accommodate the short-circuit recovery

situation. When a short-circuit is removed, the SS

ramps up as if it is a fresh soft-start process. This

prevents output voltage overshoot.

Thermal Shutdown

Thermal shutdown is implemented to prevent the

chip from thermally running away. 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,

thermal shutdown is gone so 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 about 2.4V with a hysteresis of

about 300mV. During this UVLO, the SS voltage

of the controller is reset to zero. When the UVLO

is removed, the controller follows soft-start

process.

The bootstrap capacitor is charged and regulated

to about 5V by the dedicated internal bootstrap

regulator. When the voltage between 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 free wheeling diode is on, VIN

to SW difference is the largest so it is the best

period to charge. When there is no current in the

inductor, SW equals to the output voltage VOUT

so the difference between VIN and VOUT can be

used to charge 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 charged

sufficiently.

In case the external circuit has not sufficient

voltage and time to charge the bootstrap

capacitor, extra external circuitry can be used to

ensure the bootstrap voltage in normal operation

region.

The floating driver’s UVLO is not communicated

to the controller.

The DC quiescent current of the floating driver is

about 20uA. Make sure the bleeding current at

SW node is at least higher than this number.

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 dodge the noise. Then, the

comparator compares the power switch current

with COMP voltage. When the sensed current is

higher than COMP voltage, the comparator

outputs low, turning off the power MOSFET. The

maximum current of the internal power MOSFET

is internally limited cycle by cycle.

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 rest circuitries.

While the internal supply rail is up, an internal

timer holds the power MOSFET OFF for about

50us to blank the startup glitches. When the

internal soft-start block is enabled, it first holds its

SS output low to ensure the rest circuitries are

ready and then slowly ramps up.

Three events shut down the chip: EN low, VIN

low,

thermal

shutdown.

In

the

shutdown

procedure, the signaling path is blocked first to

avoid any fault triggering. COMP voltage and the

internal supply rail are pulled down then. The

floating driver is not subject to this shutdown

command but its charging path is disabled.