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SC4510

POWER MANAGEMENT

Functional Description

Alternately if an external 12V supply is available it can be

directly connected between BST and GND. The actual gate

to source voltage of the upper Mosfet will then be

approximately be equal to (12V - VIN). This technique is

useful if the input voltage is 5V but a 12V supply is also

available in the system.

POWER GOOD MONITOR

The PGOOD circuitry monitors the FB input of the convertor

error amplifier. If the voltage on this input goes above +12%

or below -12% of the REFIN voltage the PGOOD pin is pulled

low. The PGOOD is an open drain output and can sink up

to 5 mA. The PGOOD pin is held low during the startup

sequence.

ERROR AMPLIFIER

The SC4510 is a current mode controller and operates by

matching the peak of the sensed inductor current to the

output of the voltage error amplifier. The error amplifier is

transconductance type and should be compensated

accordingly. It has a transconductance gain of 275

in the source mode. Current is sensed losslessly by taking

the weighted average of both the MOSFET drops and adding

it to the DC voltage drop across the inductor. More

information on this patented Combi Sense technique is

provided in the next section.

Current mode controllers are inherently unstable at duty

ratios above 50% and need some form of slope

compensation to operate correctly. This slope

compensation is built into the architecture of SC4510 where

a portion of the ramp is internally added to the current

sense signal. The amount of added ramp is optimised and

varies with the operating duty cycle. Larger duty ratios result

in larger ramps being added to the current sense signal.

Note that the uncorrected current signal is used for

overcurrent comparator. The current limit point is

unaffected by the slope compensation.