sc4510 Semtech Corporation, sc4510 Datasheet - Page 11

• Manufacturer Part Number: sc4510
• Description: Sc4510 High-performance Synchronous Buck Controller With Reference Tracking
• Manufacturer: Semtech Corporation

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.
POWER MANAGEMENT
Functional Description
2004 Semtech Corp.
11
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SC4510