RT2805A
Operation
The RT2805A is a constant frequency, current mode
asynchronous step-down converter. In normal operation,
the high side N-MOSFET is turned on when the S-R latch
is set by the oscillator and is turned off when the current
comparator resets the S-R latch. While the N-MOSFET
is turned off, the inductor current conducts through the
external diode.
Error Amplifier
The error amplifier adjusts its output voltage by comparing
the feedback signal (V
FB
) with the internal 1.222V
reference. When the load current increases, it causes a
drop in the feedback voltage relative to the reference, the
error amplifier's output voltage then rises to allow higher
inductor current to match the load current.
Oscillator
The internal oscillator runs at fixed frequency 500kHz. In
short circuit condition, the frequency is reduced to 150kHz
for low power consumption.
Internal Regulator
The regulator provides low voltage power to supply the
internal control circuits and the bootstrap power for high
side gate driver.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build a soft-start ramp voltage. The FB voltage will track
the internal ramp voltage during soft-start interval. The
typical soft-start time is 5ms.
Thermal Shutdown
The over temperature protection function will shut down
the switching operation when the junction temperature
exceeds 150°C. Once the junction temperature cools
down by approximately 30°C, the converter will
automatically resume switching.
Enable
The converter is turned on when the EN pin is higher than
1.4V and turned off when the EN pin is lower than 0.4V.
When the EN pin is open, it will be pulled up to logic-high
by 1μA current internally.
Copyright
©
2012 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2805A-00 November 2012
www.richtek.com
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