LTC3899_15 Datasheet, PDF (18/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Operation (Refer to the Functional Diagrams)
Boost Controller at Low SENSE Pin Common Voltage
The current comparator of the boost controller is powered
directly from the SENSE3⁺ pin and can operate to voltages
as low as 2.2V. Since this is lower than the V_BIAS UVLO of
the chip, V_BIAS can be connected to the output of the boost
controller, as illustrated in the typical application circuit in
Figure 12. This allows the boost controller to handle input
voltage transients down to 2.2V while maintaining output
voltage regulation. If SENSE3⁺ falls below 2.0V, then
switching stops and SS3 is pulled low. If SENSE3⁺ rises
back above 2.2V, the SS3 pin will be released, initiating a
new soft-start sequence.
Buck Controller Output Overvoltage Protection
The two buck channels have an overvoltage comparator
that guards against transient overshoots as well as other
more serious conditions that may overvoltage the output.
When the V_FB1,2 pin rises by more than 10% above its
regulation point of 0.800V, the top MOSFET is turned off
and the bottom MOSFET is turned on until the overvoltage
condition is cleared.
Buck Foldback Current
When the buck output voltage falls to less than 70% of
its nominal level, foldback current limiting is activated,
progressively lowering the peak current limit in proportion
to the severity of the overcurrent or short-circuit condition.
Foldback current limiting is disabled during the soft-start
interval (as long as the V_FB1,2 voltage is keeping up with
the TRACK/SS1,2 voltage). There is no foldback current
limiting for the boost channel.
3899f
18
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