LTC3859_15 Datasheet, PDF (18/42 Pages) Linear Technology – Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3859
OPERATION
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 their outputs.
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.
Channel 1 Power Good (PGOOD1)
Channel 1 has a PGOOD1 pin that is connected to an open
drain of an internal N-channel MOSFET. The MOSFET
turns on and pulls the PGOOD1 pin low when the V_FB1 pin
voltage is not within ±10% of the 0.8V reference voltage
for the buck channel. The PGOOD1 pin is also pulled low
when the RUN1 pin is low (shut down). When the V_FB1
pin voltage is within the ±10% requirement, the MOSFET
is turned off and the pin is allowed to be pulled up by an
external resistor to a source no greater than 6V.
Boost Overvoltage Indicator (OV3)
The OV3 pin is an overvoltage indicator that signals
whether the output voltage of the channel 3 boost control-
ler goes over its programmed regulated voltage. The pin
is connected to an open drain of an internal N-channel
MOSFET. The MOSFET turns on and pulls the OV3 pin low
when the V_FB3 pin voltage is less than 110% of the 1.2V
reference voltage for the boost channel. The OV3 pin is
also pulled low when the RUN3 pin is low (shut down).
When the V_FB3 pin voltage goes higher than 110% of the
1.2V reference, the MOSFET is turned off and the pin is
allowed to be pulled up by an external resistor to a source
no greater than 6V.
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_FB voltage is keeping up with
the TRACK/SS1,2 voltage). There is no foldback current
limiting for the boost channel.
THEORY AND BENEFITS OF 2-PHASE OPERATION
Why the need for 2-phase operation? Up until the 2-phase
family, constant-frequency dual switching regulators
operated both channels in phase (i.e., single-phase
operation). This means that both switches turned on at
the same time, causing current pulses of up to twice the
amplitude of those for one regulator to be drawn from the
input capacitor and battery. These large amplitude current
pulses increased the total RMS current flowing from the
input capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
With 2-phase operation, the two buck controllers of the
LTC3859 are operated 180 degrees out of phase. This
effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a significant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating efficiency.
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