LTC3350_15 Datasheet, PDF (16/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Operation
OUTPUT
VOLTAGE
CONTROLLER
VOUTSN ^LTC3350
OUTFB
V_REF
+–
30mV
BIDIRECTIONAL
SWITCHING
CONTROLLER
STEP-UP MODE
– OUTFET
+
TGATE
BGATE
ICAP
VCAP
VC
V_CAP < V_OUT
V_OUT
(TO SYSTEM)
V_CAP > V_OUT
R_SNSC
+
+
+
+
3350 F02
Figure 2. Power Path Block Diagram—Power Backup
ideal diode shuts off, the synchronous controller will turn
on immediately. If OUTFB is above 1.2V when the output
ideal diode shuts off, the load current will flow through the
body diode of the output ideal diode N-channel MOSFET for
a period of time until OUTFB falls to 1.2V. The synchronous
controller will regulate OUTFB to 1.2V when it turns on,
holding up V_OUT while the supercapacitors discharge to
ground.
The synchronous controller in step-up mode will run
nonsynchronously when V_CAP is less than 100mV below
V_OUT. It will run synchronously when V_CAP falls 200mV
below V_OUT.
Ideal Diodes
The LTC3350 has two ideal diode controllers that drive
external N-channel MOSFETs. The ideal diodes consist of
a precision amplifier that drives the gates of N-channel
MOSFETs whenever the voltage at V_OUT is approximately
30mV (V_FWD) below the voltage at V_IN or V_CAP. Within
the amplifier’s linear range, the small-signal resistance
of the ideal diode will be quite low, keeping the forward
drop near 30mV. At higher current levels, the MOSFETs
will be in full conduction.
The input ideal diode prevents the supercapacitors from
back driving V_IN during backup mode. A Fast-Off com-
parator shuts off the N-channel MOSFET if V_IN falls 30mV
below V_OUT. The PFI comparator also shuts off the MOSFET
during power failure.
The output ideal diode provides a path for the supercapaci-
tors to power V_OUT when V_IN is unavailable. In addition to a
Fast-Off comparator, the output ideal diode also has a Fast-
On comparator that turns on the external MOSFET when
V_OUT drops 65mV below V_CAP. The output ideal diode will
shut off when OUTFB is just above regulation allowing the
synchronous controller to power V_OUT in step-up mode.
3350fb
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For more information www.linear.com/LTC3350