LTC3350
16
3350fc
For more information
www.linear.com/LTC3350
operaTion
+
+
+
+
+–
+
–
30mV
OUTPUT
VOLTAGE
CONTROLLER
BIDIRECTIONAL
SWITCHING
CONTROLLER
STEP-UP MODE
V
REF
LTC3350
VOUTSN
V
OUT
(TO SYSTEM)
V
CAP
> V
OUT
V
CAP
< V
OUT
TGATE
OUTFET
OUTFB
BGATE
ICAP
VCAP
R
SNSC
3350 F02
VC
+
–
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.
