LTC3350_15 Datasheet, PDF (23/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Applications Information
V_CAP
LTC3350
CAPFB
R_FBC1
R_FBC2
3350 F04
Figure 4. V_CAP Voltage Feedback Divider
LTC3350 bidirectional controller switches to step-up mode
is programmed using a resistor divider from the V_IN pin
to SGND via the PFI pin such that:
V_IN
=


1+
R_PF1
R_{PF 2}


V_PFI(TH)
where V_PFI(TH) is 1.17V. Typical values for R_PF1 and R_PF2
are in the range of 40k to 1M. See Figure 5.
The input voltage above which the power-fail status pin
PFO is high impedance and the bidirectional controller
switches to step-down mode is:
( ) ^VIN
=


1+
R_PF1
R_{PF 2}


V_PFI(TH) + V_PFI(HYS)
where V_PFI(HYS) is the hysteresis of the PFI comparator
and is equal to 30mV.
V_IN
LTC3350
R_PF1
PFI
R_PF2
3350 F05
Figure 5. PFI Threshold Voltage Divider
Additional hysteresis can be added by switching in an
additional resistor, R_PF3, in parallel with R_PF2 when the
voltage at PFI falls below 1.17V as shown in Figure 6. The
falling V_IN threshold is the same as before but the rising
V_IN threshold becomes:
( ) ^VIN
=


1+
R_PF1
R_RP2
+
R_PF1
R_{PF 3}


V_PFI(TH) + V_PFI(HYST)
PFI
V_DD
LTC3350
PFO
V_IN
R_PF1
R_PF3
R_PF2
MP1
MN1
3350 F06
Figure 6. PFI Threshold Divider with Added Hystersis
MN1 and MP1 can be implemented with a single pack-
age N‑channel and P-channel MOSFET pair such as the
Si1555DL or Si1016CX. The drain leakage current of MN1,
when its gate voltage is at ground, can introduce an offset
in the threshold. To minimize the effect of this leakage cur-
rent R_PF1, R_PF2 and R_PF3 should be between 1k and 100k.
Setting V_OUT Voltage in Backup Mode
The output voltage for the controller in step-up mode is
set by an external feedback resistor divider, as shown in
Figure 7. The regulated output voltage is determined by:
V_OUT
=


1+
R_FBO1
R_FBO2


1.2V
Great care should be taken to route the OUTFB line away
from noise sources, such as the SW line.
V_OUT
LTC3350
OUTFB
–
+ ^VC
V_REF
R_FO R_FBO1
(OPT)
C_FO
(OPT)
R_FBO2
R_C
(OPT)
C_C
3350 F07
C_FBO1
Figure 7. V_OUT Voltage Divider and Compensation Network
For more information www.linear.com/LTC3350
3350fb
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