LTC3350
23
3350fc
For more information
www.linear.com/LTC3350
applicaTions inForMaTion
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
PF2
⎛
⎝⎜
⎞
⎠⎟
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:
V
IN
=
1
+
R
PF1
R
PF2
⎛
⎝⎜
⎞
⎠⎟
V
PFI(TH)
+
V
PFI(HYS)
(
)
where V
PFI(HYS)
is the hysteresis of the PFI comparator
and is equal to 30mV.
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.
Figure 7. V
OUT
Voltage Divider and Compensation Network
Figure 4. V
CAP
Voltage Feedback Divider
LTC3350
CAPFB
V
CAP
R
FBC1
R
FBC2
3350 F04
LTC3350
PFI
V
IN
R
PF1
R
PF2
3350 F05
Figure 5. PFI Threshold Voltage Divider
Figure 6. PFI Threshold Divider with Added Hystersis
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:
V
IN
=
1
+
R
PF1
R
RP2
+
R
PF1
R
PF3
⎛
⎝⎜
⎞
⎠⎟
V
PFI(TH)
+
V
PFI(HYST)
(
)
VC
OUTFB
LTC3350
V
REF
R
C
(OPT)
R
FBO1
R
FBO2
R
FO
(OPT)
C
FO
(OPT)
C
FBO1
V
OUT
C
C
+
–
3350 F07
LTC3350
PFI
V
DD
PFO
V
IN
R
PF1
R
PF2
R
PF3
MP1
MN1
3350 F06
