LTC4110
39
4110fb
R1
= resistor connected between DCDIV and GND
R2
= resistor connected between supply input and
DCDIV
V
BGR
= reference voltage 1.220V
For example, if supply input = 12V and backup starts when
it drops to 11V, then V
BACKUP
= 11V, V
BACKDRIVE
= 13.5V,
R2/R1 = 8.02, choose R1 = 10k, then R2 = 80.6k.
If a higher ratio than V
OVP
/V
BGR
= 1.23 is desired between
V
BACKDRIVE
and V
BACKUP
, a third resistor can be used as
shown in Figure 17.
For example, if supply input = 12V and backup starts when
it drops to 8V, calibration terminates when it rises to 16V,
and V
DC
= V
DD
= 4.75V, then R2/R1 = 21.87, R3/R1 = 3.88,
choose R1 = 10k then R2 = 221k and R3 = 39.2k.
If the noise on supply input is a problem, a capacitor can
be connected between DCDIV and GND.
PROGRAMMING CALIBRATION/BACKUP CUT-OFF
THRESHOLD
The pins V
CAL
and V
DIS
are used to calculate custom
discharge cut-off voltages for their respective operating
modes. The equations shown below are generic for both.
There is no implied relationship between V
CAL
and V
DIS
for they are independent of each other.
The equations are most helpful if you pick the V
CUTOFF
voltage you want, within the range limits offered, and then
solve for V
CAL
or V
DIS
. With the voltage value of V
CAL
or
V
DIS
calculated, determine the necessary voltage divider
network from V
REF
required to get the calculated voltage
on these pins respectively. It is recommended that one
single series resistor divider network from V
REF
to ground
be used to obtain all of the pin voltages you need. It should
be noted that custom values of V
CHG
would also affect the
divider network complexity. See Programming Charge
Voltage section for more information.
Connect the V
CAL
or V
DIS
pin to GND will set the default
calibration/backup cut-off threshold (2.75V for Li-Ion,
1.93V for SLA, 0.95V for NiMH/NiCd). These threshold
voltages can be adjusted (±400mV for Li-Ion, ±300mV
for SLA, ±200mV for NiMH/NiCd) by tying the pin to ap-
propriate voltage on the V
REF
pin resistor divider according
to the following equations:
V
VV
V
Li Ion
V
V
CUTOFF
CAL DIS
BGR
CUTOFF
C
= −
=
/
•. ( )42
AAL DIS
BGR
CUTOFF
CAL DIS
BGR
V
V
SLA
V
VV
V
/
•. ( )
/
235
= ••( / )2 NiMH NiCD
APPLICATIONS INFORMATION
Figure 17. Backup and Boost Detect Comparators
R
R
VV
V
V
BACKDRIVE BACKUP
BGR
BACKDRIVE
2
1023
= −
–
.•
––. •
.•
•
123
023
1
3
1
V
V
R
R
V
V
V
BACKUP
DC
DC
BGR
BACKD
−
=
RRIVE BACKUP
BACKDRIVE BACKUP
V
VV
V
–
–. •
.•
123
023
−
DDC
BACKDRIVE BACKUP
VV–. •123
1−
where:
V
DC
= Any regulated DC voltage available in the system
such as SMBus pull up, LED supply or LTC4110’s V
DD
voltage, must be higher than 1.7V. R3 = resistor connected
between V
DC
and DCDIV.
4110 F17
DCDIV
BACKUP
SUPPLY
INPUT
–
+
V
BGR
V
DC
R2
R1
R3
OPTIONAL RESISTOR
TO INCREASE THE
1.23 TO 1 RATIO
CMP
BOOST
1.23 • V
BGR
+
–
CMP
