LTC4110 datasheet(39/52 Pages) LINER

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LTC4110

4110fa

R1 = resistor connected between DCDIV and GND

R2 = resistor connected between supply input and

DCDIV

VBGR = reference voltage 1.220V

For example, if supply input = 12V and backup starts when

it drops to 11V, then VBACKUP = 11V, VBACKDRIVE = 13.5V,

R2/R1 = 8.02, choose R1 = 10k, then R2 = 80.6k.

If a higher ratio than VOVP/VBGR = 1.23 is desired between

VBACKDRIVE and VBACKUP, 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 VDC = VDD = 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 VCAL and VDIS 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 VCAL and VDIS

for they are independent of each other.

The equations are most helpful if you pick the VCUTOFF

voltage you want, within the range limits offered, and then

solve for VCAL or VDIS. With the voltage value of VCAL or

VDIS calculated, determine the necessary voltage divider

network from VREF required to get the calculated voltage

on these pins respectively. It is recommended that one

single series resistor divider network from VREF to ground

be used to obtain all of the pin voltages you need. It should

be noted that custom values of VCHG would also affect the

divider network complexity. See Programming Charge

Voltage section for more information.

Connect the VCAL or VDIS 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 VREF pin resistor divider according

to the following equations:

Li Ion

CUTOFF

CAL

DIS

BGR

CUTOFF

−

•.

(

)

DIS

BGR

CUTOFF

CAL

DIS

BGR

SLA

•.

(

)

235

••(

)

NiMH NiCD

APPLICATIONS INFORMATION

Figure 17. Backup and Boost Detect Comparators

BACKDRIVE

BACKUP

BGR

BACKDRIVE

10 23

−

–

.•

–

–.

•

.•

•

123

023

BACKUP

BGR

BACKD

−

RIVE

BACKUP

BACKDRIVE

BACKUP

–

–.

•

.•

123

023

−

BACKDRIVE

BACKUP

–.

•

123

−

where:

VDC = Any regulated DC voltage available in the system

such as SMBus pull up, LED supply or LTC4110’s VDD

voltage, must be higher than 1.7V. R3 = resistor connected

between VDC and DCDIV.

4110 F17

DCDIV

BACKUP

SUPPLY

INPUT

VBGR

VDC

OPTIONAL RESISTOR

TO INCREASE THE

1.23 TO 1 RATIO

CMP

BOOST

1.23 • VBGR

CMP

LTC4110 Datasheet(PDF) 39 Page - Linear Technology

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