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MAX8568A

Complete Backup-Management ICs for Lithium and NiMH Batteries

Maxim Integrated 

Complete Backup-Management ICs

for Lithium and NiMH Batteries

The lithium charger acts like a current-limited voltage

source. The battery regulation voltage for lithium mode,

VBK(LIMAX), is:

VBK(LIMAX) = 3.5 x VTERMV

If VTERMV = 1.2V, then the final charge voltage is 4.2V.

Connect TERMV to a resistor-divider from REF to GND.

Adjust VTERMV with resistors R11 and R12 (Figure 2).

Select R12 to be in the 100kΩ to 1MΩ range. Calculate

R11 as follows:

R11 =

R12







3.5 × VREF

VBK(LIMAX)



− 1



where VREF =1.25V.

Using an NiMH Backup Battery

The MAX8568 can charge NiMH backup batteries from

the main battery connected at IN. Connect NI/LI to

BKSU for NiMH backup battery charging. VTERMV sets

the maximum cell voltage and also the trip point for the

fast-charge-to-trickle-charge transition. VSTRTV sets the

trickle-to-fast-charge transition threshold.

In NiMH charge mode (NI/LI connected to BKSU), the

charger ramps the battery between two thresholds

measured at the battery connection BK, VBK(NILO) and

VBK(NIHI). When the battery falls to VBK(NILO), trickle

charging stops and fast charging starts. When the bat-

tery rises to VBK(NIHI), fast charging stops and trickle

charging begins. If, for any reason, the battery contin-

ues to rise when trickle charged, all charging ceases at

VBK(NIMAX). VBK(NILO), VBK(NIHI), and VBK(NIMAX) are

set as follows:

BK voltage where fast charge begins:

VBK(NILO) = VSTRTV

BK voltage where trickle charge begins:

VBK(NIHI) = 1.163 x VTERMV

BK voltage where all charging stops:

VBK(NIMAX) = 1.493 x VTERMV

Resistor-dividers (see Figure 3) set VSTRTV and VTERMV

by dividing down REF. To minimize operating current,

resistors between 100kΩ and 1MΩ should be used for

R14 and R16 in Figure 3. The formulas for the upper

divider-resistors in terms of VBK(NILO), VBK(NIHI), and

VBK(NIMAX) are:

R13

=

R14







VREF

VBK(NILO)



− 1



R15

=

R16







1.163 × VREF

VBK(NIHI)



− 1



Once VBK(NIHI) is selected, the maximum battery volt-

age is:

VBK(NIMAX) = 1.283 x VBK(NIHI)

16

REF

R11

15

TERMV

R12

14

STRTV

Figure 2. Resistor-Divider for Setting the Maximum Battery

Voltage, VBK(LIMAX), for Rechargeable Lithium-Type Backup

Batteries

16

REF

15

TERMV

14

STRTV

R13

R15

R16

R14

Figure 3. 2-Resistor-Dividers for Setting VBK(NILO) and VBK(NIHI)

______________________________________________________________________________________ 11

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