Datasheet

MAX846A
Cost-Saving Multichemistry
Battery-Charger System
_______________________________________________________________________________________ 9
where VX is either GND or VL, and V
F
is the per-cell
float voltage. In the circuit of Figure 1, R
VSET
is
400kΩ. R
VSET
and the internal 20kΩ resistor form a
divider, resulting in an adjustment range of approxi-
mately ±5%.
The current-regulation loop attempts to maintain the
voltage on ISET at 1.65V. Selecting resistor R
ISET
deter-
mines the reflected voltage required at the current-
sense amplifier input.
• Calculate R
CS
and R
ISET
as follows:
R
CS
= V
CS
/ I
BATT
R
ISET
(in kΩ) = 1.65V / V
CS
where the recommended value for V
CS
is 165mV.
• Connect ON to PWROK to prevent the charge current
from turning on until the voltages have settled.
Minimize power dissipation in the external pass transis-
tor. Power dissipation can be controlled by setting the
DCIN input supply as low as possible, or by making
V
DCIN
track the battery voltage.
Microprocessor-Controlled
Multichemistry Operation
The MAX846A is highly adjustable, allowing for simple
interfacing with a low-cost µC to charge Ni-based and
Li-Ion batteries using one application circuit (Figure 3).
R = 20k
4.2
1.65
V V
V 4.2
VSET
XF
F
Ω
−
−












MAX846A
I
BATT
CCI
GND
CCV
EXTERNAL PASS TRANSISTOR
CAN BE EITHER PNP OR PMOS FET.
OFFV
PGND
DRV BATTCS-
( )
10nF
0.165V
CS+
DCIN
3.7V TO 20V
ISET
CELL2
PWROK
ON
VL
DCIN
VSET
100k
R
VSET
VL

ADJUST
(UP)
(DOWN)
10k
R
ISET
VL
(2 CELLS)
(1 CELL)
0.01µF
4.7µF
0.01µF
R
DRV

660Ω
R
CS
4.7µF
Figure 2. Stand-Alone Li-Ion Charger