Datasheet

MAX846A
Cost-Saving Multichemistry
Battery-Charger System
10 ______________________________________________________________________________________
Component selection is similar to that of stand-alone
operation. By using DACs or µC PWM outputs, the float
voltage and charging current can be adjusted by the
µC. When a Ni-based battery is being charged, disable
the float-voltage regulation using the OFFV input. The
µC can also monitor the charge current through the
battery by reading the ISET output’s voltage using its
ADC. Similarly, the battery voltage can be measured
using a voltage divider from the battery.
Note that the µC only needs to configure the system for
correct voltage and current levels for the battery being
charged, and for Ni-based batteries to detect end-of-
charge and adjust the current level to trickle. The con-
troller is not burdened with the regulation task.
Float-voltage accuracy is important for battery life and
for reaching full capacity for Li-Ion batteries. Table 1
shows the accuracy attainable using the MAX846A.
For best float-voltage accuracy, set the DRV current to
1mA (R
DRV
= 660Ω for a PNP pass transistor).
High-Power Multichemistry
Offline Charger
The circuit in Figure 6 minimizes power dissipation in
the pass transistor by providing optical feedback to the
input power source. The offline AC/DC converter main-
tains 1.2V across the PNP. This allows much higher
charging currents than can be used with conventional
power sources.
P
V
DD
ADC (MEASURE I
BATT
)
PWM/DAC (CONTROL CHARGE I)
PWM/DAC (CONTROL FLOAT V)
I/O (HIGH = DISABLE FLOAT V)
I/O (HIGH = 2 Li CELLS)
I/O (LOW = TURN OFF CHARGE)
ADC (MEASURE V(BATT))
MAX846A
CCI
GND
RST
CCV
OFFV
PGND
DRV
BATT
CS-CS+
DCIN
3.7V TO 20V
ISET
VL
CELL2
PWROK
ON
DCIN
VSET
Li OR Ni
MICROCONTROLLER
Figure 3. Desktop Multichemistry Charger Concept