Datasheet

MAX712/MAX713
NiCd/NiMH Battery
Fast-Charge Controllers
6
Maxim Integrated
Getting Started
The MAX712/MAX713 are simple to use. A complete
linear-mode fast-charge circuit can be designed in a
few easy steps. A linear-mode design uses the fewest
components and supplies a load while charging.
1) Follow the battery manufacturer’s recommendations
on maximum charge currents and charge-termination
methods for the specific batteries in your application.
Table 1 provides general guidelines.
2) Decide on a charge rate (Tables 3 and 5). The slow-
est fast-charge rate for the MAX712/MAX713 is C/4,
because the maximum fast-charge timeout period is
264 minutes. A C/3 rate charges the battery in about
three hours. The current in mA required to charge at
this rate is calculated as follows:
I
FAST
= (capacity of battery in mAh)
––––––––––––––––––––––– ––
(charge time in hours)
Depending on the battery, charging efficiency can be
as low as 80%, so a C/3 fast charge could take 3 hours
and 45 minutes. This reflects the efficiency with which
electrical energy is converted to chemical energy within
the battery, and is not the same as the power-
conversion efficiency of the MAX712/MAX713.
3) Decide on the number of cells to be charged (Table 2).
If your battery stack exceeds 11 cells, see the Linear-
Mode High Series Cell Count section. Whenever
changing the number of cells to be charged, PGM0
and PGM1 must be adjusted accordingly. Attempting
to charge more or fewer cells than the number pro-
grammed can disable the voltage-slope fast-charge
termination circuitry. The internal ADC’s input volt-
age range is limited to between 1.4V and 1.9V (see
the
Electrical Characteristics ), and is equal to the
voltage across the battery divided by the number of
cells programmed (using PGM0 and PGM1, as in
Table 2). When the ADC’s input voltage falls out of
its specified range, the voltage-slope termination cir-
cuitry can be disabled.
4) Choose an external DC power source (e.g., wall
cube). Its minimum output voltage (including ripple)
must be greater than 6V and at least 1.5V higher
than the maximum battery voltage while charging.
This specification is critical because normal fast-
charge termination is ensured only if this require-
ment is maintained (see Powering the
MAX712/MAX713 section for more details).
5) For linear-mode designs, calculate the worst-case
power dissipation of the power PNP and diode (Q1
and D1 in the Typical Operating Circuit ) in watts,
using the following formula:
PD
PNP
= (maximum wall-cube voltage under
load - minimum battery voltage) x (charge current
in amps)
6) Limit current into V+ to between 5mA and 20mA. For a
fixed or narrow-range input voltage, choose R1 in the
Typical Operation Circuit using the following formula:
R1 = (minimum wall-cube voltage - 5V)/5mA
7) Choose R
SENSE
using the following formula:
RSENSE = 0.25V/(I
FAST
)
8) Consult Tables 2 and 3 to set pin-straps before
applying power. For example, to fast charge at a
rate of C/2, set the timeout to between 1.5x or 2x the
charge period, three or four hours, respectively.
Table 1. Fast-Charge Termination Methods
Charge
Rate
NiMH Batteries NiCd Batteries
ΔV/Δt and/or
temperature, MAX713
ΔV/Δt and
temperature,
MAX712 or MAX713
> 2C
2C to C/2
ΔV/Δt and/or
temperature,
MAX712 or MAX713
ΔV/Δt and/or
temperature, MAX713
ΔV/Δt and/or
temperature, MAX713
ΔV/Δt and/or
temperature, MAX712
< C/2