MAX712/MAX713
NiCd/NiMH Battery
Fast-Charge Controllers
6 _______________________________________________________________________________________
____________________Getting Started
The MAX712/MAX713 are simple to use. A complete
linear-mode or switch-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, while a switch-mode design may be neces-
sary if lower heat dissipation is desired.
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 (2V
for switch mode) than the maximum battery voltage
while charging. This specification is critical because
normal fast-charge termination is ensured only if this
requirement 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)
If the maximum power dissipation is not tolerable for
your application, refer to the
Detailed Description
or
use a switch-mode design (see
Switch-Mode
Operation
in the
Applications Information
section,
and see the MAX713 EV kit manual).
6) For both linear and switch-mode designs, limit cur-
rent 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
For designs requiring a large input voltage variation,
choose the current-limiting diode D4 in Figure 19.
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.
< C/2
∆V/∆t and/or
temperature, MAX712
∆V/∆t and/or
temperature, MAX713
NiMH Batteries NiCd Batteries
> 2C
∆V/∆t and
temperature,
MAX712 or MAX713
∆V/∆t and/or
temperature, MAX713
2C to C/2
∆V/∆t and/or
temperature,
MAX712 or MAX713
∆V/∆t and/or
temperature, MAX713
Charge
Rate
Table 1. Fast-Charge Termination Methods
