MAX712 Maxim, MAX712 Datasheet - Page 6

• Manufacturer Part Number: MAX712
• Description: NiCd/NiMH Battery Fast-Charge Controllers
• Manufacturer: Maxim

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
2) Decide on a charge rate (Tables 3 and 5). The slow-
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).
NiCd/NiMH Battery
Fast-Charge Controllers
Table 1. Fast-Charge Termination Methods
6
____________________Getting Started
2C to C/2
Charge
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
on maximum charge currents and charge-termination
methods for the specific batteries in your application.
Table 1 provides general guidelines.
< C/2
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:
Rate
> 2C
_______________________________________________________________________________________
I
FAST
∆V/∆t and
temperature,
MAX712 or MAX713
∆V/∆t and/or
temperature,
MAX712 or MAX713
∆V/∆t and/or
temperature, MAX712
= (capacity of battery in mAh)
NiMH Batteries
–––––––––––––––––––––––––
(charge time in hours)
∆V/∆t and/or
temperature, MAX713
∆V/∆t and/or
temperature, MAX713
∆V/∆t and/or
temperature, MAX713
NiCd Batteries
4) Choose an external DC power source (e.g., wall
5) For linear-mode designs, calculate the worst-case
6) For both linear and switch-mode designs, limit cur-
7) Choose R
8) Consult Tables 2 and 3 to set pin-straps before
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.
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).
power dissipation of the power PNP and diode (Q1
and D1 in the Typical Operating Circuit ) in watts,
using the following formula:
PD
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).
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:
For designs requiring a large input voltage variation,
choose the current-limiting diode D4 in Figure 19.
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.
R1 = (minimum wall-cube voltage - 5V) / 5mA
PNP
= (maximum wall-cube voltage under
SENSE
RSENSE = 0.25V / (I
using the following formula:
FAST
)