3) Select Sense Resistor. The sense resistor deter-

mines the rate at which the battery is fast-charged. The

sense pin, SNS, has an average voltage of 235mV (see

Detailed Description) and, since the charge current

lated by:

In this example, a fast-charge current of 1.7A requires

a sense resistor of about 0.14

Ω (1 watt).

4) Select TM1 and TM2. Once the charge rate is

determined, Table 4 can be used to select the TM1 and

TM2 inputs. TM1 and TM2 set the safety timeout, hold-

off time, and top-off enable (see

Fast-Charge

Termination section in the Detailed Description).

In Figure 1, a fast-charge rate of C with top-off would

is selected to be 100k

Ω and, from the following equation:

6) Select Temperature-Control Components. Most

sealed rechargeable battery packs have a built-in ther-

mistor to prevent air currents from corrupting the accurate

temperature measurements. The thermistor size and tem-

perature characteristics can be obtained from the bat-

tery-pack manufacturer, to help in designing the rest of

the circuit. Three-terminal battery packs that incorporate

a thermistor generally share a common connection for the

thermistor and the battery negative terminal. Large charg-

ing currents may produce voltage drops across the com-

mon negative connector, causing errors in thermistor

readings. Using separate contacts for the thermistor

ground sense and the battery ground sense at the nega-

tive battery terminal will reduce these errors. If an external

thermistor is to be used, take care to ensure that it is

placed in direct contact with the battery, and that the bat-

tery/thermistor set-up is placed in a sealed container.

Neither NiCd nor NiMH batteries should be fast-

charged outside the maximum and minimum tempera-

ture limits. However, some applications also require

termination using the

∆T/∆t).

Though NiCd batteries do not always require termina-

tion using the

∆T/∆t feature, it is not possible to isolate

and disable this mode. It is therefore recommended

that NiCd and NiMH batteries use the same

∆T/∆t termi-

nation parameters.

The Duracell DR17 battery pack used in our example

+50°C. These maximum temperature values will never

be reached in most cases, but are used as a safety net

to prevent battery damage. According to Duracell, the

10k

Ω thermistor inside the pack varies from 17.96kΩ at

+10°C to 4.16k

Ω at +50°C.

The circuit in Figure 1 will be designed so that a battery

temperature change of 1°C/min will result in fast-charge

termination. At 1°C/min, the battery will take 40 minutes

to change 40°C (10°C to 50°C). Since a charge rate of

C is used for this example, Table 4 shows that the

MAX2003A samples the TS pin every 68 seconds and

compares it with a value taken 136 seconds earlier. The

device will terminate fast-charge if the voltage at TS

5V). At a charge rate of 16mV every 136 seconds, the

TS pin will charge 280mV in 40 minutes (40min x

60sec/min x 16mV/136sec).

Figure 5 shows that, at any given temperature:

When the battery temperature is +10°C, the voltage is:

And at +50°C:

NiCd/NiMH Battery

Fast-Charge Controllers

______________________________________________________________________________________

13

R2

R3

R1

MCV

TCO

Figure 9. Resistor Configuration for MCV and TCO