is 5V (Figure 7). The average voltage at SNS, therefore,

is 235mV, and can be used to calculate the charge cur-

rent as follows:

charge current required.

Circuits without SNS Resistor

In some applications (shown later), SNS is tied directly

to ground. In these cases, the MOD pin remains on

until any one charge-termination condition is exceeded

(Figure 8). A reasonable external current limit (such as

a current-limited DC source) must be provided for

these applications, to prevent battery damage due to

excessive charge currents.

Charge Termination

The MAX2003 has several charge-termination methods.

The termination method selected depends on the type

of battery and charge rate used. Table 1 summarizes

the conditions used to terminate fast-charge with differ-

ent battery types and charge rates.

Temperature Rate Termination

The Temperature Rate Termination (

∆T/∆t) method ter-

minates fast-charge when a particular rate-of-change in

temperature is exceeded. As the battery begins fast-

charge, its temperature increases at a slow rate. When

the battery nears full capacity, this rate of temperature

change increases. When the rate of temperature

change exceeds a preset number, fast-charge is termi-

nated. This method of fast-charge termination can be

used for both NiCd and NiMH batteries.

The MAX2003 samples the voltage at the TS pin every 34

seconds and compares it with a value taken 68 seconds

earlier. Since an NTC thermistor is used for temperature

measurements, a gradual rise in temperature will result in

successively lower voltage readings. If the new reading is

reading, fast-charge is terminated.

The MAX2003A varies the sampling interval as a function

of charge rate (Table 4). As the charge rate increases,

the sampling interval decreases, thereby allowing more

accurate termination of fast charge.

Note: This method of charge termination is valid only

NiCd/NiMH Battery

Fast-Charge Controllers

10

______________________________________________________________________________________

TIME

TIME

TIME

0

0

0

MOD

SNS

0.044

0.050

FAST CHARGE

FAST-CHARGE

TERMINATE

Figure 7. Current Regulation with an SNS Resistor

TIME

TIME

SNS = 0V

TIME

0

0

MOD

SNS

0.044

0.050

FAST CHARGE

FAST-CHARGE

TERMINATE

Figure 8. Current Regulation without an SNS Resistor