Temperature Measurement

The MAX2003/MAX2003A employs a negative tempera-

ture-coefficient (NTC) thermistor to measure the bat-

tery’s temperature. This temperature value can be used

to determine start and termination of fast-charge. The

two temperature conditions that can be used for fast-

charge termination are:

• Maximum Temperature

• Rate-of-Change of Temperature (

∆T/∆t)

Figure 5 shows the various temperature cutoff points and

the typical voltages that the device will see at the TS pin.

age at TS when the battery temperature is too low, and

temperature cutoff. If the voltage is outside these limits,

the MAX2003/MAX2003A will not enter fast-charge

mode. After fast-charge is initiated, the termination

TEMP LED status.

determine the high-temperature cutoff after fast-charge

Thermistors are inherently nonlinear with respect to

temperature. This nonlinearity is especially noticed

when

∆T/∆t measurements are made to determine

charge termination. The simplest way around this is to

place a resistor-divider network in parallel with the ther-

mistor (Figure 6) to reduce the effects of nonlinearity.

ates high-frequency noise on the signal seen by TS.

Charge Pending

Before fast-charge is initiated, the cell voltage and tem-

perature of the battery pack must be within the

assigned limits. If the voltage or temperature is outside

these limits, the device is said to be in a “charge-pend-

ing” state. During this mode, the CHG pin will cycle low

(LED on) for 0.125sec and high (LED off) for 1.375sec.

Fast-charge is normally initiated if the cell voltage is

the trickle current brings the voltage up before fast-

Temperature Measurement section).

Initiate Fast-Charge

If the MAX2003/MAX2003A are out of the charge-pend-

ing state, fast-charge can be initiated upon one of the

following conditions:

• Battery Replacement

• Applying Power to the MAX2003/MAX2003A

(battery already present)

• Digital Control Signal

During fast-charge, the CHG pin will be continuously

low (LED on). For the initial period of fast-charge (the

hold-off time), the voltage charge-termination methods

are disabled. The hold-off time is a function of the

charge rate selected by TM1 and TM2 (see Table 4).

NiCd/NiMH Battery

Fast-Charge Controllers

8

_______________________________________________________________________________________

OFF

ON

ON

TEMP LED

STATUS

Figure 5. Temperature Measurement Scale

RC FILTER

100k

0.1

µF

NTC

TS

SNS

RT2

RT1

MAX2003

MAX2003A

Figure 6. Thermistor Configuration for Temperature Measurement