NiCd/NiMH Battery

Fast-Charge Controllers

12

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The voltage loop is stabilized by the output filter

capacitor. A large filter capacitor is required only if the

load is going to be supplied by the MAX712/MAX713 in

(10,000 recommended)

Current Loop

Figure 6 shows the current-regulation loop for a linear-

mode circuit. To ensure loop stability, make sure that

gm = 0.0018 Siemens

The pole frequency of the PNP pass transistor, Q1, can

be determined by assuming a single-pole current gain

data sheet for the particular transistor used for Q1.

Condition for Stability of Current-Regulation Loop:

The MAX712/MAX713 dissipate power due to the cur-

rent-voltage product at DRV. Do not allow the power

dissipation to exceed the specifications shown in the

Absolute Maximum Ratings. DRV power dissipation can

be reduced by using the cascode connection shown in

Figure 5 or by using a switch-mode circuit.

Power dissipation due to DRV sink current =

(current into DRV) x (voltage on DRV)

Voltage-Slope Cutoff

The MAX712/MAX713’s internal analog-to-digital con-

verter has 2.5mV of resolution. It determines if the bat-

tery voltage is rising, falling, or unchanging by

comparing the battery’s voltage at two different times.

to 168sec passes (see Table 3 and Figure 8), then a

battery voltage measurement is taken. It takes 5ms to

perform a measurement. After the first measurement is

second measurement is taken. The two measurements

are compared, and a decision whether to terminate

charge is made. If charge is not terminated, another full

two-measurement cycle is repeated until charge is

and two voltage measurements.

The MAX712 terminates fast charge when a compari-

son shows that the battery voltage is unchanging. The

MAX713 terminates when a conversion shows the bat-

tery voltage has fallen by at least 2.5mV per cell. This is

the only difference between the MAX712 and MAX713.

Temperature Charge Cutoff

Figure 9a shows how the MAX712/MAX713 detect over-

and under-temperature battery conditions using negative

temperature coefficient thermistors. Use the same model

thermistor for T1 and T2 so that both have the same

nominal resistance. The voltage at TEMP is 1V (referred

to BATT-) when the battery is at ambient temperature.

The threshold chosen for THI sets the point at which

fast charging terminates. As soon as the voltage-on

TEMP rises above THI, fast charge ends, and does not

restart after TEMP falls below THI.

The threshold chosen for TLO determines the tem-

perature below which fast charging will be inhibited.

If TLO > TEMP when the MAX712/MAX713 start up, fast

charge will not start until TLO goes below TEMP.

The cold temperature charge inhibition can be disabled

by removing R5, T3, and the 0.022

µF capacitor; and by

tying TLO to BATT-.

To disable the entire temperature comparator charge-

cutoff mechanism, remove T1, T2, T3, R3, R4, and R5,

and their associated capacitors, and connect THI to V+

and TLO to BATT-. Also, place a 68kQ resistor from

REF to TEMP, and a 22k

Ωresistor from BATT- to TEMP.

Some battery packs come with a temperature-detecting

thermistor connected to the battery pack’s negative

POSITIVE

RESIDUAL

5ms

5ms

5ms

5ms

5ms

5ms

INTERVAL

NOTE: SLOPE PROPORTIONAL TO VBATT

INTERVAL

INTERVAL

INTERVAL

INTERVAL

INTERVAL

NEGATIVE

RESIDUAL

ZERO

RESIDUAL

VOLTAGE

RISES

0t

ZERO

VOLTAGE

SLOPE

CUTOFF FOR MAX712

NEGATIVE

VOLTAGE

SLOPE

CUTOFF FOR MAX712

OR MAX713

Figure 8. Voltage Slope Detection