NiCd/NiMH Battery

Fast-Charge Controllers

10

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The 1.5V of overhead is needed to allow for worst-case

voltage drops across the pass transistor (Q1 of Typical

Operating Circuit), the diode (D1), and the sense

ment is critical, because violating it can inhibit proper

termination of the fast-charge cycle. A safe rule of

thumb is to choose a source that has a minimum input

voltage = 1.5V + (1.9V x the maximum number of cells

to be charged). When the input voltage at DC IN drops

below the 1.5V + (1.9V x number of cells), the part

oscillates between fast charge and trickle charge and

might never completely terminate fast-charge.

The MAX712/MAX713 are inactive without the wall cube

attached, drawing 5µA (max) from the battery. Diode

D1 prevents current conduction into the DRV pin. When

the wall cube is connected, it charges C1 through R1

(see Typical Operating Circuit) or the current-limiting

diode (Figure 19). Once C1 charges to 5V, the internal

shunt regulator sinks current to regulate V+ to 5V, and

fast charge commences. The MAX712/MAX713 fast

charge until one of the three fast-charge terminating

conditions is triggered.

If DC IN exceeds 20V, add a cascode connection in

series with the DRV pin as shown in Figure 5 to prevent

exceeding DRV’s absolute maximum ratings.

Furthermore, if Figure 19’s DC IN exceeds 15V, a tran-

sistor level-shifter is needed to provide the proper volt-

age swing to the MOSFET gate. See the MAX713 EV kit

manual for details.

Select the current-limiting component (R1 or D4) to

pass at least 5mA at the minimum DC IN voltage (see

step 6 in the Getting Started section). The maximum

current into V+ determines power dissipation in the

MAX712/MAX713.

maximum current into V+ =

(maximum DC IN voltage - 5V) / R1

power dissipation due to shunt regulator =

5V x (maximum current into V+)

Sink current into the DRV pin also causes power dissipa-

tion. Do not allow the total power dissipation to exceed

the specifications shown in the Absolute Maximum

Ratings.

Fast Charge

The MAX712/MAX713 enter the fast-charge state under

one of the following conditions:

1) Upon application of power (batteries already

installed), with battery current detection (i.e., GND

voltage is less than BATT- voltage), and TEMP

higher than TLO and less than THI and cell voltage

higher than the UVLO voltage.

2) Upon insertion of a battery, with TEMP higher than

TLO and lower than THI and cell voltage higher than

the UVLO voltage.

fast charge, the voltage difference between the BATT-

and GND pins is regulated to 250mV. DRV current

increases its sink current if this voltage difference falls

below 250mV, and decreases its sink current if the volt-

age difference exceeds 250mV.

Trickle Charge

4C ensures a C/16 trickle-charge current. Other fast-

charge rates can be used, but the trickle-charge

current will not be exactly C/16.

BATT-

X

V+

OPEN

REF

BATT-

1

0

0

0

0

8

512

256

128

64

CURRENT-SENSE AMPLIFIER

PGM3 FAST_CHARGE Av

1.25V

V+

DC IN

GND

DRV

GND

CC

BATT-

D1

REF

VLIMIT

CELL_VOLTAGE

BATT-

BATT-

IN_REGULATION

C2

Figure 6. Current and Voltage Regulator (linear mode)