NiCd/NiMH Battery

Fast-Charge Controllers

_______________________________________________________________________________________

9

The MAX712/MAX713 can be configured so that voltage

slope and/or battery temperature detects full charge.

Figure 4 shows a charging event in which a battery is

inserted into an already powered-up MAX712/MAX713.

During time 1, the charger’s output voltage is regulated

at the number of cells times VLIMIT. Upon insertion of

the battery (time 2), the MAX712/MAX713 detect cur-

rent flow into the battery and switch to fast-charge

state. Once full charge is detected, the device reverts

to trickle charge (time 3). If the battery is removed (time

4), the MAX712/MAX713 remain in trickle charge and

the output voltage is once again regulated as in time 1.

Powering the MAX712/MAX713

AC-to-DC wall-cube adapters typically consist of a trans-

former, a full-wave bridge rectifier, and a capacitor.

Figures 10–12 show the characteristics of three con-

sumer product wall cubes. All three exhibit substantial

120Hz output voltage ripple. When choosing an adapter

for use with the MAX712/MAX713, make sure the lowest

wall-cube voltage level during fast charge and full load is

at least 1.5V higher (2V for switch mode) than the maxi-

mum battery voltage while being fast charged. Typically,

the voltage on the battery pack is higher during a fast-

charge cycle than while in trickle charge or while supply-

ing a load. The voltage across some battery packs may

approach 1.9V/cell.

2N3904

D1

Q1

V+

DRV

DC IN

R1

R2

MAX712

MAX713

Figure 5. DRV Pin Cascode Connection (for high DC IN voltage

or to reduce MAX712/MAX713 power dissipation in linear mode)

↑

1

x

UNDER_VOLTAGE

IIN

N_

_R

RE

EG

GU

UL

LA

AT

TIIO

ON

N

0

x

x

↑

x

x

P

PO

OW

WE

ER

R_

_O

ON

N_

_R

RE

ES

SE

ET

T

↑

x

1

↑

0

0

↑

x

x

1

0

0

1

0

0

1

0

↓

1

↓

0

1

0

0

1

0

0

1

x

x

1

x

x

1

0

↑

1

↑

0

x

C

CO

OL

LD

D

x

0

x

1

x

↓

1

1

1

↑

1

x

0

x

x

x

H

HO

OT

T

x

x

x

1

0

1

1

1

1

1

↑

0

x

x

x

No change

RESULT*

Set trickle

No change

No change

Set fast

No change***

No change

No change

Set fast

Set fast

Set fast**

No change***

Trickle to fast transition inhibited

Trickle to fast transition inhibited

Set trickle

Set trickle

1

x

x

x

↓

Set trickle

† Only two states exist: fast charge and trickle charge.

* Regardless of the status of the other logic lines, a timeout or a voltage-slope detection will set trickle charge.

** If the battery is cold at power-up, the first rising edge on COLD will trigger fast charge; however, a second rising edge will

have no effect.

*** Batteries that are too hot when inserted (or when circuit is powered up) will not enter fast charge until they cool and power is recycled.