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 than the maximum 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 supplying a load. The volt-

age across some battery packs may approach 1.9V/cell.

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

voltage drops across the pass transistor (Q1 of Typical

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.