NiCd/NiMH Battery
Fast-Charge Controllers
MAX712/MAX713
Table 5. Trickle-Charge Current
Determination from PGM3
PGM3
V+
OPEN
REF
BATT-
Fast-Charge Rate
4C
2C
C
C/2
Trickle-Charge
Current (I
TRICKLE
)
I
FAST
/64
I
FAST
/32
I
FAST
/16
I
FAST
/8
Q1
DC IN
V+
DRV
10k
Q2
10k
D1
R7
BATTERY
MAX712
MAX713
FASTCHG
The MAX712/MAX713 internally set the trickle-charge
current by increasing the current amplifier gain (Figure
6), which adjusts the voltage across R
SENSE
(see
Trickle-Charge V
SENSE
in the
Electrical Characteristics
table).
R
SENSE
GND
Figure 7. Reduction of Trickle Current for NiMH Batteries
(linear mode)
Nonstandard Trickle-Charge
Current Example
Configuration:
Typical Operating Circuit
2 x Panasonic P-50AA 500mAh AA NiCd batteries
C/3 fast-charge rate
264-minute timeout
Negative voltage-slope cutoff enabled
Minimum DC IN voltage of 6V
Settings:
Use MAX713
PGM0 = V+, PGM1 = open, PGM2 = BATT-,
PGM3 = BATT-, R
SENSE
= 1.5Ω (fast-charge current,
I
FAST
= 167mA), R1 = (6V - 5V) / 5mA = 200Ω
Since PGM3 = BATT-, the voltage on R
SENSE
is regulat-
ed to 31.3mV during trickle charge, and the current is
20.7mA. Thus the trickle current is actually C/25, not
C/16.
Regulation Loop
The regulation loop controls the output voltage between
the BATT+ and BATT- terminals and the current
through the battery via the voltage between BATT- and
GND. The sink current from DRV is reduced when the
output voltage exceeds the number of cells times
V
LIMIT
, or when the battery current exceeds the pro-
grammed charging current.
For a linear-mode circuit, this loop provides the following
functions:
1) When the charger is powered, the battery can be
removed without interrupting power to the load.
2) If the load is connected as shown in the
Typical
Operating Circuit,
the battery current is regulated
regardless of the load current (provided the input
power source can supply both).
Further Reduction of Trickle-Charge
Current for NiMH Batteries
The trickle-charge current can be reduced to less than
C/16 using the circuit in Figure 7. In trickle charge,
some of the current will be shunted around the battery,
since Q2 is turned on. Select the value of R7 as follows:
R7 = (V
BATT
+ 0.4V) / (l
TRlCKLE
- I
BATT
)
where
V
BATT
= battery voltage when charged
I
TRlCKLE
= MAX712/MAX713 trickle-charge
current setting
I
BATT
= desired battery trickle-charge current
Voltage Loop
The voltage loop sets the maximum output voltage
between BATT+ and BATT-. If V
LIMIT
is set to less than
2.5V, then:
Maximum BATT+ voltage (referred to BATT-) = V
LIMIT
x
(number of cells as determined by PGM0, PGM1)
VLIMIT should be set between 1.9V and 2.5V. If VLIMIT
is set below the maximum cell voltage, proper
termination of the fast-charge cycle might not occur.
Cell voltage can approach 1.9V/cell, under fast charge,
in some battery packs. Tie V
LIMIT
to V
REF
for normal
operation .
With the battery removed, the MAX712/MAX713 do not
provide constant current; they regulate BATT+ to the
maximum voltage as determined above.
11
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