NiCd /NiMH Ba t t e ry
Fa s t -Ch a rg e Co n t ro lle rs
The volta g e loop is s ta b ilize d b y the outp ut filte r
capacitor. A large filter capacitor is required only if the
load is going to be supplied by the MAX712/MAX713 in
terminated. Note that each cycle has two t intervals
A
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.
the absence of a battery. In this case, set C
as:
OUT
C
(in farads) = (50 x I
) / (V
x BW
)
OUT
LOAD
OUT
VRL
where BW
= loop bandwidth in Hz
VRL
(10,000 recommended)
C
> 10µF
Te m p e ra t u re Ch a rg e Cu t o ff
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.
OUT
I
= external load current in amps
= programmed output voltage
LOAD
V
OUT
(V
LIMIT
x number of cells)
Cu rre n t Lo o p
Figure 6 shows the current-regulation loop for a linear-
mode circuit. To ensure loop stability, make sure that
the bandwidth of the current regulation loop (BW
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.
2/MAX713
) is
CRL
lower than the pole frequency of transistor Q1 (f ). Set
B
BW
by selecting C2.
CRL
BW
in Hz = gm / C2, C2 in farads,
gm = 0.0018 Siemens
The thre shold c hose n for TLO d e te rmine s the te m-
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.
CRL
The pole frequency of the PNP pass transistor, Q1, can
be determined by assuming a single-pole current gain
response. Both f and B should be specified on the
data sheet for the particular transistor used for Q1.
The cold temperature charge inhibition can be disabled
by removing R5, T3, and the 0.022µF capacitor; and by
tying TLO to BATT-.
T
o
f in Hz = f / B , f in Hz, B = DC current gain
B
T
o
T
o
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 22kΩresistor from BATT- to TEMP.
Condition for Stability of Current-Regulation Loop:
BW < f
CRL
B
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.
Some battery packs come with a temperature-detecting
thermistor connected to the battery pack’s negative
Power dissipation due to DRV sink current =
(current into DRV) x (voltage on DRV)
NEGATIVE
ZERO
VOLTAGE
SLOPE
VOLTAGE
SLOPE
CUTOFF FOR MAX712
OR MAX713
Vo lt a g e -S lo p e Cu t o ff
The MAX712/MAX713’s internal analog-to-digital con-
verter has 2.5mV of resolution. It determines if the bat-
te ry volta g e is ris ing , fa lling , or unc ha ng ing b y
comparing the battery’s voltage at two different times.
CUTOFF FOR MAX712
VOLTAGE
RISES
ZERO
RESIDUAL
NEGATIVE
RESIDUAL
After power-up, a time interval of t ranging from 21sec
A
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
0
t
POSITIVE
RESIDUAL
5ms
5ms
5ms
5ms
5ms
5ms
t
A
t
A
t
A
t
A
t
A
t
A
c omp le te , a nothe r t inte rva l p a s s e s , a nd the n a
A
INTERVAL INTERVAL INTERVAL INTERVAL INTERVAL INTERVAL
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
NOTE: SLOPE PROPORTIONAL TO VBATT
Figure 8. Voltage Slope Detection
12 ______________________________________________________________________________________
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