NiCd/NiMH Battery
Fast-Charge Controllers
MAX712/MAX713
DC IN
V+
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
CC
C2
REF
DRV
VLIMIT
D1
GND
CELL_VOLTAGE
CURRENT-SENSE AMPLIFIER
PGM3 FAST_CHARGE Av
X
V+
OPEN
REF
BATT-
1
0
0
0
0
8
512
256
128
64
BATT-
R
SENSE
BATT-
IN_REGULATION
1.25V
BATT-
BATT-
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.
GND
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.
R
SENSE
sets the fast-charge current into the battery. In
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.
fast-charge current (I
FAST
) = 0.25V / R
SENSE
Figure 6. Current and Voltage Regulator (linear mode)
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
resistor (R
SENSE
). This minimum input voltage require-
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 5µ (max) from the battery. Diode D1
A
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
10
Trickle Charge
Selecting a fast-charge current (I
FAST
) of C/2, C, 2C, or
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.
______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
