MAX712 Maxim, MAX712 Datasheet - Page 10

• Manufacturer Part Number: MAX712
• Description: NiCd/NiMH Battery Fast-Charge Controllers
• Manufacturer: Maxim

NiCd/NiMH Battery
Fast-Charge Controllers
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
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 5 A (max) from the battery. Diode D1
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
DC IN
GND
______________________________________________________________________________________
D1
BATT-
R
SENSE
DRV
SENSE
GND
CURRENT-SENSE AMPLIFIER
PGM3 FAST_CHARGE Av
BATT-
OPEN
REF
V+
X
). This minimum input voltage require-
V+
1
0
0
0
0
BATT-
512
256
128
64
8
VLIMIT
CELL_VOLTAGE
REF
1.25V
BATT-
IN_REGULATION
BATT-
CC
C2
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.
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 .
The MAX712/MAX713 enter the fast-charge state under
one of the following conditions:
1) Upon application of power (batteries already
2) Upon insertion of a battery, with TEMP higher than
R
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.
Selecting a fast-charge current (I
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.
SENSE
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.
TLO and lower than THI and cell voltage higher than
the UVLO voltage.
fast-charge current (I
sets the fast-charge current into the battery. In
maximum current into V+ =
(maximum DC IN voltage - 5V) / R1
power dissipation due to shunt regulator =
5V x (maximum current into V+)
FAST
) = 0.25V / R
FAST
Trickle Charge
) of C/2, C, 2C, or
Fast Charge
SENSE