MAX2003 Maxim Integrated Products, MAX2003 Datasheet - Page 13

• Manufacturer Part Number: MAX2003
• Description: Nicd/nimh Battery Fast-charge Controllers
• Manufacturer: Maxim Integrated Products

3) Select Sense Resistor. The sense resistor deter-
mines the rate at which the battery is fast-charged. The
sense pin, SNS, has an average voltage of 235mV (see
Detailed Description ) and, since the charge current
(I
lated by:
In this example, a fast-charge current of 1.7A requires
a sense resistor of about 0.14Ω (1 watt).
4) Select TM1 and TM2. Once the charge rate is
determined, Table 4 can be used to select the TM1 and
TM2 inputs. TM1 and TM2 set the safety timeout, hold-
off time, and top-off enable (see Fast-Charge
Termination section in the Detailed Description ).
In Figure 1, a fast-charge rate of C with top-off would
require TM1 to be GND and TM2 to be V
5) Select R
user to select R
cells in the battery. The total resistance value (R
R
any problems with noise. In Figure 1 (with six cells) R
is selected to be 100kΩ and, from the following equation:
R
6) Select Temperature-Control Components. Most
sealed rechargeable battery packs have a built-in ther-
mistor to prevent air currents from corrupting the accurate
temperature measurements. The thermistor size and tem-
perature characteristics can be obtained from the bat-
tery-pack manufacturer, to help in designing the rest of
the circuit. Three-terminal battery packs that incorporate
a thermistor generally share a common connection for the
thermistor and the battery negative terminal. Large charg-
ing currents may produce voltage drops across the com-
mon negative connector, causing errors in thermistor
readings. Using separate contacts for the thermistor
ground sense and the battery ground sense at the nega-
tive battery terminal will reduce these errors. If an external
thermistor is to be used, take care to ensure that it is
placed in direct contact with the battery, and that the bat-
tery/thermistor set-up is placed in a sealed container.
Neither NiCd nor NiMH batteries should be fast-
charged outside the maximum and minimum tempera-
ture limits. However, some applications also require
termination using the ∆T/∆t criterion. The resistors R
and R
(V
Though NiCd batteries do not always require termina-
tion using the ∆T/∆t feature, it is not possible to isolate
and disable this mode. It is therefore recommended
FAST
B2
B2
TCO
R
) should be between 100kΩ and 500kΩ to prevent
B2
can be calculated to be 20kΩ.
T2
) is known from above, the resistor can be calcu-
) and the rate-of-change of temperature (∆T/∆t).
= R
(Figure 1) will determine the temperature cutoff
R
B1
SNS
B1
/ (Number of Cells - 1) = 100kΩ / (6 - 1)
and R
= V
B1
______________________________________________________________________________________
and R
SNS
B2
/ I
. The MAX2003A requires the
B2
FAST
to indicate the number of
= 0.235 / I
CC
FAST
.
B1
B1
T1
+
Fast-Charge Controllers
that NiCd and NiMH batteries use the same ∆T/∆t termi-
nation parameters.
The Duracell DR17 battery pack used in our example
circuit recommended a low fault temperature (V
+10°C and a maximum temperature cutoff (V
+50°C. These maximum temperature values will never
be reached in most cases, but are used as a safety net
to prevent battery damage. According to Duracell, the
10kΩ thermistor inside the pack varies from 17.96kΩ at
+10°C to 4.16kΩ at +50°C.
The circuit in Figure 1 will be designed so that a battery
temperature change of 1°C/min will result in fast-charge
termination. At 1°C/min, the battery will take 40 minutes
to change 40°C (10°C to 50°C). Since a charge rate of
C is used for this example, Table 4 shows that the
MAX2003A samples the TS pin every 68 seconds and
compares it with a value taken 136 seconds earlier. The
device will terminate fast-charge if the voltage at TS
changes by more than 0.0032V
5V). At a charge rate of 16mV every 136 seconds, the
TS pin will charge 280mV in 40 minutes (40min x
60sec/min x 16mV/136sec).
The low fault temperature (V
0.4V
ture cutoff voltage (V
Figure 5 shows that, at any given temperature:
When the battery temperature is +10°C, the voltage is:
V
And at +50°C:
V
Figure 9. Resistor Configuration for MCV and TCO
TS10
TS50
V
CC
TS
= V
= V
, which is 2.0V for a supply of 5V. The tempera-
= V
CC
CC
NiCd/NiMH Battery
V
CC
TCO
(R
(R
(R
T2
T2
T2
= (2.00V - 0.28V) = 1.72V
|| R
|| R
|| R
TCO
NTC10
NTC50
NTC
R2
R1
R3
) will be 280mV below V
V
CC
) / [(R
) / [(R
) / [(R
LTF
T2
T2
T2
MCV
TCO
CC
) is set internally at
|| R
|| R
|| R
(16mV for V
NTC
NTC10
NTC50
) + R
) + R
) + R
T1
TCO
LTF
LTF
]
T1
T1
CC
, or:
) of
) of
]
]
13
=