CM9112 Datasheet, PDF (13/18 Pages) California Micro Devices Corp – Dual Inputs Dual Outputs High Accurate Fast Charger

PRELIMINARY
CM9112
Application Information (cont’d)
VIN, as shown in Figure 6. To determine the proper
value for Rtherm, the thermistor used in the battery-
pack should follow the 7:1 ratio on the Resistance vs.
Temperature curve (for example, Vishay Dale’s R-T
Curve 2):
-R----^c---^o---^l-^d---⁽---a----t--^φ·---0----^°---C----⁾-= 7
R_{h o t} ( a t φ 50 ° C )
A thermistor with a room temperature value of about
10kΩ, or higher, will keep the interface current drain
from VIN low. Choose the Rtherm value equal to R_hot,
with a 0.5% tolerance. A metal film resistor is best for
temperature stability.
For example, a typically used thermistor for this appli­
cation is Vishay Dale’s NTHS0603N02N1002J. This
thermistor has a R_hot (50°C) = 4kΩ and R_cold (0°C) =
28kΩ. The thermistor interface will work properly if
Rtherm is 4.02kΩ, 0.5%. At 25°C the thermistor value
is 10kΩ. Therefore, a value of voltage at the THERM
pin will be:
Vtherm= ^10kΩ ×5V =3.57V 25^oC
14kΩ
Vtherm= ^4kΩ×5V =2.5V 50^oC
8kΩ
Vtherm= ^28kΩ ×5V =4.375V 0^oC
32kΩ
VIN
Thermistor
Interface
CM9112
Charger
Rtherm (4k)
THERM
NTC
VOUT _Battery
Pack
BSEN
Vishay
NTHS0603N02N1002 J
Figure 6. NTC Thermistor Interface
Because the thermistor is typically located on the bat­
tery-pack, removal of the battery-pack will remove the
thermistor, and cause value of voltage at the THERM
pin to go above the window and thus stop charging.
This allows the THERM interface to function also as a
battery present detector.
When using the CM9112 with a dummy battery, without
a thermistor attached, this function can be disabled by
connecting the THERM pin to GND. In this case, the
THERM interface will never provide a fault condition to
stop charge.
If there is no need for the thermistor interface, the
THERM pin could be used as a second ENABLE pin
for charging control. If the system has an additional
control condition for stop charge, then the THERM pin
could be used as a second control input. Connecting
the THERM pin to VIN will stop charging, while pulling
to GND will resume charging.
Timeout Intervals
A programmable timer is used to terminate the Pre-
charge and Termination charge modes. There are
three modes in a normal charging procedure; Pre-
charge, Fast-charge (or CC Mode), and Termination (or
CV Mode). Because the first and the third modes take
place at low currents, any failure of the battery (for
example, excessive leakage current) could cause
these modes to continue indefinitely if there was not a
Timeout limit.
CM9112 provides two Timeout intervals: Timeout1,
which limits Precharge time and Timeout2, which limits
the Termination time. These intervals are digitally pro­
duced based on an internal clock signal. Timeout1
counts 131072 (2¹⁷) clock cycles and Timeout2 counts
262144 (2¹⁸) clock cycles. The ratio of Timeout2/
Timeout1 = 2 is fixed by the design, but the absolute
Timeout values are programmable by an external
capacitor, Ct, connected between the CT pin and GND.
This capacitor is responsible for the clock cycle rate.
Timeout1 time can be calculated as:
Timeout 1 = 2¹⁷ x ^{13.6ms ⋅ Ct} (in minutes)
0.1uF ⋅ 60
© 2006 California Micro Devices Corp. All rights reserved.
07/06/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 lTel: [phone] lFax: [phone] lwww.cmd.com
13