Datasheet

MAX8903A–E/G/H/J/N/Y
2A 1-Cell Li+ DC-DC Chargers
for USB and Adapter Power
21
Maxim Integrated
VL Regulator
VL is a 5V linear regulator that powers the MAX8903’s
internal circuitry and charges the BST capacitor. VL is
used externally to bias the battery’s thermistor. VL takes
its input power from USB or DC. When input power is
available from both USB and DC, VL takes power from
DC. VL is enabled whenever the input voltage at USB
or DC is greater than ~1.5V. VL does not turn off when
the input voltage is above the overvoltage threshold.
Similarly, VL does not turn off when the charger is dis-
abled (CEN = high). Connect a 1µF ceramic capacitor
from VL to GND.
Thermistor Input (THM)
The THM input connects to an external negative tem-
perature coefficient (NTC) thermistor to monitor battery
or system temperature. Charging is suspended when
the thermistor temperature is out of range. The charge
timers are suspended and hold their state but no fault is
indicated. When the thermistor comes back into range,
charging resumes and the charge timer continues from
where it left off. Connecting THM to GND disables the
thermistor monitoring function. Table 3 lists the fault
temperature of different thermistors.
Since the thermistor monitoring circuit employs an exter-
nal bias resistor from THM to VL (R
TB
, Figure 7), the ther-
mistor is not limited only to 10kΩ (at +25°C). Any
resistance thermistor can be used as long as the value is
equivalent to the thermistor’s +25°C resistance. For
example, with a 10kΩ at +25°C thermistor, use 10kΩ at
R
TB
, and with a 100kΩ at +25°C thermistor, use 100kΩ.
For a typical 10kΩ (at +25°C) thermistor and a 10kΩ
R
TB
resistor, the charger enters a temperature suspend
state when the thermistor resistance falls below 3.97kΩ
(too hot) or rises above 28.7kΩ (too cold). This corre-
sponds to a 0°C to +50°C range when using a 10kΩ
NTC thermistor with a beta of 3500. The general relation
of thermistor resistance to temperature is defined by
the following equation:
RR e
T
TC C
=×
+°
−
°
⎛
⎝
⎜
⎞
⎠
⎟
⎧
⎨
⎩
⎪
⎫
⎬
25
1
273
1
298
β
⎭⎭
⎪
THM
GND
THM
OUT OF
RANGE
DISABLE
CHARGER
VL
CEN
VL
0.74 VL
0.87 VL
0.28 VL
0.03 VL
COLD
HOT
R
TB
THERMISTOR
CIRCUITRY
THERMISTOR
DETECTOR
MAX8903B/MAX8903E/
MAX8903G ONLY
ENABLE THM
R
T
R
TS
R
TP
ALTERNATE
THERMISTOR
CONNECTION
R
T
ALL COMPARATORS
60mV HYSTERESIS
MAX8903_
Figure 7. Thermistor Monitor Circuitry
Thermistor β (K) 3000 3250 3500 3750 4250
R
TB
(kΩ) (Figure 7) 10 10 10 10 10
Resistance at +25°C
(kΩ)
10 10 10 10 10
Resistance at +50°C
(kΩ)
4.59 4.30 4.03 3.78 3.316
Resistance at 0°C (kΩ) 25.14 27.15 29.32 31.66 36.91
Nominal Hot Trip
Temperature (°C)
55 53 50 49 46
Nominal Cold Trip
Temperature (°C)
-3 -1 0 2 4.5
Table 3. Fault Temperatures for Different
Thermistors