MAX8903NETI+T datasheet(21/30 Pages) MAXIM

21 / 30 page

MAX8903A–E/G/H/J/N/Y

2A 1-Cell Li+ DC-DC Chargers

for USB and Adapter Power

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 (RTB, Figure 7), the ther-

mistor is not limited only to 10k

Ω (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 10k

Ω at +25°C thermistor, use 10kΩ at

RTB, and with a 100kΩ at +25°C thermistor, use 100kΩ.

For a typical 10k

Ω (at +25°C) thermistor and a 10kΩ

RTB resistor, the charger enters a temperature suspend

state when the thermistor resistance falls below 3.97k

Ω

(too hot) or rises above 28.7k

Ω (too cold). This corre-

sponds to a 0°C to +50°C range when using a 10k

Ω

NTC thermistor with a beta of 3500. The general relation

of thermistor resistance to temperature is defined by

the following equation:

=×

+°

−

⎛

⎝⎜

⎞

⎠⎟

⎧

⎨

⎩⎪

⎫

⎬

273

298

⎭⎭⎪

THM

GND

THM

OUT OF

RANGE

DISABLE

CHARGER

CEN

0.74 VL

0.87 VL

0.28 VL

0.03 VL

COLD

HOT

RTB

THERMISTOR

CIRCUITRY

THERMISTOR

DETECTOR

MAX8903B/MAX8903E/

MAX8903G ONLY

ENABLE THM

RTS

RTP

ALTERNATE

THERMISTOR

CONNECTION

ALL COMPARATORS

60mV HYSTERESIS

MAX8903_

Figure 7. Thermistor Monitor Circuitry

Thermistor

β (K)

3000

3250

3500

3750

4250

RTB (k

Ω) (Figure 7)

Resistance at +25°C

Ω)

Resistance at +50°C

Ω)

4.59

4.30

4.03

3.78

3.316

Resistance at 0°C (k

Ω) 25.14 27.15 29.32 31.66 36.91

Nominal Hot Trip

Temperature (°C)

Nominal Cold Trip

Temperature (°C)

-3

-1

4.5

Table 3. Fault Temperatures for Different

Thermistors

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