mode. To ensure stable transition from CCM to CVM

choose a capacitor with the following ESR:

cally 30mV). Tantalum capacitors are recommended.

However a ceramic capacitor (typically 10µF) with a

series resistor can also be used.

MOSFET Selection

The MAX1925/MAX1926 drive an external P-channel

MOSFET’s gate from IN to GND. Choose a P-channel

for typical MAX1925/MAX1926 applications resistive

and diode losses dominate switching losses. For this

tive losses are:

is the inductor resistance. The MOSFET’s power dissi-

Diode Selection

In the event of a short-circuited source, the body diode

inherent in the external PFET allows the cell to dis-

charge. To prevent this and to protect against negative

input voltages, add a Schottky or silicon diode between

the power source and IN.

The MAX1925/MAX1926 use a diode for catching the

inductor current during the off cycle. Select a Schottky

Dropout Behavior

The MAX1925/MAX1926 regulate charging current by

ramping inductor current between upper and lower

input voltages near dropout (4.6V at IN for the typical

circuit), the inductor current ramp waveform becomes

somewhat flattened as inductor, MOSFET, input diode,

and battery resistance limit inductor current. When the

inductor current waveform flattens, it’s average value

rises with respect to the upper and lower current

thresholds. This creates a slight peak (about 5%) in

charging current at high battery voltages as seen in the

Charging Current vs. Battery Voltage plot in the Typical

Operating Characteristics. Charging current is still con-

trolled in dropout and the charger operates normally.

The dropout current peak can be minimized by reduc-

ing MOSFET and inductor resistance, as well as for-

ward voltage in the input diode.

Thermistor Interface

An external thermistor inhibits charging by setting a

fault flag when the cell is cold (<0°C) or hot (>+50°C).

The THRM time-multiplexes two sense currents to test

for both hot and cold qualification. Connect the thermis-

tor between THRM and GND. If no temperature qualifi-

cation is desired, replace the thermistor with a 10k

Ω

resistor connected through the battery-latch mecha-

nism. The thermistor should be 10k

Ω at +25°C and

have a negative temperature coefficient, as defined by

the expression below:

Table 3 shows nominal fault detection temperatures

that result from a wide range of available thermistor

temperature curves.

For a given thermistor characteristic, it is possible to

adjust the fault-detection temperatures by adding a

resistor in series with the thermistor or a parallel resistor

from THRM to GND.

Chip Information

TRANSISTOR COUNT: 5722

PROCESS: BiCMOS

RR

T

TC

e

=

+













































°×

−

25

1

273

1

298

β

R

V

V

R

ESR

VHIST

IHIST

SET

>×

THERMISTOR BETA

3000

3250

3500

3750

Resistance at +25

°C

10000

Ω

10000

Ω

10000

Ω

10000

Ω

Resistance at +50

°C

4587.78

Ω

4299.35

Ω

4029.06

Ω

3775.75

Ω

Resistance at 0

°C

25140.55

Ω

27148.09

Ω

29315.94

Ω

31656.90

Ω

Nominal Hot Trip Temperature

55.14

°C

52.60

°C

50.46

°C

48.63

°C

Nominal Cold Trip Temperature

-3.24

°C

-1.26

°C

0.46

°C

1.97

°C

Switch-Mode 1-Cell Li+ Chargers

14

______________________________________________________________________________________

Table 3. Fault Temperature for Different Thermistors