and a new battery is connected during either fast-charge

or full topoff modes, the charger begins with full charge

current without prequalification unless the part is reset.

Detect battery insertion by connecting THRM to a ther-

mistor on the battery, if a thermistor is used, or to a 10k

Ω

resistor linked to a battery door mechanism.

Constant-Current Mode (CCM)

When the battery voltage is below 4.2V, the

MAX1925/MAX1926 regulate the charging current by

controlling the peak and valley inductor currents. When

MAX1925/MAX1926 turn the external PFET off. When

MAX1925/MAX1926 turns the external PFET on, but

only if the battery voltage is below regulation. The maxi-

mum cell charging current is programmed by selecting

between BATT and CS. Select the external resistor

The accuracy of the charge current is a function of

input voltage, battery voltage, inductance, and com-

parator delay (300ns typ). Determine the charge-cur-

rent error according to the following equation:

where

the current-sense comparator delay.

For this reason choose L for an on-time and off-time greater

Constant-Voltage Mode (CVM)

In constant-voltage mode (CVM), the controller regu-

lates the peak and valley of the output ripple. The maxi-

mum cell voltage is regulated to 4.2V. If, for any reason,

the cell voltage exceeds 4.35V, a fault alarm is set, the

CHG output blinks, and the PFET power switch is held

off. The charger can then be restarted only by cycling

input power or the EN input.

Indication of Charge Completion (CHG)

The CHG output is a 10mA current-sink output that indi-

cates the cell’s charging status. Connect an LED from

IN to CHG for a visible indicator. Alternatively, a pullup

resistor (typically 200k

Ω) from a logic supply to CHG

provides a logic-level output. Table 1 relates the status

of the LED to the condition of the charger and battery.

ACON Output

The ACON open-drain output indicates when usable

exceeds ACON threshold (nominally 4.25V with IN ris-

ing—see the Electrical Characteristics table), ACON

goes low. In the MAX1925, ACON goes low when the

input voltage is between 4.5V and 6.1V (see Table 2).

Re-Initiating a Charging Cycle

The MAX1925/MAX1926 feature automatic restart that

resumes charging when the cell voltage drops to 4V and

∆=

−×

×

I

VV

t

L

CHG

IN

BATT

IDelay

2

2

Switch-Mode 1-Cell Li+ Chargers

12

______________________________________________________________________________________

STATE

CONDITION

CHG

OFF

EN low or no battery or input power

High impedance (LED off)

PREQUAL1

Charge current = 4mA until BATT reaches 2V.

Low (LED on)

PREQUAL2

Charge current = C/10 until BATT reaches 3V.

Low (LED on)

FAST CHARGE

Low (LED on)

FULL CHARGE

Charge current has fallen to C/8.

High impedance (LED off)

FAULT PREQUAL1

BATT does not reach 2V before PREQUAL1 timeout.

FAULT PREQUAL2

BATT does not reach 3V before PREQUAL2 timeout.

FAULT FULL

Charge current does not drop to C/8 before FULL CHARGE

timeout.

only be cleared by cycling input power,

THRM, or EN.

FAULT BATT

VOLTAGE

Battery voltage has exceeded 4.35V.

MAX1925—High impedance (LED off)

FAULT TEMP

Temperature has risen above +50

°C or fallen below 0°C.

Temp fault clears by itself.

MAX1926—Blinking (LED on 50% 0.5Hz)

NONE

Initial power-up or enable with battery not present.

Blinking at rapid rate as charger cycles

through RESET, PREQUAL1, and DONE.

Table 1. CHG Output States