ing when the battery voltage drops, the MAX1925/MAX1926

ensure that the cell does not remain partially charged after

use when charger power is available.

Charging also restarts if input power is cycled or if the

charger is restarted by the EN or THRM input. If a new

battery is inserted, the charger must be restarted. If the

THRM functionality is used, the charger is automatically

restarted upon battery insertion. When THRM is not

used, toggle EN or connect THRM through a resistor to

be grounded with a battery-door latch switch.

Applications Information

Timer Capacitor and Fault Modes

The on-chip timer checks charge progress and issues

an alarm signal through a blinking CHG output when one

of the safety timers times out (see Table 1). All timers are

set by one external capacitor at CT. A 100nF value sets

If the charger enters full-charge state (after the charging

current has fallen below C/8) before the full-charge timer

expires, no fault occurs, but if the timer expires before full

charge is reached, a fault is indicated (see Table 1).

A fault is also indicated if the battery voltage exceeds

4.35V. When the cell voltage exceeds 4.35V a fault alarm

is set, the CHG output blinks, and the PFET turns off.

To restart the charger after a fault occurs, the fault state

must be cleared by toggling EN, or by cycling input power

at IN (see Figure 1). Temperature faults do not need to be

cleared by EN. The MAX1925/MAX1926 resume charging

after the temperature returns to within the set window.

Inductor Selection

Because the MAX1925/MAX1926 is hysteretic, the con-

stant-current mode switching frequency is a function of

the inductance, sense resistance, and current-sense

hysteresis (30mV, from the Electrical Characteristics).

To minimize charge-current error:

where

and should usually be less than 1/4th the full charge

er calculated value for L.

The resulting switching frequency in CCM is:

Choose an inductor with an RMS and saturation current

rating according to the following equation:

Output Capacitor Selection

The ESR of the output capacitor influences the switch-

ing frequency of the charger during voltage regulation

I

V

R

VV

t

L

SAT RMS

IPK

SET

IN

BATT IDelay

/

−

f

IL t

V

VV

t

IL t

V

V

V

Switch

HYST

IDelay BATT

IN

BATT

IDelay

HYST

IDelay

IN

BATT

BATT

>

×+

+×

+





×+

×





−

−

−

2

1

()

L

VV

t

I

IN

BATT

Delay

CHG

>

×

×∆

2

Switch-Mode 1-Cell Li+ Chargers

______________________________________________________________________________________________________

13

PART

ACON

CHARGING

CHG LED

LOW

YES

ON

(until charge complete)

(Note: This state should never occur)

High Z

NO

OFF

MAX1925

High Z

NO

OFF

LOW

YES

ON

(until charge complete)

(Note: This state should never occur)

High Z

NO

OFF

MAX1926

High Z

NO

OFF