LT1769IGN datasheet(11/16 Pages) LINER

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LT1769

APPLICATIONS INFORMATION

When power is on, there is about 200

µA of current flowing

out of the BAT and SENSE pins. If the battery is removed

during charging, and total load including R3 and R4 is less

than 200

µA, VBAT could float up to VCC even though the

loop has turned switching off. To keep VBAT regulated to

the battery voltage in this condition, R3 and R4 can be

chosen to draw 0.5mA and Q3 can be added to disconnect

them when power is off (Figure 4). R5 isolates the OVP pin

from any high frequency noise on VIN. An alternative method

is to use a Zener diode with a breakdown voltage two or three

volts higher than battery voltage to clamp the VBAT voltage.

Lithium-Ion Charging

The 2A Lithium-Ion Battery Charger (Figure 1) charges at

a constant 2A until battery voltage reaches a limit set by R3

and R4. The charger will then automatically go into a

constant-voltage mode with current decreasing to near

zero over time as the battery reaches full charge. This is the

normal regimen for lithium-ion charging, with the charger

holding the battery at “float” voltage indefinitely. In this

case no external sensing of full charge is needed.

Battery Voltage Sense Resistors Selection

To minimize battery drain when the charger is off, current

through the R3/R4 divider is set at 15

µA. The input current

to the OVP pin is 3nA and the error can be neglected.

With divider current set at 15

µA, VBAT = 8.4V, R4 =

2.465/15

µA = 162k and,

R4 V

2.465

162k 8.4 2.465

2.465

390k

BAT

= ()

−

()

−

()

Li-Ion batteries typically require float voltage accuracy of

1% to 2%. Accuracy of the LT1769 OVP voltage is

±0.5%

at 25

°C and ±1% over full temperature. This leads to the

possibility that very accurate (0.1%) resistors might be

needed for R3 and R4. Actually, the temperature of the

LT1769 will rarely exceed 50

°C in float mode because

charging currents have tapered off to a low level, so 0.25%

resistors will normally provide the required level of overall

accuracy.

Figure 4. Disconnecting Voltage Divider

PWM

RPROG

4.7k

300

Ω

PROG

CPROG

µF

VN2222

LT1769

1769 F03

IBAT = (DC)(2A)

Figure 3. PWM Current Programming

12k

0.25%

4.99k

0.25%

OVP

VIN

8.4V

VBAT

VN2222

LT1769

1769 F04

220k

Some battery manufacturers recommend terminating the

constant-voltage float mode after charge current has

dropped below a specified level (typically around 10% of

the full current)

and a further time out period of 30 to 90

minutes has elapsed. This may extend battery life, so

check with the manufacturer for details. The circuit in

Figure 5 will detect when charge current has dropped

below 270mA. This logic signal is used to initiate a timeout

period, after which the LT1769 can be shut down by

pulling the VC pin low with an open collector or drain.

Some external means must be used to detect the need for

additional charging or the charger may be turned on

periodically to complete a short float-voltage cycle.

Current trip level is determined by the battery voltage, R1

through R3 and the sense resistor (RS1). D2 generates

hysteresis in the trip level to avoid multiple comparator

transitions.

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