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LT1511

Constant-Current/Constant-Voltage 3A Battery Charger with Input Current Limiting

Linear Technology 

LT1511

APPLICATIONS INFORMATION

LT1511

PROG

300Ω

5V

0V

PWM

IBAT = (DC)(3A)

RPROG

4.7k

Q1

VN2222

CPROG

1µF

1511 • F03

Figure 3. PWM Current Programming

Lithium-Ion Charging

The 3A Lithium Battery Charger (Figure 1) charges lithium-

ion batteries at a constant 3A until battery voltage reaches

a limit set by R3 and R4. The charger will then automati-

cally go into a constant-voltage mode with current de-

creasing to zero over time as the battery reaches full

charge. This is the normal regimen for lithium-ion charg-

ing, 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 15µA. The input current

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

With divider current set at 15µA, R4 = 2.465/15µA = 162k

and,

R3 = (R4)(VBAT − 2.465) = 162k(8.4 − 2.465)

2.465

2.465

= 390k

Li-Ion batteries typically require float voltage accuracy of

1% to 2%. Accuracy of the LT1511 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

LT1511 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.

When power is on, there is about 200µA 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 the 200µA, 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 way is

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

volts higher than battery voltage to clamp the VBAT voltage.

R3

12k

0.25%

LT1511

Q3

VN2222

VIN

R5

220k

OVP

R4

4.99k

0.25%

VBAT

+

4.2V

–

+

4.2V

–

LT1511 • F04

Figure 4. Disconnecting Voltage Divider

Some battery manufacturers recommend termination of

constant-voltage float mode after charging current has

dropped below a specified level (typically around 10% of

the full current) and a further time out period of 30 minutes

to 90 minutes has elapsed. This may extend the life of the

battery, so check with manufacturers for details. The

circuit in Figure 5 will detect when charging current has

dropped below 400mA. This logic signal is used to initiate

a timeout period, after which the LT1511 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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