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LT1511

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

Linear Technology 

LT1511

PIN FUNCTIONS

GND (Pins 1, 4, 5, 7, 16, 23, 24): Ground Pin.

SW (Pin 2): Switch Output. The Schottky catch diode must

be placed with very short lead length in close proximity to

SW pin and GND.

BOOST (Pin 3): This pin is used to bootstrap and drive the

switch power NPN transistor to a low on-voltage for low

power dissipation. In normal operation, VBOOST = VCC +

VBAT when switch is on. Maximum allowable VBOOST is

55V.

UV (Pin 6): Undervoltage Lockout Input. The rising thresh-

old is at 6.7V with a hysteresis of 0.5V. Switching stops in

undervoltage lockout. When the supply (normally the wall

adapter output) to the chip is removed, the UV pin has to

be pulled down to below 0.7V (a 5k resistor from adapter

output to GND is required) otherwise the reverse battery

current drained by the chip will be approximately 200µA

instead of 3µA. Do not leave UV pin floating. If it is

connected to VIN with no resistor divider, the built-in 6.7V

undervoltage lockout will be effective.

OVP (Pin 8): This is the input to the amplifier VA with a

threshold of 2.465V. Typical input current is about 3nA out

of pin. For charging lithium-ion batteries, VA monitors the

battery voltage and reduces charging when battery voltage

reaches the preset value. If it is not used, the OVP pin

should be grounded.

CLP (Pin 9): This is the positive input to the supply current

limit amplifier CL1. The threshold is set at 100mV. When

used to limit supply current, a filter is needed to filter out

the 200kHz switching noise.

CLN (Pin 10): This is the negative input to the amplifier

CL1.

COMP1 (Pin 11): This is the compensation node for the

amplifier CL1. A 200pF capacitor is required from this pin

to GND if input current amplifier CL1 is used. At input

adapter current limit, this node rises to 1V. By forcing

COMP1 low with an external transistor, amplifier CL1 will

be defeated (no adapter current limit). COMP1 can source

200µA.

SENSE (Pin 12): Current Amplifier CA1 Input. Sensing can

be at either terminal of the battery.

SPIN (Pin 13): This pin is for the internal amplifier CA1

bias. It has to be connected to RS1 as shown in the 3A

Lithium Battery Charger (Figure 1).

BAT (Pin 14): Current Amplifier CA1 Input.

COMP2 (Pin 15): This is also a compensation node for the

amplifier CL1. It gets up to 2.8V at input adapter current

limit and/or at constant-voltage charging.

UVOUT (Pin 17): This is an open collector output for

undervoltage lockout status. It stays low in undervoltage

state. With an external pull-up resistor , it goes high at valid

VCC. Note that the base drive of the open collector NPN

comes from CLN pin. UVOUT stays low only when CLN is

higher than 2V. Pull-up current should be kept under

100µA.

VC (Pin 18): This is the control signal of the inner loop of

the current mode PWM. Switching starts at 0.7V. Higher

VC corresponds to higher charging current in normal

operation. A capacitor of at least 0.33µF to GND filters out

noise and controls the rate of soft-start. To shut down

switching, pull this pin low. Typical output current is 30µA.

PROG (Pin 19): This pin is for programming the charging

current and for system loop compensation. During normal

operation, VPROG stays close to 2.465V. If it is shorted to

GND the switching will stop. When a microprocessor

controlled DAC is used to program charging current, it

must be capable of sinking current at a compliance up to

2.465V.

VCC (Pins 20, 21, 22): This is the supply of the chip. For

good bypass, a low ESR capacitor of 20µF or higher is

required, with the lead length kept to a minimum. VCC

should be between 8V and 28V and at least 3V higher than

VBAT. Undervoltage lockout starts and switching stops

when VCC goes below 7V. Note that there is a parasitic

diode inside from SW pin to VCC pin. Do not force VCC

below SW by more than 0.7V with battery present. All three

VCC pins should be shorted together close to the pins.

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