LT1505 Просмотр технического описания (PDF) - Linear Technology

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LT1505

Constant-Current/Voltage High Efficiency Battery Charger

Linear Technology 

LT1505

PIN FUNCTIONS

the noninverting input to the amplifier, VA in the Block

Diagram, that controls the charging current when the

device operates in constant voltage mode. The amplifier

VA controls the charging current to maintain the voltage

on the VFB pin at the reference voltage (2.465V). Input bias

current for VA is approximately 3nA. The LT1505 incorpo-

rates a resistor divider that can be used to select the

correct voltage for either three or four 4.1V or 4.2V

lithium-ion cells. For three cells the 3CELL pin is shorted

to the VFB pin. For four cells the 3CELL pin is not con-

nected. For 4.1V cells the 4.1V pin is connected to the VFB

pin and the 4.2V pin is not connected. For 4.2V cells the

4.2V pin is connected to VFB and the 4.1V pin is not

connected. See the table below.

PRESET BATTERY VOLTAGE

12.3V (3 × 4.1V Cell)

16.4V (4 × 4.1V Cell)

12.6V (3 × 4.2V Cell)

16.8V (4 × 4.2V Cell)

PIN SELECTION

4.1V, VFB, 3CELL Short Together

4.1V, VFB, Short Together, 3CELL Floats

4.2V, VFB, 3CELL Short Together

4.2V, VFB, Short Together, 3CELL Floats

For battery voltages other than the preset values, an

external resistor divider can be used. If an external divider

is used then the 4.1V, 4.2V and 3CELL pins should not be

connected and BAT2 pin should be grounded. To maintain

the tight voltage tolerance, the external resistors should

have better than 0.25% tolerance. Note that the VFB pin will

float high and inhibit switching if it is left open.

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

the current mode PWM. Switching starts at 0.9V, higher

VC corresponds to higher charging current in normal

operation and reaches 1.1V at full charging current. A

capacitor of at least 0.33µF to GND filters out noise and

controls the rate of soft start. Pulling this pin low will stop

switching. Typical output current is 60µA.

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

current and for system loop compensation. During normal

operation, VPROG stays at 2.465V. If it is shorted to GND or

more than 1mA is drawn out of the pin, switching will stop.

When a microprocessor controlled DAC is used to pro-

gram charging current, it must be capable of sinking

current at a compliance up to 2.465V.

BAT2 (Pin 20): This pin is used to connect the battery to

the internal preset voltage setting resistor. An internal

switch disconnects the internal divider from the battery

when the device is in shutdown or when power is discon-

nected. This disconnect function eliminates the current

drain due to the resistor divider. This pin should be

connected to the positive node of the battery if the internal

preset divider is used. This pin should be grounded if an

external divider is used. Maximum input voltage on this

pin is 20V.

SENSE (Pin 21): This pin is the noninverting input to the

current amplifier CA1 in the Block Diagram. Typical bias

current is – 50µA.

SPIN (Pin 22): This pin is for the internal amplifier CA1

bias. It must be connected as shown in the application

circuit.

BAT (Pin 23): Current Amplifier CA1 Inverting Input.

Typical bias current is – 50µA.

VCC (Pin 24): Input Supply. For good bypass, a low ESR

capacitor of 10µF or higher is required. Keep the lead

length to a minimum. VCC should be between 11V and 24V.

Do not force VCC below VBAT by more than 1V with the

battery present.

BOOSTC (Pin 25): This pin is used to bootstrap and supply

the current sense amplifier CA1 for very low dropout

condition. VCC can be as low as only 0.4V above the battery

voltage. A diode and a capacitor are needed to get the

voltage from VBOOST. If low dropout is not needed and VCC

is always 3V or higher than VBAT, this pin can be left

floating or tied to VCC. Do not force this pin to a voltage

lower than VCC. Typical input current is 1mA.

GBIAS (Pin 26): This is the output of the internal 8.6V

regulator to power the drivers and control circuits. This pin

must be bypassed to a ground plane with a minimum of

2.2µF ceramic capacitor. Switching will stop when VGBIAS

drops below 7V.

BGATE (Pin 27): Low Side Power MOSFET Drive.

PGND (Pin 28): MOSFET Driver Power Ground. A solid

system ground plane is very important. See the LT1505

Demo Manual for further information.

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