LTC1759CG データシートの表示（PDF） - Linear Technology

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LTC1759CG

Smart Battery Charger

Linear Technology 

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OPERATIO

Overview (Refer to Block Diagram and Figure 10)

The LTC1759 is composed of a battery charger section, a

charger controller, two 10-bit DACs to control charger

parameters, a thermistor decoder, limit decoder and an

SMBus controller block. If no battery is present, the

thermistor decoder indicates a THERM_OR condition and

charging is disabled by the charger controller (CHGEN =

Low). Charging will also be disabled if AC_PRESENT is

low, or the battery thermistor is decoded as THERM_HOT.

If a battery is inserted or AC power is connected, the

battery will be charged with an 80mA “wake-up” current.

The wake-up current is discontinued after three minutes

if the thermistor is decoded as THERM_UR or

THERM_COLD, and the battery or host doesn’t transmit

charging commands.

The SMBus controller block receives ChargingCurrent()

and ChargingVoltage() commands via the SMBus. If

ChargingCurrent() and ChargingVoltage() command pairs

are received within a three-minute interval, the values are

stored in the current and voltage DACs and the charger

controller asserts the CHGEN line if the decoded ther-

mistor value will allow charging to commence.

ChargingCurrent () and ChargingVoltage() values are com-

pared against limits programmed by the limit decoder

block; if the commands exceed the programmed limits

these limits are substituted and overrange flags are set.

The charger controller will assert INTB whenever a status

change is detected. The host may query the charger, via

the SMBus, to obtain ChargerStatus() information. INTB

will be deasserted upon a successful read of

ChargerStatus() or a successful Alert Response Address

(ARA) request.

Battery Charger Section

The LTC1759 is synchronous current mode PWM step-

down (Buck) switcher. The battery DC charging current is

programmed with a current DAC via the SMBus interface.

Amplifier CA1 converts the charging current through

RSENSE to a much lower current IPROG (IPROG = IBAT •

RSENSE/RS2) fed into the PROG pin. Amplifier CA2 com-

pares the output of CA1 with the programmed current and

drives the PWM loop to force them to be equal. High DC

LTC1759

accuracy is achieved with averaging capacitor CPROG.

Note that IPROG has both AC and DC components. IPROG

generates a ramp signal that is fed to the PWM control

comparator C1 through buffer B1 and level shift resistors

forming the current mode inner loop. The BOOST pin

supplies the top power switch gate drive. The LTC1759

generates a 8.9V VGBIAS for bootstrapping VBOOST and

VBOOSTC as well as to drive the bottom power FET. The

BOOSTC pin supplies the current amplifier CA1 with a

voltage higher than VCC for low dropout applications.

Amplifier VA reduces the charging current when the bat-

tery voltage reaches the set voltage programmed by the

VDAC and the 2.465V reference voltage.

The amplifier CL1 monitors and limits the input current,

normally from the AC adapter, to a preset level (92mV/

RCL). At input current limit, CL1 will supply the program-

ming current IPROG and thus reduce battery charging

current.

The INFET pin drives an external input P-channel FET for

low dropout applications.

SMBus Interface

All communications over the SMBus are interpreted by the

SMBus controller block. The SMBus controller is an

SMBus slave device. All internal LTC1759 registers may

be updated and accessed through the SMBus controller,

and charger controller as required. The SMBus protocol is

a derivative of the I2CTM bus (Reference “I2C-Bus and How

to Use It, V1.0” by Philips and “System Management Bus

Specification” by the Smart Battery System Organiza-

tion*, for a complete description of the bus protocol

requirements.)

All data is clocked into the shift register on the rising edge

of SCL. All data is clocked out of the shift register on the

falling edge of SCL. Detection of an SMBus Stop condition,

or power-on reset via the VDD undervoltage lockout, will

reset the controller to an initial state at any time.

The LTC1759 command set is interpreted by the SMBus

controller and passed onto the charger controller block as

control signals or updates to internal registers.

I2C is a trademark of Philips Electronics N.V.

*http://www. SBS-FORUM.org

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