LTC4100

17

sn4100 4100is

OPERATIO

The Current DAC Block

The current DAC is a delta-sigma modulator which con-

to set the current limit of the charger. Figure 7 is a

simplified diagram of the DAC operation. The delta-sigma

modulator and switch convert the ChargingCurrent() value,

received via the SMBus, to a variable resistance equal to:

Therefore, programmed current is equal to:

When a value less than 1/16th of the maximum current

current DAC enters a different mode of operation called

LOWI. The current DAC output is pulse width modulated

with a high frequency clock having a duty cycle value of

1/8. Therefore, the maximum output current provided by

low duty cycle signal on and off. The delta-sigma shift

registers are then clocked at a slower rate, about 45ms/bit,

The resulting average charging current is equal to that

requested by the ChargingCurrent() value.

Note: The LOWI mode can be disabled by setting the

NO_LOWI bit in the LTC0() function.

When wake-up is asserted to the current DAC block, the

delta-sigma is then fixed at a value equal to 80mA, inde-

Input FET

The input FET circuit performs two functions. It enables

the charger if the input voltage is higher than the CLP pin,

and provides an indication of this condition at both the

CHGEN pin and the PWR_FAIL bit in the ChargerStatus()

register. It also controls the gate of the input FET to keep

a low forward voltage drop when charging and prevents

reverse current flow through the input FET.

CHGEN pin is forced low unless this condition is met. The

gate of the input FET is driven to a voltage sufficient to keep

a low forward voltage drop from drain to source. If the

voltage between DCIN and CLP drops to less than 25mV,

the input FET is turned off slowly. If the voltage between

DCIN and CLP is ever less than –25mV, then the input FET

is turned off quickly to prevent significant reverse current

from flowing in the input FET. In this condition the CHGEN

pin is driven low and the charger is disabled.

(FROM CA1 AMP)

4100 F07

CHARGING_CURRENT

VALUE

∆-∑

MODULATOR

Figure 7. Current DAC Operation

AVERAGE CHARGER CURRENT

4100 F08

~40ms

0

Figure 8. Charging Current Waveform in Low Current Mode

PROGRAMMED VALUE (V)

0

25

20

15

10

5

0

10

20

25

4100 F06

5

15

30

35

NOTE: THE LTC4100 CAN BE PROGRAMMED WITH ChargingVoltage() FUNCTION VALUES

BETWEEN 1.184V AND 2.9V, HOWEVER, THE BATTERY CHARGER CONTROLLER OUTPUT

VOLTAGE MAY BE ZERO WITH PROGRAMMED VALUES BELOW 2.9V.

Figure 6. Transfer Function of Charger