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LTC1759

Table 7. VLIMIT Trip Points and Ranges

NOMINAL

EXTERNAL

CHARGING

RESISTOR

VOLTAGE (VOUT)

(RVLIMIT)VLIMIT VOLTAGE

RANGE

GRANULARITY

0VVLIMIT < 0.09VVCCP 2465mV < VOUT

16mV

< 8432mV

10k

±1%

0.17VVDD < VVLIMIT

2465mV < VOUT

16mV

< 0.34VVDD

< 12640mV

33k

±1%

0.42VVCCP < VVLIMIT 2465mV < VOUT

32mV

< 0.59VVDD

< 16864mV

100k

±1% 0.66VVDD < VVLIMIT 2465mV < VOUT

32mV

< 0.84VVDD

< 21056mV

Open or

0.91VVDD < VVLIMIT

2465mV < VOUT

32mV

Tied to VDD

< 32768mV

PROGRAMMED VALUE (V)

NOTE: THE USER MUST ADJUST THE VALUE OF

THE EXTERNAL CURRENT SENSING COMPONENTS

(RS1, RS2, RSENSE, RSET) TO MAINTAIN CONSISTENCY

WITH ILIMIT RANGES. SEE APPLICATIONS INFORMATION

1759 F06

Figure 6. Transfer Function of Charger

IPROG

(FROM CA1 AMP)

1759 F07

RSET

VREF

ISET

PROG

ERROR

AMP

CHARGINGCURRENT()

VALUE

∆-∑

MODULATOR

Figure 7. Current DAC Operation

The Voltage DAC Block

Note that the charge output voltage is offset by VREF.

Therefore, the value of VREF is subtracted from the SMBus

ChargingVoltage() value in order for the output voltage to

be programmed properly (without offset). If the

ChargingVoltage() value is below the nominal reference

voltage of the charger, nominally 2.465V, the charger

output voltage is programmed to zero. In addition, if the

ChargingVoltage() value is above the limit set by the VLIMIT

pin, then the charger output voltage is set to the value

determined by the VLIMIT resistor and the VOLTAGE_OR

bit is set. These limits are demonstrated in Figure 6.

The Current DAC Block

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

trols the effective value of an external resistor, RSET, used

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:

1.25RSET/ChargingCurrent()/ILIMIT[x])

Therefore, programmed current is equal to:

0.8VREF/RSET (ChargingCurrent()/ILIMIT[x]),

for ChargingCurrent() < ILIMIT[x].

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

allowed by ILIMIT is applied to the current DAC input, the

current DAC enters a different mode of operation. The

current DAC output is pulse width modulated with a high

ILIMIT/8

AVERAGE CHARGER CURRENT

1750 F08

~40ms

Figure 8. Charging Current Waveform in Low Current Mode

OPERATIO

frequency clock having a duty cycle value of 1/8. There-

fore, the maximum output current provided by the charger

is IMAX/8. The delta-sigma output gates this low duty cycle

signal on and off. The delta-sigma shift registers are then

clocked at a slower rate, about 45ms/bit, so that the

charger has time to settle to the IMAX/8 value. The resulting

average charging current is equal to that requested by the

ChargingCurrent() value.

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