LTC4100 datasheet(35/48 Pages) LINER

35 / 48 page

LTC1760

1760fa

OPERATION

The delta-sigma modulator and switch SWV convert the

VDAC value to a variable resistance equal to (11/8)RVSET/

(VDAC(VALUE)/2047). In regulation, VSET is servo driven to

the 0.8V reference voltage, VREF.

Capacitors CB1 and CB2 are used to average the voltage

present at the VSET pin as well as provide a zero in the

voltage loop to help stability and transient response time

to voltage variations.

10 The Current DAC Block

The current DAC is a delta-sigma modulator which controls

the effective value of an internal resistor, RSET = 18.77k,

used to program the maximum charger current. Figure 8

is a simplified diagram of the DAC operation. The delta-

sigma modulator and switch convert the IDAC value to a

variable resistance equal to 1.25RSET/(IDAC(VALUE)/1023).

In regulation, ISET is servo driven to the 0.8V reference

voltage, VREF,andthecurrentfromRSETismatchedagainst

a current derived from the voltage between pins CSP and

CSN. This current is (VCSP – VCSN)/3k.

Therefore programmed current is:

ICHG = VREF • 3k/(1.25 RSNS RSET) • (IDAC(VALUE)/1023)

= (102.3mV/RSNS) • (IDAC(VALUE)/1023)

During wake-up current operation, the current DAC enters

a low current mode. 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 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 40ms/bit, so that the charger has time to settle

to the IMAX/8 value.

–

CB1

CB2

CSN

1760 F07

VREF

VSET

RVSET

7.2k

RVF

405.3k

ΔΣ

MODULATOR

SWV

ITH

DAC

VALUE

(11 BITS)

–

1760 F08

VREF

ISET

RSET

18.77k

ΔΣ

MODULATOR

ITH

DAC

VALUE

(10 BITS)

CSET

(VCSP – VCSN)

3kΩ

(FROM CA1 AMPLIFIER)

Figure 7. Voltage DAC Operation

Figure 8. Current Dac Operation

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