LTC1760 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1760' PDF : 44 Pages View PDF
LTC1760
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
CSN
CB2
VSET
CB1
RVF
405.3k
VREF
RVSET
7.2k
–
EA
+
SWV
ΔΣ
11
MODULATOR
TO
ITH
DAC
VALUE
(11 BITS)
1760 F07
Figure 7. Voltage DAC Operation
voltage, VREF, and the current from RSET is matched against
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.
(VCSP – VCSN)
3kΩ
(FROM CA1 AMPLIFIER)
ISET
+
CSET
VREF –
RSET
18.77k
ΔΣ
10
MODULATOR
Figure 8. Current Dac Operation
TO
ITH
DAC
VALUE
(10 BITS)
1760 F08
1760fa
35
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