Datasheet

LTC1760
35
1760fa
OPERATION
The delta-sigma modulator and switch SWV convert the
V
DAC
value to a variable resistance equal to (11/8)R
VSET
/
(V
DAC(VALUE)
/2047). In regulation, V
SET
is servo driven to
the 0.8V reference voltage, V
REF
.
Capacitors C
B1
and C
B2
are used to average the voltage
present at the V
SET
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, R
SET
= 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 I
DAC
value to a
variable resistance equal to 1.25R
SET
/(I
DAC(VALUE)
/1023).
In regulation, I
SET
is servo driven to the 0.8V reference
voltage, V
REF
, and the current from R
SET
is matched against
a current derived from the voltage between pins CSP and
CSN. This current is (V
CSP
– V
CSN
)/3k.
Therefore programmed current is:
I
CHG
= V
REF
• 3k/(1.25 RSNS R
SET
) • (I
DAC(VALUE)
/1023)
= (102.3mV/R
SNS
) • (I
DAC(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 I
MAX
/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 I
MAX
/8 value.
–
+
C
B1
C
B2
CSN
1760 F07
EA
V
REF
V
SET
R
VSET
7.2k
R
VF
405.3k
ΔΣ
MODULATOR
SWV
11
TO
I
TH
DAC
VALUE
(11 BITS)
–
+
1760 F08
V
REF
I
SET
R
SET
18.77k
ΔΣ
MODULATOR
10
TO
I
TH
DAC
VALUE
(10 BITS)
C
SET
(V
CSP
– V
CSN
)
3kΩ
(FROM CA1 AMPLIFIER)
Figure 7. Voltage DAC Operation Figure 8. Current Dac Operation