LTC1960CUHF-TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1960CUHF-TRPBF' PDF : 28 Pages View PDF
LTC1960
OPERATION
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 6 is
a simplified diagram of the DAC operation. The delta-sigma
modulator and switch convert the IDAC value, received
via SPI communication, to a variable resistance equal to
1.25RSET/(IDAC(VALUE)/1023). In regulation, ISET is servo
driven to the 0.8V reference voltage, VREF , and the cur-
rent 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:
IAVG
=
VREF • 3k
(1.25RSNS RSET )
•
IDAC(VALUE)
1023
When the low current mode bit (D10) is set to 1, the current
DAC enters a different mode of operation. 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. The resulting average
charging current is equal to 1/8 of the current programmed
in normal mode. Dual battery charging is disabled in low
current mode. If both batteries are selected for charging,
then only BAT1 will charge.
(VCSP – VCSN)
3kΩ
(FROM CA1 AMPLIFIER)
ISET
+
CSET
VREF
–
RSET
18.77k
∆Σ
10
MODULATOR
TO
ITH
DAC
VALUE
(10 BITS)
1960 F06
Figure 6. Current DAC Operation
IMAX/8
0
AVERAGE CHARGER CURRENT
~40ms
1960 F07
Figure 7. Charging Current Waveform in Low Current Mode
1960fb
18
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