Datasheet
21
LTC1759
Table 7. V
LIMIT
Trip Points and Ranges
NOMINAL
EXTERNAL CHARGING
RESISTOR VOLTAGE (V
OUT
)
(R
VLIMIT
)V
LIMIT
VOLTAGE RANGE GRANULARITY
0V
VLIMIT
< 0.09V
VCCP
2465mV < V
OUT
16mV
< 8432mV
10k ±1% 0.17V
VDD
< V
VLIMIT
2465mV < V
OUT
16mV
< 0.34V
VDD
< 12640mV
33k ±1% 0.42V
VCCP
< V
VLIMIT
2465mV < V
OUT
32mV
< 0.59V
VDD
< 16864mV
100k ±1% 0.66V
VDD
< V
VLIMIT
2465mV < V
OUT
32mV
< 0.84V
VDD
< 21056mV
Open or 0.91V
VDD
< V
VLIMIT
2465mV < V
OUT
32mV
Tied to V
DD
< 32768mV
PROGRAMMED VALUE (V)
NOTE: THE USER MUST ADJUST THE VALUE OF
THE EXTERNAL CURRENT SENSING COMPONENTS
(R
S1
, R
S2
, R
SENSE
, R
SET
) TO MAINTAIN CONSISTENCY
WITH I
LIMIT
RANGES. SEE APPLICATIONS INFORMATION
0
CHARGER V
OUT
(V)
25
20
15
10
5
0
10
20
25
1759 F06
5
15
30
35
Figure 6. Transfer Function of Charger
I
PROG
(FROM CA1 AMP)
1759 F07
+
–
R
SET
V
REF
I
SET
PROG
TO
ERROR
AMP
CHARGINGCURRENT()
VALUE
∆-∑
MODULATOR
Figure 7. Current DAC Operation
The Voltage DAC Block
Note that the charge output voltage is offset by V
REF
.
Therefore, the value of V
REF
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 V
LIMIT
pin, then the charger output voltage is set to the value
determined by the V
LIMIT
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, R
SET
, 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:
1.25R
SET
/ChargingCurrent()/I
LIMIT[x]
)
Therefore, programmed current is equal to:
0.8V
REF
/R
SET
(ChargingCurrent()/I
LIMIT
[x]),
for ChargingCurrent() < I
LIMIT[x]
.
When a value less than 1/16th of the maximum current
allowed by I
LIMIT
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
I
LIMIT
/8
AVERAGE CHARGER CURRENT
1750 F08
~40ms
0
Figure 8. Charging Current Waveform in Low Current Mode
OPERATIO
U
frequency clock having a duty cycle value of 1/8. There-
fore, 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 45ms/bit, so that the
charger has time to settle to the I
MAX
/8 value. The resulting
average charging current is equal to that requested by the
ChargingCurrent() value.