LTC1759 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1759' PDF : 28 Pages View PDF
U
OPERATIO
Table 7. VLIMIT Trip Points and Ranges
EXTERNAL
RESISTOR
(RVLIMIT)
VLIMIT VOLTAGE
NOMINAL
CHARGING
VOLTAGE (VOUT)
RANGE
0
VVLIMIT < 0.09VVCCP 2465mV < VOUT
< 8432mV
10k ±1% 0.17VVDD < VVLIMIT
< 0.34VVDD
33k ±1% 0.42VVCCP < VVLIMIT
< 0.59VVDD
100k ±1% 0.66VVDD < VVLIMIT
< 0.84VVDD
Open or 0.91VVDD < VVLIMIT
Tied to VDD
2465mV < VOUT
< 12640mV
2465mV < VOUT
< 16864mV
2465mV < VOUT
< 21056mV
2465mV < VOUT
< 32768mV
GRANULARITY
16mV
16mV
32mV
32mV
32mV
The Voltage DAC Block
Note that the charge output voltage is offset by VREF.
Therefore, the value of VREF 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 VLIMIT
pin, then the charger output voltage is set to the value
determined by the VLIMIT resistor and the VOLTAGE_OR
bit is set. These limits are demonstrated in Figure 6.
LTC1759
25
20
15
10
5
0
0 5 10 15 20 25 30 35
PROGRAMMED VALUE (V)
NOTE: THE USER MUST ADJUST THE VALUE OF
THE EXTERNAL CURRENT SENSING COMPONENTS
(RS1, RS2, RSENSE, RSET) TO MAINTAIN CONSISTENCY
WITH ILIMIT RANGES. SEE APPLICATIONS INFORMATION
1759 F06
Figure 6. Transfer Function of Charger
IPROG
(FROM CA1 AMP)
PROG
+
RSET
–
VREF
ISET
∆-∑
MODULATOR
TO
ERROR
AMP
CHARGINGCURRENT()
VALUE
1759 F07
The Current DAC Block
The current DAC is a delta-sigma modulator which con-
trols the effective value of an external resistor, RSET, 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:
1.25RSET/ChargingCurrent()/ILIMIT[x])
Therefore, programmed current is equal to:
0.8VREF/RSET (ChargingCurrent()/ILIMIT[x]),
for ChargingCurrent() < ILIMIT[x].
When a value less than 1/16th of the maximum current
allowed by ILIMIT 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
Figure 7. Current DAC Operation
ILIMIT/8
0
~40ms
AVERAGE CHARGER CURRENT
1750 F08
Figure 8. Charging Current Waveform in Low Current Mode
frequency clock having a duty cycle value of 1/8. There-
fore, 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 45ms/bit, so that the
charger has time to settle to the IMAX/8 value. The resulting
average charging current is equal to that requested by the
ChargingCurrent() value.
21
Share Link: 
