Datasheet

14
LTC1325
FUNCTIONAL DESCRIPTIO
UU
U
–
+
–
+
V
DD

4.5V TO 16V
P1
IRF9Z30
N1
IRFZ34
R
TRK
D1
1N5818
PGATE
DIS
DISCHARGE
R
F

1k
R2
125k
R1
500k
REG
3.072V
C1
16pF
FILTER
LTC1325 • F04
SENSE
S4
BATTERY
L1
R
DIS
R
SENSE
C
F
S2
S1
S3
GG
DR0 TO
DR2
A1
A2
GG
0
0
0
0
1
VR1
0
0
1
1
X
VR0
0
1
0
1
X
DAC VOLTAGE
18mV
34mV
55mV
160mV
0mV
DAC
VR0, VR1 GG
(GAS GAUGE)
CHIP
BOUNDARY
2
V
DAC
R
SQ
DUTY RATIO
GENERATOR
ONE SHOT
111kHz
OSCILLATOR
3
CHARGE
TO
ADC MUX
Figure 4. Charging Loop Block Diagram
rise as does the voltage across the sense resistor. When
the voltage across the sense resistor is greater than the
output of the integrator, comparator A2 changes state.
This resets the flip-flop and P1 is turned off. Catch diode
D1 clamps the drain of P1 one diode drop below ground
when the inductor flies back and the current through the
inductor starts to drop. The voltage across the sense
resistor also drops and may reach zero and stay there until
the next clock cycle begins.
The average charging current is set by the output of the
DAC (V
DAC
) and the duty ratio generator. V
DAC
can be
programmed to one of four values with the following
ratios: 1, 1/3, 1/5 or 1/10. The duty ratio can be set to
1/16, 1/8, 1/4, 1/2 or 1. When the duty ratio is 1, the duty
ratio generator output is always low and the charge loop
operates continuously (see Figure 4). At other duty ratio
settings, the duty generator output is a square wave with
a period of 42 seconds. The time for which the generator
output is low varies with the duty ratio setting. For ex-
ample, if a duty ratio of 1/2 is programmed, the generator
output is low only for 42/2 = 21 seconds. Since the loop
operates for only 21 out of every 42 seconds, the average
charging current is halved. In general, the average charg-
ing current is:
I
CHRG
= V
DAC
(Duty Ratio)/R
SENSE
Gated PFET Controller
When using an external current regulator or current lim-
ited wall pack, simply remove the inductor L1 and catch
diode D1. Set the DAC control bits VR1 = 1 and VR0 = 1,
and select the desired duty ratio. By insuring that the
voltage at the Sense pin is never greater than 140mV, the
output of the integrator A1 will saturate high and the
comparator A2 will never trip and turn the loop off. This
can be achieved by removing the sense resistor and
grounding the Sense pin or if the gas gauge is to be used,
selecting R
SENSE
so that R
SENSE
/I
CHRG
< 140mV.