Datasheet
10 M:
-
+
V
OUT
10 M:
10 M:
V
IN
10 M:
10 M:
270 pF 270 pF
270 pF
CR2032 Coin Cell
225 mAh = 5 circuits @ 9.5 yrs. each
60 Hz Twin T Notch Filter
A
V
= 2 V/V
10 M:
270 pF
V
BATT
V
BATT
= 3V o
2V @ end of life
LPV521
www.ti.com
SNOSB14C –AUGUST 2009–REVISED FEBRUARY 2013
Figure 62. 60 Hz Notch Filter
Figure 63. 60 Hz Notch Filter Waveform
BATTERY CURRENT SENSING
The rail-to-rail common mode input range and the very low quiescent current make the LPV521 ideal to use in
high side and low side battery current sensing applications. The high side current sensing circuit in Figure 64 is
commonly used in a battery charger to monitor the charging current in order to prevent over charging. A sense
resistor R
SENSE
is connected in series with the battery. The theoretical output voltage of the circuit is V
OUT
= [
(R
SENSE
× R
3
) / R
1
] × I
CHARGE
. In reality, however, due to the finite Current Gain, β, of the transistor the current
that travels through R
3
will not be I
CHARGE
, but instead, will be α × I
CHARGE
or β/( β+1) × I
CHARGE
. A Darlington pair
can be used to increase the β and performance of the measuring circuit. Using the components shown in
Figure 64 will result in V
OUT
≈ 4000 Ω × I
CHARGE
. This is ideal to amplify a 1 mA I
CHARGE
to near full scale of an
ADC with V
REF
at 4.1V. A resistor, R2 is used at the non-inverting input of the amplifier, with the same value as
R1 to minimize offset voltage. Selecting values per Figure 64 will limit the current traveling through the R
1
– Q1 –
R
3
leg of the circuit to under 1 µA which is on the same order as the LPV521 supply current. Increasing resistors
R
1
, R
2
, and R
3
will decrease the measuring circuit supply current and extend battery life. Decreasing R
SENSE
will
minimize error due to resistor tolerance, however, this will also decrease V
SENSE
= I
CHARGE
× R
SENSE
, and in turn
the amplifier offset voltage will have a more significant contribution to the total error of the circuit. With the
components shown in Figure 64 the measurement circuit supply current can be kept below 1.5 µA and measure
100 µA to 1 mA..
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