LT2940
14
2940f
APPLICATIONS INFORMATION
Figure 6. LATCH Pin Protective Damping
below full-scale also affords scaling fl exibility. Line (D)
along I
PMON
= 100µA covers a 4:1 range like (A), but the
maximum V
I
is 100mV, which reduces voltage drop and
dissipation in the sense resistor.
Variable power applications comprise power measuring,
whether battery charging, energy metering or motor
monitoring, variable load-boxes, and other circuits where
the signifi cant metric is not a single value, and voltage
and current may be independent of each other. Design in
this case requires mapping the LT2940’s sense ranges to
cover the maximum voltage and the maximum current,
while considering whether the power represented is at,
above, or below full-scale I
PMON
. For example, setting it
at full-scale puts all values in the accurate range, setting
it above puts more accuracy in nominal power levels and
less accuracy in perhaps rarely encountered high levels,
and setting below might afford fl exibility to lower dissipa-
tion in the current sense resistor.
Output Filtering and Integration
Lowpass fi ltering the output power or current signal is as
simple as adding a capacitor in parallel with the output
voltage scaling resistor at PMON or IMON. For example,
adding 1nF in parallel with the PMON load resistor on the
front page application creates a lowpass corner frequency
of approximately 6.4kHz on the power monitor voltage.
Loaded by only a capacitor, the PMON pin voltage is pro-
portional to the time-integral of power, which is energy.
The integrating watt-hour meter application shown on
the back page takes advantage of this convenience. In a
similar way, a capacitor load on IMON produces a volt-
age proportional to charge that can be used to create a
coulomb counter.
Comparator Function
The LT2940’s integrated comparator features an internal
fi xed reference, complementary open-collector outputs
and confi gurable latching. A rising voltage at the CMP
+
pin is compared to the internal 1.24V threshold. 35mV
(typical) negative hysteresis provides glitch protection and
makes falling inputs trip the comparator at about 1.21V.
The comparator result drives the open-collector CMPOUT
and CMPOUT pins which, when pulling down, sink at
least 3mA down to 0.4V. See the Typical Performance
Characteristics for more information. Complementary
comparator outputs save external components in some
applications. The CMPOUT and CMPOUT pins may be
pulled up externally to 36V maximum.
Comparator Latching
The LATCH pin controls the behavior of the comparator
outputs. When the LATCH pin is open, the comparator
output latch is transparent. Leakage currents up to ±10µA
will not change the decoded state of the LATCH pin. Internal
circuits weakly drive the pin to about 1.5V. Adding a 10nF
capacitor between LATCH and GND protects against high
dV/dt on adjacent pins and traces. Where more than 30V
and long inductive leads will be connected to LATCH,
damp potentially damaging ringing with a circuit like that
shown in Figure 6.
LATCH
I
–
I
+
2940 F06
C2
10nF
R9A
20k
RESET
4V TO 80V
LT2940
GND
R9B
49.9k
LONG
WIRE
