Datasheet

+IN
-IN
V
+
R
S
V
-
R
G
V
OUT
LMP8645
R
IN
R
IN
I
S
-+
+
-
+
-
ADC
-+
V
A
ACTIVE
DEVICE
LMP8645, LMP8645HV
www.ti.com SNOSB29F –NOVEMBER 2009–REVISED MARCH 2013
Figure 20. LMP8645 in current monitor application
Example 2
The LMP8645 is used to monitor the current in a high brightness LED application together with LM340x LED
drivers. The LMP8645 is supplied at 3.3V and the max voltage at LED’s string is 30V, the LED brightness is
controlled with the dimming (MOSFET in parallel to the LED). The approach of the high current sense is used
especially in automotive application where the cathode of the LED needs to be directly connected to the chassis
of the car (Ground of the system). Even though LMP8645 will work in all 3 Ranges as in the previous example,
R
GAIN
will be calculated according to Range 3 because the purpose is regulating the current in the LEDs when
the external MOSFET is OFF (LMP8645 at high V
CM
). Even if this approach makes the LMP8645 able to sense
high peak current only in Range 3 where the dynamic output is higher than Range 1 the current resolution is
maximized. At each switch ON/OFF of the MOSFET the LMP8645 goes from Range 1 (MOSFET ON, string of
LED OFF), to Range 3 (MOSFET OFF, string of LED ON) passing through Range 2 (MOSFET OFF, string of
LED OFF). Since the purpose of the application is to sense the current with high precision when the LED string is
ON, the R
GAIN
will be calculated according to the Range 3.
To summarize, the R
GAIN
will be calculated according to the range of operation in which the application will
mainly work. Once selected the range, will be taken in account the more stringent constraint
DRIVING ADC
The input stage of an Analog to Digital converter can be modeled with a resistor and a capacitance versus
ground. So if the voltage source doesn't have a low impedance an error in the amplitude's measurement will
occur. In this Range a buffer is needed to drive the ADC. The LMP8645 has an internal output buffer able to
drive a capacitance load up to 30 pF or the input stage of an ADC. If required an external low pass RC filter can
be added at the output of the LMP8645 to reduce the noise and the bandwidth of the current sense. Any other
filter solution which implies a capacitance connected to the R
G
pin is not suggested due to the high impedance of
that pin.
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