Datasheet

+IN
-IN
V
+
R
S
V
-
R
GAIN
V
OUT
LMP8645HV
R
IN
R
IN
I
F
+
-
+
-
HB/HP
(3W-25W)
LM3406HV
Constant Current
Buck Regulator
SW
CS
R
G
V
IN
V
CC
L
I
F
= 200 mV/(R
S
*Gain)
Gain = R
GAIN
*Gm
+IN
-IN
V
+
R
S
V
-
R
G
V
OUT
LMP8645
R
IN
R
IN
I
S
-+
+
-
+
-
L
o
a
d
ADC
R
F
C
F
f
-3dB
= 1/(2SR
F
C
F
)
-+
V
A
R
GAIN
LMP8645, LMP8645HV
SNOSB29F –NOVEMBER 2009–REVISED MARCH 2013 www.ti.com
Figure 21. LMP8645 to ADC interface
SENSING CURRENT IN LED DRIVER APPLICATIONS
The LMP8645 is the right choice in the applications which requires high side current sense, such as High
Brightness LED for automotive where the LED's cathode has to be connected to the Range (ground) of the car.
In this Range the classical low side current sense with a shunt resistor connected between the LED's cathode
and the Range doesn't specify the ground connection. In Figure 22, the LMP8645 monitors the current for the
LM3406 a constant current buck regulator. The LMP8645 is supplied by the internal LDO of the LM3406
thorough the pin VCC, the current which flows in the LED is programmed according the following formula: I
F
=
V
CS
/(R
S
*Gain), where Gain = R
GAIN
*Gm and V
CS
=200 mV. In this application the current which flows in the HB
LED is in the Range between 350 mA and 1A, so in order to reduce the power dissipation on the shunt resistor
and have a good accuracy, the R
S
should be in the Range between 50 mΩ and 200 mΩ. In the table below two
examples are analyzed.
I
F
=350mA I
F
=1A
R
GAIN
40kΩ 36kΩ
R
S
77mΩ 27mΩ
Dissipated Power 9.5mW 27mW
Total Accuracy ≊5% ≊5%
Figure 22. High Side Current Sensing in Driving HP/HB LED
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