LMP8645HVMK/NOPB National Semiconductor, LMP8645HVMK/NOPB Datasheet - Page 12

• Manufacturer Part Number: LMP8645HVMK/NOPB
• Description: IC AMP CURRENT SENSE 6TSOT
• Manufacturer: National Semiconductor

Specifications of LMP8645HVMK/NOPB

• Amplifier Type: Current Sense
• Number Of Circuits: 1
• Slew Rate: 0.6 V/µs
• -3db Bandwidth: 860kHz
• Current - Input Bias: 13µA
• Voltage - Input Offset: 1700µV
• Current - Supply: 2.2mA
• Current - Output / Channel: 5mA
• Voltage - Supply, Single/dual (±): 2.7 V ~ 12 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: TSOT-23-6, TSOT-6
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Output Type: -
• Gain Bandwidth Product: -
• Other names: LMP8645HVMK

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Example 2
The LMP8645 is used to monitor the current in a high bright-
ness 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 dim-
ming (MOSFET in parallel to the LED). The approach of the
high current sense is used especially in automotive applica-
tion where the cathode of the LED needs to be directly con-
nected 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
cause the purpose is regulating the current in the LEDs when
the external MOSFET is OFF (LMP8645 at high V
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
to the Range 3.
To summarize, the R
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 mod-
eled 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 in-
ternal 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
pin.
G
pin is not suggested due to the high impedance of that
FIGURE 6. LMP8645 to ADC interface
GAIN
will be calculated according to Range 3 be-
GAIN
will be calculated according to the
GAIN
will be calculated according
CM
). Even
30071631
12
SENSING CURRENT IN LED DRIVER APPLICATIONS
The LMP8645 is the right choice in the applications which re-
quires 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 be-
tween the LED's cathode and the Range doesn't guarantee
the ground connection. In
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
(R
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 ac-
curacy, the R
200 mΩ. In the table below two examples are analyzed.
R
R
Dissipated Power 9.5mW
Total Accuracy
GAIN
S
S
*Gain), where Gain = R
S
should be in the Range between 50 mΩ and
I
40kΩ
77mΩ
≊
F
=350mA
5%
GAIN
Figure
*Gm and V
7, the LMP8645 monitors
I
36kΩ
27mΩ
27mW
≊
F
CS
5%
=1A
=200 mV. In this
F
= V
CS
/