LMP8601MAEVAL/NOPB National Semiconductor, LMP8601MAEVAL/NOPB Datasheet - Page 13

• Manufacturer Part Number: LMP8601MAEVAL/NOPB
• Description: BOARD EVALUATION FOR LMP8601MA
• Manufacturer: National Semiconductor
• Series: LMP®r

Specifications of LMP8601MAEVAL/NOPB

• Channels Per Ic: 1 - Single
• Amplifier Type: Current Sense
• Output Type: Single-Ended
• Slew Rate: 0.83 V/µs
• Current - Output / Channel: 48mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 1.1mA
• Voltage - Supply, Single/dual (±): 3 V ~ 5.5 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMP8601
• Silicon Manufacturer: National
• Silicon Core Number: LMP8601, LMP8601Q
• Kit Application Type: Amplifier
• Application Sub Type: Current-Sense Amplifier
• Kit Contents: Board, Doc
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Other names: LMP8601MAEVAL

ADDITIONAL SECOND ORDER LOW PASS FILTER
The LMP8601/LMP8601Q has a third order Butterworth low-
pass characteristic with a typical bandwidth of 60 kHz inte-
grated in the preamplifier stage of the part. The bandwidth of
the output buffer can be reduced by adding a capacitor on the
A1 pin to create a first order low pass filter with a time constant
determined by the 100 kΩ internal resistor and the external
filter capacitor.
It is also possible to create an additional second order Sallen-
Key low pass filter by adding external components R
C
in
characteristic.
When the corner frequency of the additional filter is much
lower than 60 kHz, the transfer function of the described am-
plifier van be written as:
Where K
the buffer amplifier.
The above equation can be written in the normalized frequen-
cy response for a 2
The cutt-off frequency ω
cut-off frequency in Hz) is given by:
2
Figure
. Together with the internal 100 kΩ resistor R
1
2, this circuit creates a second order low-pass filter
equals the gain of the preamplifier and K
nd
order low pass filter:
o
in rad/sec (divide by 2
1
as illustrated
π
to get the
2
, C
2
that of
1
and
13
and the quality factor of the filter is given by:
With K
With this filter gain K2= 2x, the design procedure can be very
simple if the two capacitors are chosen to be equal, C
In this case, given the predetermined value of R1 = 100kΩ
( the internal resistor), the quality factor is set solely by the
value of the resistor R
R
first step of the design procedure with the following equation:
For instance, the value of Q can be set to 0.5
Butterworth response, to 1/
or a 0.5 to create a critically damped response. Once the
value of R
design procedure is to calculate the required value of C to give
the desired low-pass cut-off frequency using:
Note that the frequency response achieved using this proce-
dure will only be accurate if the cut-off frequency of the second
order filter is much smaller than the intrinsic 60 kHz low-pass
filter. In other words, to have the frequency response of the
LMP8601/LMP8601Q circuit chosen such that the internal
poles do not affect the external second order filter.
2
can be calculated based on the desired value of Q as the
2
= 2x, the above equation transforms results in:
2
has been found, the second and last step of the
2
.
√
3 to create a Bessel response,
√
2 to create a
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1
=C
2
=C.