Datasheet

LMP8646EB User’s Guide
November 2011 Page 5 of 20 SNOU010
Note: Refer to the “Selection of the Sense Resistor, R
SENSE
” section of the LMP8646 datasheet to
select your own R
SENSE
if 55 mOhm is not desired.
3. Step 3: Choose the Bandwidth Capacitance, C
G
.
The product of C
G
and R
G
determines the bandwidth for the LMP8646. Refer to the Typical
Performance Characteristics plots of the LMP8646 datasheet to see the range for the LMP8646
bandwidth and gain. Since each application is very unique, the LMP8646 bandwidth capacitance, C
G
,
needs to be adjusted to fit the appropriate application.
Bench data has been collected for the supercap application with the LM3102 regulator. We found
that this application works best for a bandwidth of 500 Hz to 3 kHz. Operating outside of this
recommended bandwidth range might create an undesirable load current ringing. We recommend
choosing a bandwidth that is in the middle of this range and using the following equation to find C
G:
  
  
nFCg
kHzkOhm
Cg
BandwidthR
Cg
G
8.1
75.1502
1
2
1





Once C
G
is chosen, capture the output regulator current plot and adjust C
G
to get the desired value.
If adjusting C
G
isn’t enough, we also recommend adjusting the regulator’s C
OUT
to reduce the current
ringing. We found that this application works best for a C
OUT
value of 47 µF.
4. Step 4: Calculate the Output Accuracy and Choose a Tolerable System Error
Since the LMP8646 is a precision current limiter, the output current accuracy is extremely important.
This accuracy is affected by the system error contributed by the LMP8646 device error and other
errors contributed by the regulator and external resistances, such as R
SENSE
and R
G
. However, we
cannot control for external errors, but we can predict the LMP8646 device error using the following
equations:




  
  
kOhmR
AmOhm
kOhmV
R
IR
kOhmV
R
kOhm
R
VA
R
cectranscondu
R
Gainwhere
GainIRV
G
G
LIMITSENSE
OUT
G
GGG
LIMITSENSEOUT
50
5.155
58.0
5
5/200/1tan/1
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

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
