LMP8646_15 Datasheet, PDF (22/31 Pages) Texas Instruments – LMP8646 Precision Current Limiter

LMP8646
SNOSC63B – FEBRUARY 2012 – REVISED DECEMBER 2014
www.ti.com
Typical Applications (continued)
R_G is chosen from I_LIMIT. As stated, V_OUT = (R_SENSE * I_LIMIT) * (R_G / 5kOhm). Since V_OUT = ADJ = 0.6V, I_LIMIT = 1A,
and R_SENSE = 58 mOhm , R_G can be calculated as:
R_G = (V_OUT * 5 kOhm) / (R_SENSE * I_LIMIT)
(19)
R_G = (0.6 * 5 kOhm) / (58 mOhm* 1A) = 51.7 kOhm
(20)
Step 4: 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 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 this resistive load application with the LP38501 regulator, and we found that
this application works best for a bandwidth of 50 Hz to 300 Hz. Operating anything larger than this recommended
bandwidth might prevent the LMP8646 from quickly limiting the current. We recommend choosing a bandwidth
that is in the middle of this range and using the equation: C_G = 1/(2*pi*R_G*Bandwidth) to find C_G (this example
uses a C_G value of 10 nF). After this selection, capture the plot for I_SENSE and adjust C_G until a desired sense
current plot is obtained.
Step 5: Choose the output resistor, ROUT, for the LMP8646
ROUT plays a very small role in the overall system performance for the resistive load application. ROUT was
important in the supercap application because it affects the initial current error. Because current is directly
proportional to voltage for a resistive load, the output current is not large at start-up. The bigger the ROUT, the
longer it takes for the output voltage to reach its final value. We recommend that the value for ROUT is at least
50 Ohm, which is the value we used for this example.
Step 6: Adjusting Components
Capture the output current and output voltage plots and adjust the components as necessary. The most common
component to adjust is C_G for the bandwidth. An example plot of the output current and voltage can be seen in
Figure 33.
8.2.3.3 Application Curve
2.5
2.5
Vclose_loop
I_limit
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
-10 10 30 50 70 90 110 130 150 170
TIME (ms)
Figure 33. Plot for the Resistive Load Application With the LP38501 LDO Regulator
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