LMP8646 Datasheet, PDF (18/23 Pages) Texas Instruments – Precision Current Limiter

APPLICATION #3: CURRENT LIMITER WITH A LOW-DROPOUT REGULATOR AND RESISTIVE LOAD
FIGURE 17. Resistive Load Application with LP38501 Regulator
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This next example is the same as the last example, except
that the regulator is now a low-dropout regulator, the
LP38501, as seen in Figure 17. For this example, we will let
the open-loop current to be 1.25A and the close-loop current,
I_LIMIT, to be 1A.
Step 1: Choose the components for the Regulator.
Refer to the LP38501 application note (AN-1830) to select the
appropriate components for the LP38501.
Step 2: Choose the sense resistor, R_SENSE
R_SENSE sets the voltage V_SENSE between +IN and -IN and has
the following equation:
R_SENSE = V_OUT / [(I_LIMIT) * (R_G / 5kOhm)]
In general, R_SENSE depends on the output voltage, limit cur-
rent, and gain. Refer to section SELECTION OF THE SENSE
RESISTOR, R_SENSE to choose the appropriate R_SENSE value;
this example uses 58 mOhm.
Step 3: Choose the gain resistor, R_G, for LMP8646
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)
R_G = (0.6 * 5 kOhm) / (58 mOhm* 1A) = 51.7 kOhm
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 band-
width capacitance, C_G, needs to be adjusted to fit the appro-
priate application.
Bench data has been collected for this resistive load applica-
tion with the LP38501 regulator, and we found that this appli-
cation 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*Band-
width) 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 perfor-
mance for the resistive load application. ROUT was important
in the supercap application because it affects the initial cur-
rent error. Because current is directly proportional to voltage
for a resistive load, the output current is not large at startup.
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 compo-
nent to adjust is C_G for the bandwidth. An example plot of the
output current and voltage can be seen in Figure 18.
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