Texas Instruments | AN-2218 Precision Current Limiting with the LMP8646 and LM3102 (Rev. A) | Application notes | Texas Instruments AN-2218 Precision Current Limiting with the LMP8646 and LM3102 (Rev. A) Application notes

Texas Instruments AN-2218 Precision Current Limiting with the LMP8646 and LM3102 (Rev. A) Application notes
Application Report
SNAA137A – February 2012 – Revised April 2013
AN-2218 Precision Current Limiting with the
LMP8646 and LM3102
This application report discusses how to design the Texas Instruments LMP8646 with the LM3102 voltage
regulator and a supercap load application.
Overview ..................................................................................................................... 2
Example ...................................................................................................................... 2
List of Figures
LMP8646 Output Accuracy Equation .....................................................................................
SuperCap Application with LM3102 Regulator Plot ....................................................................
SuperCap Application with LM3102 Regulator and LMP8646 Precision Current Limiter
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SNAA137A – February 2012 – Revised April 2013
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AN-2218 Precision Current Limiting With the LMP8646 and LM3102
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The LMP8646 is a precision current limiter used to improve the current limit accuracy of any switching or
linear regulator with an available feedback node. Many regulators might have an internal current limiter,
but its output accuracy is often as high as 30%. The output accuracy of the LMP8646 can be as low as
3%, making it a preferred current limiter for many regulator applications. The SuperCap application with
LM3102 regulator and LMP8646 precision current limiter is shown in Figure 1.
Figure 1. SuperCap Application with LM3102 Regulator and
LMP8646 Precision Current Limiter
A supercap application requires a very high capacitive load to be charged. This example assumes the
output capacitor is 5F with a limited sense current, ILIMIT, at 1.5A. The LM3102 provides the current to
charge the supercap, and the LMP8646 monitors this current to make sure it does not exceed the desired
1.5A value.
This is done by connecting the LMP8646 output to the feedback pin of the LM3102, as shown in Figure 1.
This feedback voltage at the FB pin is compared to a 0.8V internal reference. Any voltage above this 0.8V
means the output current is above the desired value of 1.5A, and the LM3102 will reduce its output current
to maintain the desired 0.8V at the FB pin. The following steps show the design procedures for this
supercap application. In summary, the steps consist of selecting the components for the voltage regulator,
integrating the LMP8646 and selecting the proper values for its gain, bandwidth, and output resistor, and
adjusting these components to yield the desired performance.
Step 1: Choose the components for the Regulator.
To select the appropriate components for the LM3102 voltage regulator, see AN-1646 LM3102
Demonstration Board Reference Design (SNVA248).
AN-2218 Precision Current Limiting With the LMP8646 and LM3102
SNAA137A – February 2012 – Revised April 2013
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Step 2: Step 2: Choose the sense resistor, RSENSE
RSENSE sets the voltage VSENSE between +IN and -IN and has the following equation:
RSENSE = VOUT / [(ILIMIT) × (RG / 5kOhm)]
In general, RSENSE depends on the output voltage, limit current, and gain. To choose the appropriate RSENSE
value, see the Selection of the Sense Resistor, RSENSE section in LMP8646 Precision Current Limiter
(SNOSC63). Typically, RSENSE is a power resistor in the mOhm range. In this example, we will use
55 mOhm.
Step 3: Choose the gain resistor, RG, for LMP8646
RG is chosen from the limited sense current. As stated in Equation 1, since VOUT = VFB = 0.8V, the limited
sense current is 1.5A and RSENSE is 55 mOhm, RG can be calculated as:
RG = (VOUT × 5 kOhm) / (RSENSE × ILIMIT)
RG = (0.8 × 5 kOhm) / (50 mOhm × 1A) = 50 kOhm (approximate)
Step 4: Choose the Bandwidth Capacitance, CG.
The product of CG and RG determines the bandwidth for the LMP8646. To see the range for the LMP8646
bandwidth and gain, see the Typical Performance Characteristics plots in LMP8646 Precision Current
Limiter (SNOSC63). Since each application is very unique, the LMP8646 bandwidth capacitance, CG,
needs to be adjusted to fit the appropriate application.
Bench data has been collected for the supercap application with the LM3102 regulator, and 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 equation:
CG = 1/(2 × pi × RG × Bandwidth)
to find CG. For example, if the bandwidth is 1.75 kHz and RG is 50 kOhm, then CG is approximately 1.8 nF.
After selecting an initial CG value, capture the plot for lLIMIT and adjust CG until a desired load current plot is
Step 5: Calculate the Output Accuracy and 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 external resistances, such as RSENSE and RG.
In this application, VSENSE = ILIMIT × RSENSE = 1.5A × 55 mOhm = 0.0825V, and RG = 50 kOhm. From the
LMP8646 Electrical Characteristics Table, it is known that VOFFSET = 1 mV and Gm_Accuracy = 2%. Using
the equations in Figure 2, the output accuracy can be calculated as 3.24%.
Figure 2. LMP8646 Output Accuracy Equation
SNAA137A – February 2012 – Revised April 2013
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AN-2218 Precision Current Limiting With the LMP8646 and LM3102
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Step 6: Choose the Output Resistor, ROUT
At startup, the capacitor is not charged yet and thus the output voltage of the LM3102 is very small.
Therefore, at startup, the output current is at its maximum (IMAX). When the output voltage is at its nominal,
then the output current will settle to the desired limited value. Because a large current error is not desired,
ROUT needs to be chosen to stabilize the loop with minimal initial startup current error. Follow the
equations and example below to choose the appropriate value for ROUT to minimize this initial error.
Assume that the tolerable current error, IERROR, is 5%, where IERROR = (IMAX - ILIMIT)/IMAX (%). Therefore, the
maximum allowable current is calculated as:
IMAX = ILIMIT (1+ IERROR) = 1.5A × (1 + 5/100) = 1.575 A.
Next, use the following formula to calculate for ROUT:
For example, assume the minimum LM3102 output voltage, VO_REG_MIN, is 0.6V, then ROUT can be
calculated as:
ROUT = [1.575A × 55 mOhm × (49.9k / 5k) - 0.8] / [ (0.8 / 2k) - (0.6 - 0.8) / 10k] = 153.6 Ohm.
Populate ROUT with a resistor that is as close as possible to 153.6 Ohm (this application uses 160 Ohm).
If the current exceeds 1.575A at any point in time, then adjust this ROUT value to obtain the desired limit
current. We recommend that the value for ROUT is at least 50 Ohm.
Step 7: Adjusting the Components
Capture the output current and output voltage plots and adjust the components as necessary. The most
common components to adjust are CG to decrease the current ripple and ROUT to get a low current error.
An example output current and voltage plot can be seen in Figure 3.
TIME (s)
Figure 3. SuperCap Application with LM3102 Regulator Plot
AN-2218 Precision Current Limiting With the LMP8646 and LM3102
SNAA137A – February 2012 – Revised April 2013
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Copyright © 2012–2013, Texas Instruments Incorporated
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