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Chapter 09 - MPLAB® Mindi™ Analog Simulator - Peak Current Mode Step-up LED Current Regulators

Last modified by Microchip on 2023/11/09 09:06

 

This chapter presents an introduction to Microchip’s Peak Current Mode Step-Up (Boost) Converters for driving LEDs in lighting applications. The goal of these case studies is to understand the impact of the input voltage, load current, and passive components on the quality and stability of the output current.

9.1 Prerequisites

9.2 Case Study: LED Dimming with a Constant Current PWM Regulator

The MCP1643 regulates current by sensing the voltage across a shunt resistor in series with the LEDs. The EN pin can be used for LED dimming by driving it with a variable duty cycle PWM signal, as seen in the accompanying figure on the left. By varying the duty cycle, the average LED current changes proportionally, as shown in the accompanying figure on the right.

LED dimming by variable duty cycle diagram and graph

9.2.1 LED brightness with PWM Dimming

Open the Synchronous boost example, Generic LED dimming application schematic from Power Management > Switching Regulators > MCP1643.
Edit the source driving the EN pin to match the graph above.

Change the LED current by modifying RLOAD1 to be 0.12/200 m, which is the 0.12 V internal reference divided by the desired 200 mA LED current.

Edit Waveform dialog

Run a transient analysis with a stop time of 3 ms and use the cursors to measure key parameters.

transient analysis graph

From the Measure > Mean/cycle menu, determine the mean LED current.

Select Curve dialog and Mean/cycle

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9.2.2 Additional Exercises

Repeat this simulation with different VEN source parameters, such as duty cycle and frequency, and find the value of the mean/cycle ILED output current.
Adjust the value of RSET and rerun the simulations to observe the LED current.

The goal of this section is to understand how Output Overvoltage Protection works with the MCP1643. Overvoltage protection is designed to save the MCP1643 if the output voltage exceeds 5.0 V. If the load is disconnected (an LED fails), the output voltage increases rapidly, because this topology is regulating the current, which is 0. The protection circuit trips stop the switching and periodically monitor the output to verify that the fault is still present.

This feature does not protect the LED. An optional Zener diode can be added between VOUT and VFB pins to clamp the output voltage and protects the LED against excessive voltage and current. The MCP1643's response to the open load event is presented in the below waveform:

MCP1643's response to open load event waveform

Open the Synchronous boost example, startup application schematic from Power Management > Switching Regulators > MCP1643.
Remove the wire between the Cathode of DLED1 and RLOAD.

Choose the transient analysis with a stop time of 20 ms and run it.

transient analysis with a stop time of 20 ms schematic

Using cursors, measure VOUT and identify where the limitation occurs. Notice the waveform shape corresponding to the periodic restart attempts.

VOUT waveform shape graph

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9.4 Case Study: LED Model Experiments

The goal of this section is to improve your ability to include LEDs in simulations. A real LED is a two-pin nonlinear semiconductor device, as seen in the accompanying figure. Each LED has slightly different characteristics, which makes modeling the nonlinearity challenging.

Forward Current and Forward Voltage graph

The MPLAB Mindi Analog Simulator LED model utilizes a PWL block, which can be customized based either on the LED’s datasheet or parameters obtained by characterizing the device. The nonlinear curve is approximated by linear segments that are defined by a table of vertices. The model accuracy can, therefore, be controlled by the number of segments.

The LED model included in the MCP1643 example schematics can be easily placed into other devices' schematics as long as the LED model block in the same folder is copied as well.

What you need for customizing the LED model is the I/V graph from its datasheet as shown in the accompanying example. This is for an XLAMP7090XRE LED type.

To update the default model with the custom LED's data:

Open the Synchronous boost example, steady state application schematic from Power Management > Switching Regulators > MCP1643.
Edit the LED parameters and click on the LED V-I Curve tab to bring up the window.
As mentioned above, the accuracy of the model depends on how many segments the I/V curve is made of. The default value is 10 segments. The first point is the reverse current, the second one is the zero point for both voltage and current and then the actual forward drop / direct current data begins.

With all segments entered, it is recommended to validate the LED model in a separate setup before the actual schematic is created. This is easily done by preparing a very basic test schematic in MPLAB Mindi analog simulator. It should include a variable voltage source, a current probe, and a voltage probe, as seen in the accompanying figure.

Microchip Single LED Model using PWL window

Edit Waveform dialog and Generic LED DLED1 schematic

Choose transient analysis with a stop time of 10 ms and run it. When the graphs are plotted, stack the curves to observe the accompanying figure.

The resulted graph looks quite similar to the one in the datasheet, where the input values were taken from. Now the LED Model is ready to be placed in the real schematic.

transient analysis plotted graphs

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9.5 References

Datasheets

Evaluation Boards

Application Notes

​Learn More

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