The goal of this chapter is to understand how to use and analyze Sequential Linear LED drivers. MPLAB® Mindi™ will be used to investigate the impact of the input voltage, number of LEDs in each TAP and their configuration (series or parallel), Total output LED current (shape and stability), and Total Lumen Power.
10.1 Prerequisites
- Chapter 1 of this workbook: Getting Started with MPLAB® Mindi™ Analog Simulator.
- Review CL8800 Datasheet.
10.2 Case Study: CL8800 LED Driver Input Voltage and tap configuration
The goal of this section is to understand start up from input voltage (VIN) by using MPLAB Mindi analog simulator.
a
Open the 'Sequential Linear LED Driver' application schematic from Power Management > High-Voltage LED Drivers > CL8800.
b
Add a Differential Voltage Probe, VIN_LINE, on the input voltage.
c
Set VIN=120 VAC (Sine, F=60 Hz, Initial= -120*1.414, Pulse= 120*1.414).
d
Set VIN=135 VAC (Sine, F=60 Hz, Initial= -135*1.414, Pulse= 135*1.414).
e
Repeat the simulation with the input line voltage set to 90 VAC, the lowest allowed line voltage. It is the minimum voltage to allow all LEDs from TAP1 to TAP4 to turn on (the LEDs from TAP5 are off).
10.2.1 Start-up Simulation Examples
The goal of this section is to understand how to modify the number of the LEDs for each TAP and to observe the impact. A string of series/parallel LEDs is tapped at six locations, allowing various configurations of LEDs. Six linear current regulators sink current at each tap and are sequentially turned on and off, tracking the input sine wave voltage.
NS = the number of LEDs in series. NP = the number of LEDs in parallel.
a
Open the 'Sequential Linear LED Driver' application schematic from Power Management > High-Voltage LED Drivers > CL8800.
b
Set DSEG1 to NS=2, NP=1. Total Lumens: 1098.
c
Run the simulation and observe the resulting waveforms.
d
Set DSEG1 to NS=3, NP=1. Total Lumens: 1050.
e
Run the simulation and observe the resulting waveforms.
10.2.2 Additional Exercises
a
Modify DSEG1 LEDs to NS=1, NP=1 and DSEG2 LEDs to NS=2, NP=2.
b
What is the resulting total lumens and LED current?
10.3 Case Study: Using LEDs or Zener Diodes on the Higher Taps
The goal of this section is to show how to reduce the cost of the application while still maintaining the same performance. With each schematic modification, the LED current and IC power will be measured.
a
Open the 'Sequential Linear LED Driver' application schematic from Power Management > High-Voltage LED Drivers > CL8800.
b
Run the simulation with the initial parameters: 1 Zener on TAP 6) VIN = 117 VAC, Total Lumens = 1103 Lm and LED Current = 90 mA, Power on IC = 1.5 W.
c
Replace D6 with an LED (DSeg6).
d
Run the simulation. Total Lumens = 1113 Lm and LED Current = 90m A. We can see that we gained only 10 Lm by using one LED, which is not optimum considering the cost of solution.
e
Replace DSeg5 with a ZENER diode D6 in the original schematic. Be sure to ground the LM terminal of DSeg4.
f
Run the simulation. We can see that the total lumens reduces from 1103 to 985, which means a difference of 118 Lm. The LED Current and IC power remain the same at 90 mA, and 1.5 W. This means that we must keep an LED on TAP5 to obtain the requested lumen power.
For superior TAPS, because they are triggered only in higher input voltages and the active period is short, we can use ZENER diodes instead of LEDs (two pieces for 230 VAC Line and one piece for 120 VAC Line voltage). At rated input voltage the difference in lumens is very small (in this case 10 Lm) so it is a good choice to use ZENER diodes in certain conditions on superior TAPS.
10.4 Case Study: Modifying taps series resistors and the impact on application parameters
The goal of this section is to understand the impact on the performance of current through the LED strip.
10.4.1 Analyze application parameter table in different configuration of the series resistors.
a
Open the 'Sequential Linear LED Driver' application schematic from Power Management > High-Voltage LED Drivers > CL8800.
b
Run the simulation without modification, stack all curves, and delete the VIN curve.
c
Double the Rset resistors and only put 25 Ω in Rset5.
d
Run the simulation, delete the VIN curve and then stack all curves.
e
Select all of the curves and add an RMS measurement.
f
Unselect all curves and from the ‘Curves’ menu select curves and move to obtain the below graph.
We can see that the parameters of the application significantly reduce the LED Current from 90 to 63 mA and as a result, the total lumen power decreases from 1098 to 831 Lm. The IC is less stressed, because the dissipated power reduces from 1.49 to 1.11 W. However, this modification is not recommended because of reduced performances and low utilization of the LEDs.
10.4.2 Additional Exercises
g
Reduce the Rset resistors to half of their original value.
h
Run the simulation.
i
This new configuration increases the LED current significantly, from 90 to 168 mA, and the total lumen power increases from 1098 to 1645 Lm.
The IC is too stressed because dissipated power increase from 1.49 to 3.6 W, which is not sustainable since the IC can dissipate a maximum of 2 W.
This modification is not recommended, because it stresses the IC and the LEDs.
10.5 References
a
Datasheet