The goal of this chapter is to understand how to use buck LED drivers using open-loop, peak-current mode control. In order to showcase the functionality of the parts, the MPLAB® Mindi™ simulation tool will be used to explore the HV9910B/C models.
11.1 Prerequisites
- Chapter 1 of this workbook, Getting Started with MPLAB® Mindi™.
- Review HV9910B and HB9910C datasheets.
11.2 Case Study: HV9910C Led Driver (120 VAC/DC and 230V AC/DC)
The goal of this section is to understand and analyze using MPLAB® Mindi™ analog simulator how to set the input voltage Vin for a specific AC Offline input.
a
Vin=120 VAC (Sine, F=60 Hz, Initial= -120*1.414, Pulse= 120*1.414).
b
Vin=230 VAC (Sine, F=50 Hz, Initial= -230*1.414, Pulse= 230*1.414).
11.2.1 Open Loop Peak Current Controller
The goal of this section is to understand and analyze what an open loop peak current controller does. Throughout these exercises, the benefits of this control method will be presented.
A peak-current-controlled buck converter can give reasonable LED current variation over a wide range of input and LED voltages. It needs little effort in feedback control design. An open loop, peak current mode average current can be calculated by:
11.2.2 Start-up Simulation Examples
a
Open the HV9910C application schematic from Power Management > High-Voltage LED Drivers > HV9910C.
b
Add two differential voltage probes for VIN and VLED.
c
Add one inline current probe with the LED.
d
Modify the names for ILED and VLED to use the same graph name.
e
Run the simulation and observe the results.
f
Modify the number of series and parallel LEDs in the LED string (DLED1), as seen in the figure below.
g
Run the simulation and observed the changes to the waveforms.
11.3 Case Study: Constant Frequency or Constant Off-Time Modes
This section illustrates the differences between constant frequency and constant off-time operation.
a
Open the HV9910C application schematic from Power Management > High-Voltage LED Drivers > HV9910C.
b
Connect the resistor (R1) to GND to enable constant frequency mode.
c
Change the value of R1 to 1 Meg.
d
Run the simulation.
e
Select 'VGATE' and stack the select curve. Do the same for the VRT curve.
f
Use the cursors to measure the desired parameter.
In-constant frequency is easier to design the EMI filter for the application.
g
To enable constant off-time mode, connect the resistor R1 to the GATE.
h
Run the simulation and analyze the waveforms as before.
In constant TOFF mode, TS variation depends on duty cycle:
- Large Duty cycle => Large variation in TS with VIN
- Small Duty cycle => Small variation in TS with VIN
11.4 Case Study: Linear and PWM Dimming
The linear dimming pin (LD) is used to control the LED current. It is useful when we cannot find the exact R1 value required for obtaining the LED current and when adjusting the current level is desired. In these cases, an external voltage divider from the VDD pin can be connected to the LD pin to obtain a voltage (less than 250 mV) corresponding to the desired voltage across R1.
PWM Dimming can be achieved by driving the PWMD pin with a low-frequency square wave signal. When the PWM signal is zero, the GATE driver is turned off; when the PWMD signal is high, the GATE driver is enabled.
11.4.1 Linear Dimming Start-Up Example
a
Open the HV9910C application schematic from Power Management > High-Voltage LED Drivers > HV9910C.
b
HV9910B has two current sense threshold voltages, an internally set 250 mV and an external voltage at the LD pin. The actual threshold voltage will be the lower of these two.
c
The default configuration is linear dimming, as the PWMD pin is tied high at 5 V.
d
Change the value of wiper position to 90%.
e
Run the simulation and display the VLED and ILED curves.
f
Change the value of wiper position to 10%.
g
Run the simulation again and display the new VLED and ILED curves to see the difference between the two duty cycles.
When using the LD pin, it is not possible to obtain zero LED current, even if the LD pin is pulled to GND. This is because of the minimum on time for the FET (450 ns). To get zero LED current, the PWMD pin needs to be used.
11.4.2 PWM Dimming Start-Up Example
h
HV9910C includes a TTL-compatible, PWM-dimming input that can accept an external control signal with a duty ratio of 0 – 100% and a frequency of up to a few kilohertz.
i
Remove the connection of LD with the wiper potentiometer and connect it at VDD.
j
Replace the power supply with a Waveform Generator V1 (Place > Voltage Sources > Waveform Generator) and configure it a Pulse generator (F=5 kHz, Duty=10%, Pulse voltage=5 V, Rise and fall times=200 ns).
k
Run the simulation and display VPWMD, VLED, and ILED.
l
Change the value of the duty cycle to 90%.
m
Run the schematic and display the new VPWMD, VLED, and ILED curves to see the difference in comparison with 10% Duty Cycle.
These plots show you that the PWM-dimming response is limited only by the rate of rise of the inductor current, enabling a very fast rise and fall times of the LED current.
This happens because the PWMD signal does not turn off the other parts of the IC, therefore, the response of HV9910C to the PWMD signal is almost instantaneous.
11.5 References
a
Datasheets