One-chip LED Street Lighting Solution with PowerPSoC™ | Cypress Semiconductor
LED based Solar Street Lights offer benefits in terms of higher energy efficiency compared to CFL/Halogen lights and higher reliability in operation. For similar luminosity outputs LED based street lights consume up to 70% lesser electricity. LED lights have been known to have life spans of over 50,000 hours up to 100,000 hours and less than 1% of these lights experience catastrophic failure within the first 6,000 hours of life.
The Block Diagram shown below is implemented using a Floating Buck Topology, which is explained in the next section.
A typical LED lighting system is a complex mix of power peripherals, LED lights, discrete components and a microcontroller. The block diagram shows a system operating under Floating Load Buck topology.
Floating Load Buck Topology
High Brightness LEDs (HBLEDs) are typically rated within 250 mA to 750 mA. They require, by nature of their composition, a constant current source in order maintain a set color and intensity. Operating within the manufacturer’s set current limits, plays an important role in ensuring longevity of LEDs.
A Floating Load Buck SMPS (Switched Mode Power Supply) is an ideal HBLED driver when the LED string voltage is lower than input supply voltage. The SMPS however operates on DC voltage rather than AC. However an SMPS can regulate the output voltage across the LED string and thus provide much better design flexibility to the designer.
The block diagram above, can be broken down into two functional sections:
- The Power Converter section consists of an n-channel Field Effect Transistor (Q1), an Inductor (L1) and a Diode (D1).
- The Control section consists of the Current Sense Amplifier (CSA) for load current feedback and the Hysteretic Controller, which generates control signals for the transistor switch (Q1).
The Power Converter section
When FET (Q1) is turned on, current forms a forward path through the sense resistor (Rsense), LEDs 1, 2, and 3 and the FET (Q1).
When FET (Q1) is turned off, the energy stored in the inductor (L1) is manifested as current through the diode (D1), sense resistor (Rsense) and the LED string.
The Control Section
This section needs to ensure that a constant current is maintained through the LED string by switching FET (Q1) on and off. A Hysteretic Controller provides a simple and cost effective way to achieve this. FET (Q1) is turned-on and off such that the current through the inductor (L1) is maintained between user defined upper and lower thresholds.
Figure 2: Inductor Current Waveform
The inductor Mean current is defined as the midpoint of the upper and lower thresholds. tON and tOFF are set such that this current remains constant. As a design guideline, the upper and lower thresholds are set to lie between 15% and 30% of the mean current value.
As seen in Figure 1, the inductor (or LED) current is measured by placing a sense resistor (RSENSE) which is read by a Current Sense Amplifier (CSA). Reference DACs (Digital to Analog Converters) can be used to provide reference signals corresponding to the upper and lower thresholds of the Hysteresis Controller. These reference signals are compared with the load current to generate digital signals when upper or lower thresholds are breached.
Figure 3: Generating Hysteretic Control Function
Additional functions of the Hysteretic Controller can include controlling the minimum on-time and off-time to protect the circuit against high frequency oscillation.
The output to the gate driver can be gated by functions implementing Trip, Enable and Dimming Modulation as long as they follow TTL logic (logic high and logic low).
Figure 4: Hysteresis Output Signal
Inductors can consume a lot of board space and must be chosen carefully:
- A small inductor will have smaller voltage drop than a large inductor, hence better system efficiency.
- A smaller inductor can be chosen if the system supports high hysteretic switching frequencies.
- Saturation current of the inductor should exceed the peak current of the system.
While choosing Rsense, the designer should consider a value,
- That minimizes power dissipation
- That ensures that the peak-to-peak voltage can be read by the Current Sense Amplifier.
For a system with i(LED) = 1A, typically 100 mΩ sense resistor is used.
PowerPSoC is Cypress only Programmable System on-Chip which integrates power peripherals typically used to make LED lighting solutions, on the same silicon.
Up to 4 Internal 1A MOSFETs (32V, RDS(ON) - 0.5 Ω)
Up to 4 low side Gate Drivers with programmable drive strength
Up to 4 Current Sense Amplifiers
Up to 4 independent Hysteretic Controllers with switching frequency up to 2 MHz
Up to 8 DACs to provide High and Low Threshold to Hysteretic Controllers
Up to 6 Comparators for Hysteresis Control
Built-in 5V Switching regulator
Three 16-bit LED dimming modulators: PrISM, DMM and PWM
M8C Core, 8-bit CPU with 4MIPS at 24 MHz
16 K Flash, 2 K SRAM
Capacitive Sensing Capability
Full Duplex UARTs
Multiple SPI masters or slaves
Up to 12-bit Incremental ADCs
Up to 9-bit DACs
Programmable Gain Amplifier, Filters and Comparators
8 to 32-bit Timer/Counter and PWMs
To know more about PowerPSoC, please click here.
Software and Drivers
- PSoC Creator
PSoC Creator is a state-of-the-art software development IDE combined with a revolutionary graphical design editor to form a uniquely powerful hardware/software co-design environment.
- PSoC Designer
PSoC Designer is the revolutionary Integrated Design Environment (IDE) that you can use to customize PSoC to meet your specific application requirements. PSoC Designer software accelerates system bring-up and time-to-market.