Single power LEDs are typically rated for 500mA up to 2000mA. Compared to classic signal LEDs with a maximum current of 20mA, these power LED can emit an insane amount of light and are often found in LED torches, emergency lights, and high power illuminations.
LDO6AJSA is a tiny breakout board and constant current LED driver that can drive one LED with currents of up to 1.5A (multiple LEDs only when connected in parallel).
Internally, it uses the CN5711 constant current controller chip.
CN5711
The CN5711 is a current regulation IC operating from an input voltage of 2.8V to 6V. The constant current output can be set by an external resistor (in the case of the breakout board, this is a small potentiometer) up to a maximum of 1.5A.
The on-chip power MOSFET and current sense block greatly reduces the number of external components. The chip protects itself via temperature regulation (versus )temperature protection) and has a chip enable pin that can be used to turn the LED on and off, even in rapid succession, i.e. to implement light patterns as in the case of emergency vehicle lamps. Other use cases are flashlights and headlights.
Required Input Voltage
The chip accepts an input voltage in the range of 2.8-6.0V. The input voltage is the maximum available output voltage, so it must be at least the LED forward voltage.
For example, if you are driving an LED with a forward voltage of 3.5V, then the input voltage must be at least 3.5V.
Because of this, you can only drive one LED with this chip: when you connect more than one LED in series, their forward voltages add up and exceed the maximum input voltage of 6V.
You could connect more than one LED to the chip if you connect the LEDs in parallel in which case their current requirements would add up. For example, if you connected ten 20mA signal LEDs in parallel, they would require a combined current of 200mA and could all be driven by this controller.
When you connect LED in parallel, you no longer control current per LED. While the overall current is controlled, there may now be imbalances in between the LED, and one could draw more current, while another draws less. That’s why it is recommeded to use current limiting resistors in series with each LED when you connect in parallel.
Specs
| Spec | Value |
|---|---|
| Input voltage | 2.8-6.0V |
| Dropout Voltage | 0.37V @ 1.5A |
| Output Accuracy | 5% |
| Chip Temperature Regulation | >135C junction temp: current is reduced |
| LED Over-Current Protection | 1.5-2.3A (1.9A typ.) |
| Chip Enable | diable via low, power consumption reduced to 1uA, max frequency 2kHz |
Pin Out
| Pin | Description |
|---|---|
| VCC | input voltage (2.8-6V), connect both VCC pins |
| LED | output current, connect both LED pins to LED anode |
| GND | ground |
| exposed pad on backside | ground |
| ISET | resistor sets output current |
| CE | connect to VCC to enable output. Pull to GND disables the output. Do not let this pin float (do not leave unconnected) |
| NC | not connected |
Output Current
The constant output current is set by a resistor that is connected to ISET and GND. The resistor value is calculated with the formula Iled = 1800V / Riset.
The maximum resistor value is 30K: 1800/30000 results in a minimum current of 60mA. The maximum current is 1.5A.
| Resistor | Constant Current |
|---|---|
| 30K | 60mA |
| 1.8K | 1.000mA (1A) |
| 1.2K | 1.500mA (1.5A) |
Over-current protection kicks in inbetween 1.5-2.3A.
Dimming
LEDs can be dimmed in three ways:
- PWM: apply a PWM signal to CE (up to 2kHz). Frequencies above 200Hz are not recognized by the human eye or cameras.
- Iset: Add an NMOS transistor in parallel to Rset and vary the resistor, thus effectively varying the constant current. Current controls LED brightness. In contrast to PWM, this type of dimming is completely flicker-free.
- Iset potentiometer: use a potentiometer as Rset: varying the potentiometer setting will manually dim the LED.
Heat Sink
The chip needs to be able to sink heat in order to provide the maximum constant current. Without proper heat sink, once the internal junction temperature exceeds 135C, the output current is reduced.
The thermal path is through the chip ground plate, and the PCB copper traces act as immediate heat sink.
If you want to improve the heat sink, an external heat sink would need to go on the back side of the PCB (not on top of the chip package).
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(content created Sep 08, 2024)