Using a power distribution switch to drive LEDs

LEDs are increasingly being used in a variety of applications, including automotive lighting, camera phone flashes, stage lighting, venue lighting, night scene lighting, flashlights, bicycle lights, and car running lights. This article will discuss the various technical issues of using power distribution switches in LED applications.

Currently, current-limited power distribution switches are one of the best choices for designers to drive LEDs. This is because power distribution switches are simple to operate and can better control the distribution of power to various loads in the system. Its function is simple: when the switch is on, it allows current to flow to the load, and when the switch is off, it prevents current from flowing to the load.

This switch also has a current limiting function, that is, when the switch is turned on, the maximum current of the load is equal to the current limit value set by the switch. This current limiting function is necessary to design a constant current LED driver.

Why do designers need to drive LEDs with constant current? Because when the current flowing through the LED remains constant, the light intensity of the LED is also constant, even if the voltage flowing through the LED changes. This is necessary for various applications that require stable light intensity.

Figure 1 shows an example of a Micrel MIC2007 current-limiting power distribution switch.

Figure 1: MIC2007 block diagram The MIC2007 has the highest power-to-package ratio of all current-limiting switches, while the resistance is typically only 77mΩ.

Figure 2: MIC2007 driving LED light

Using a power distribution switch as a constant current LED driver

Figure 2 shows the power distribution switch connected to the output and LED. MIC2007 can operate at an input voltage of 2.5V-5.5V. It sets the current limit value through an external resistor Rset. The current limit value is equal to CLF/Rset, where CLF is the current limit factor, which is generally 245 (please refer to the MIC2007 data sheet and electrical performance table), and Rset is equal to 0.7 kΩ. Therefore, the current limit value is 245/0.7 kΩ = 350 mA.

When the switch is on, the LED will try to draw as much current as possible, but due to the switch's limitations, the maximum current is only 350mA. Therefore, as long as the switch is on and the switch input voltage (Vin) is within the range of 2.5V-5.5V, the current flowing through the LED can be maintained at 350mA.

With the current limiting switch, the current limit of the LED can be adjusted between 200 mA and 2 A by changing the value of the Rset resistor. Such a wide current limit range can be applied to various high-power LEDs and enable them to be used in various applications.

This IC has a Cslew pin, through which a capacitor is connected between Cslew and Vin. The Cslew capacitor can be used to extend the on-time of the LED. It also has a built-in load discharge transistor (P-channel FET). When the switch is closed, this FET will discharge the output current. Figure 3: Using MIC2007 to gradually turn on and off the LED

In some applications, it is necessary to gradually turn on and off the LED so that the audience will not be temporarily blinded by sudden changes in lighting conditions. Figure 3 shows a solution that can set the on and off time arbitrarily.

Here, the turn-on input is related to Vin, and the turn-on and turn-off control is accomplished by sending a 5 V signal to the Ilimit pin (the Cslew pin is not shown in Figure 3).

To calculate the current limit for the circuit in Figure 3, add 0.5 kΩ and 0.2 kΩ (the equivalent resistance of the 3.33 kΩ and 0.212 kΩ resistors) to get 0.7 kΩ. This gives a current limit of 245/0.7 kΩ = 350 mA. In addition, the turn-on and turn-off times are set at 300 ms with a 680 μF capacitor value. This time is calculated as: 2.2 x R x C = 2.2 x 0.2 kΩ x 680 μF = 300 ms. The result is shown in Figure 4 below.

Figure 4: Channel 1 is the 0.5 V signal added to Ilimit, while Channel 4 is the current through the LED (Iled; on time = off time = 300 ms).

MIC2007 can adjust the current limit value after the switch is turned on and the LED is lit. By changing the current limit value, the light intensity of the LED can be dynamically adjusted. As shown in Figure 5, there are 4 resistors connected to the Ilimit pin.

Figure 5: 4-level LED light intensity control

If the resistance values ​​of the four resistors are the same, the LED light intensity can have four different levels. By connecting several resistors to the Ilimit pin, you can get several levels of LED light intensity.

If the resistance values ​​of the resistors are different, the light intensity of the LED can be adjusted by using different combinations of resistors. For example, if five resistors with resistance values ​​of R, R/2, R/4, R/8 and R/16 are used, the light intensity of the LED will be lower when the three resistors R, R/2 and R/4 are combined, while the light intensity of the LED will be higher when the three resistors R/4, R/8 and R/16 are combined.

Micrel's MIC2000 series current-limited power distribution switch can allow a maximum current of 2A to flow to the load, which is sufficient for most loads in the system. The current-limited power distribution switch can provide loads for high-power LEDs currently used by many designers to meet the growing requirements of various lighting applications in today's market.