Versatile Dimming Controls Using an Advanced Power LED Driver

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Power LEDs such as LUXEON® devices from Philips Lumileds are penetrating applications previously dominated by incandescent lamps, including low voltage domestic and architectural lighting, signage and automotive lighting. Advantages include greater longevity, higher reliability and more versatile operating modes, but active control of the LED current is required to ensure consistent light output and to avoid exceeding the maximum rated current.

LED driver modules for these applications usually provide on-chip current control circuitry. However, the Zetex ZXLD1350 also supports a number of dimming techniques including Pulse Width Modulation (PWM) dimming, which produces consistent color temperature as the LED brightness is varied. Simpler dimming modes, using inexpensive external components, are also supported.

Figure 1 shows the block diagram for the ZXLD1350. Dimming control is achieved via the external ADJ pin, but the device can be programmed to maintain a constant LED current by selecting the value of the external sensing resistor (R_S) and leaving ADJ unconnected. The current sense circuitry detects the LED current flowing through R_S and produces a proportional voltage at the input of the comparator. When this reaches the threshold defined by the internal 1.25V reference, the comparator switches low and the internal transistor MN turns off.

 

 

When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to V_IN. The current decays at a rate determined by the LED and diode forward voltages to produce a falling voltage at the input of the comparator. When this voltage again becomes equal to the internal reference voltage, the comparator output switches MN on again. In this mode the ZXLD1350 maintains a constant LED current, with a small amount of hysteresis provided by R3.

DC or pulse signals applied to the ZXLD1350’s external ADJ pin will change the switching threshold and adjust the output current dynamically, to produce dimming.

 

 

 

PWM Dimming
A PWM signal with duty-cycle (D_PWM) will adjust the output current to a value above or below the nominal average value set by the resistor (R_S). This is best applied via a switching transistor as shown in Figure 2, to control the amplitude of the PWM waveform. The ZXLD1350’s internal 200kΩ resistor acts as a pull-up. If the PWM frequency is sufficiently low, the instantaneous LED driving current remains constant, thereby ensuring a consistent color temperature as the LED brightness is varied. Low frequency dimming also allows the brightness to be adjusted down to just 1%, enabling a dimming range of 100:1.

In this case, the average value of the output current is given by:

IOUT ≈ 0.1 x DPWM x RS [for 0 < DPWM < 1]

A suitable maximum PWM frequency is around 1kHz. At the top of this range, audible noise may be generated in the inductor, particularly if there are loose windings. On the other hand, the frequency should be greater than 100Hz to avoid visible flicker in the LED.

If the PWM frequency is set higher still, say between 1kHz and 10kHz, the ZXLD1350’s internal low pass filter will integrate the PWM signal to produce a DC dimming control. This may be desirable if the system requirements demand low radiated emissions or low input and output ripple.

 

 

Dimming with External DC Voltage Control
Connecting a variable resistor, (R_ADJ) between ADJ and ground provides a straightforward, low cost dimming control. However, continuous dimming is not possible, since setting R_ADJ to very low levels will breach the shutdown threshold, thereby reducing the output current to zero.

The current output can be determined by:

IOUT = (0.08/RS) x RADJ /(RADJ + 200kΩ)

For more accurate current control, a 1% regulator, such as a Zetex ZTLV431 precision reference, may be used as an external shunt regulator. A variable resistor connected between the external regulator and the ADJ pin, as shown in Figure 3, controls the LED current.

 

 

 

The ZXLD1350EV2 is an evaluation board for the ZLD1350 350mA LED driver with internal switch. The board can be used to drive 1, 2 or 3 one-watt LEDs, or an external choice of LEDs. The number of external LEDs which can be connected depends on their combined forward voltage drop.

The operating voltage is nominally 24V. For three 1W series connected LEDs, the voltage can be from 12V minimum to 30V maximum. The 100uH inductor used in the circuit is based on a nominal 24V supply, which should be connected across the +V_IN and GND pins.

Note: The board does not have reverse battery protection. The nominal current through the LEDs is set at 300mA with a 0.33Ω sense resistor, Rs.

 

Mathematical Simulations

ZXLD1350 Performance Evaluation Calculator

The above calculator is in MS Excel. To avoid problems whilst saving, we recommend selecting the "Save" option at the prompt to save the file to your local PC then opening it, rather than opening the file directly in your browser.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
ZXLD1350ET5TA		7V-30V Input, 350mA LED Driver with Internal Switch

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