AL1791/1792/1793/1794

Document number:37957 Rev. 3 - 2

14 of 18

www.diodes.com

September 2015

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AL1791/1792/1793/1794

Support Higher Current

with higher current by connecting the cathodes of the LED emitter string to two or more LED channel outputs (LEDx) and associated PWM pins

(PWMx) with the same PWM signal from the MCU. The result current through the LED emitter string is the sum of each current through the

individual channels. For example, one can achieve a total of 1.5A by connecting the only one LED emitter string to all four LED channel outputs

AL1793 or AL1794 for a fixed CCT lighting application.

Similarly, it is also possible to further increase the current for a specific channel by connecting two or more AL179x IC chips in parallel and drive all

associated PWM pins with a common PWM signal.

Dimming Performance

For PWM-dimmed light sources, there are two important performance aspects to pay attention to, namely, PWM frequency and deep dimming

performance. While human eyes cannot discern any flickering caused by PWM-dimming light sources over 200Hz, electronic devices such as

cameras for smartphones can detect flickering (referred to as e-Flicker) caused by certain much higher PWM-dimmed light sources. Generally

speaking, the higher the PWM dimming signals applied to the light source, the less e-Flicker is detected by electronic components. The Smart

Light bulb design could leverage the fast response time enabled by AL179x by applying PWM signals over 4KHz, which has been experimented to

be free of e-Flicker.

Measured light is linearly proportional to the measured current and PWM duty cycle. The perceived light by human eyes has a non-linear

relationship with measured light. Human eyes are also found to be more sensitive to low light situations. For example, 10% measured light is

perceived to be 32% of the original full light. 5% measured light is perceived to be 22% of the original full light. 1% measured light is perceived to

be 10% of the original full light. Therefore, deeper dimming such as less-than 0.4% at 4KHz PWM frequency can save significant energy as well

as to provide adequate visible light output.

Special System Design Considerations

Three key Smart Light Bulb system design issues need to be carefully planned for system efficiency consideration. Any headroom voltage (over

total voltage drops for all LED emitters in a string) in each channel will result in additional power consumption for AL179x, which needs to be

dissipated by AL179x. System designers are advised to carefully plan for forward-voltage-matched LED strings with Constant Voltage (CV)

regulators to ensure that AL179x can properly dissipate heat for normal operations.

(1)

system efficiency.

(2)

Multiple CVs to Match Emitter Module Design: The AC to DC Power Conversion block may need to support multiple CV outputs to power

all emitter strings in a Smart Light Bulb design.

(3)

Overall Thermal Management: Though there are various protection mechanisms built into AL179x, it is good practice to check the maximum

dimming patterns to avoid Thermal Shutdown (when Al179x junction temperature exceeds +160°C).

Thermal resistivity θja (Junction-to-

Ambient) of AL179x is +55°C/Watt. AL179x has to absorb any additional voltage headroom provided by the voltage input to the emitter strings.

Therefore, thermal management of AL179x must follow the following guidelines:



PDmax (Maximum Power Dissipation of AL179x) = (160ºC Junction Shutdown Temperature - Ambient Temperature, e.g. 85°C) /

55°C/W = 1.364Watts



PAmax (Power to be Absorbed by AL179x)

Cycle) for all channels



Thermal Management Guideline

PAmax < Pdmax