Power Supply

Replace AMC7135 – Higher Efficiency and PWM

The hunt for parts to replace AMC7135 has been on for a long time now. Because of the low cost and simplicity, it is extremely difficult to find a drop in replacement from a trusted manufacturer.
However, here is one solution that I have used that comes very close to the AMC7135 and has a lot of advantages over the AMC7135.

AMC7135 1W LED Driver Schematic

The AMC7135 current sink can drive a 1W LED with very little additional circuitry. All you need is decoupling and the LED itself. You might need an external dropper resistor in some cases to distribute the heat dissipated by the circuit if the source voltage is higher than the maximum voltage allowed for AMC7135.

Here is the “typical” AMC7135 schematic used with a 1W or 350mA single LED. This is the circuit you will find in most devices like low cost flashlights and other such equipment.

AMC7135 1w led driver schematic circuit diagram
Schematic for 1W LED driver using AMC7135

The Problem with AMC7135 Circuits

There are a bunch of problems that come with using the low cost and simple AMC7135 constant current sink. You cannot get around those, and they can sometimes really hurt your application.

Heat Dissipation

Heat Dissipation can be a major problem with this chip. For example, if you are powering an LED with 350mA current, and the LED draws that current at 3.2V across the LED, this means that the AMC7135 is burning almost (0.35A * 1.8V) = 0.63W when powered from a 5V source!

If you burn 0.6W to light up an LED that is 1W, you are getting down to only 60% efficiency without even taking into account tons of other factors that cause energy to be wasted. There is no way to work around this as the AMC7135 is a linear current sink.

Low Efficiency and Battery Life

As mentioned above, the AMC7135 wastes a lot of energy by dropping it across the chip and therefore the efficiency is only about 60% in most cases.

This wasted energy causes the battery to last only half as long as it should have.
There is no way to work around this issue.

No PWM or Brightness Control

There is no proper way to use PWM with this AMC7135. Granted you might not always need dimming in 1W applications, but even if you had dimming with this regulator, the efficiency will still be very low and the light would flicker after a certain PWM frequency.

Replace AMC7135?

You can do that, definitely not at the price point that the AMC7135 offers because of its simplicity, lack of peripehral components and extremely low price of the chip itself. However, a switching LED driver can be a very simple and efficient solution that solves all the problems of using AMC7135.
Here is a circuit based on the Diodes Incorporated ZXSC400.
lipo 1W LED driver schematic to replace AMC7135
ZXSC400 based high efficiency 1W LED driver schematic to replace AMC7135
Granted that the circuit is much more complex that the AMC7135 schematic is. However, you get PWM control with this circuit by using a simple resistor as the feedback path for current control is exposed. The ZXSC400 datasheet includes a lot of information on how to add PWM.
The efficiency of operation in the configuration is also above 80%, which is pretty good given the cost and simplicity of this circuit.
For that reason, this circuit is also one of my favorites for designing LCD panel backlight drivers.
Change Log
  • Initial Release: 15 May 2021
References

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1 comment

Polaris July 10, 2024 - 10:25 am

While the base idear is certainly a good one, the choosen chip serves a different function, since the AMC drives LEDs at a lower than source voltage and the zxsc drives led at higher than source voltage.
With a 5V supply, this mandates that the amc only drives 1 led, while the zxsc drives a minimum of 2 leds. (The amc can controll an led string of up to 24v before the internal protection triggers, the vin still has to be under 6v ).
A closer replacement would be the PAM2804 as a stepdown with the same voltage range as the amc or a PAM2804 if a step up is needed to drive a single led.

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