The Right Power Supply Can Simplify Automotive ECU Design

The perfect automotive power supply unit has 36V or more input voltage rating. Very low drop out that its load voltage can to survive cold crank pulses. Very low operating quiescent current and still high efficiency. At 12V 100uA input you will get maximum 100uA output with a linear regulator. In reality a little bit less because the linear regulator need some current for himself. A switch mode regulator will provide you with more current at 12V 100uA input because you still have an active conversion.

A synchronous step down configuration will have the highest efficiency. The LT8610 family is a perfect example. For comparison I will use 3.3V as output voltage. Except for USB there is not much 5V circuitry left in actual ECUs. On a 3.3V supply you can survive cold crank pulses with a step down topology.

The LT8610 data sheet shows a 12V in and 3.3V out efficiency curve over a very wide range. In Figure 1, two decades of current on one log scale block are shown. Total over 6 decades in this case about 128dB of current dynamic.

 

I took some measurements using the stock LT8610 demo board at 700kHz with only 6 decades shown to get a bit more resolution. I changed the input voltage to a more realistic 13.5V. Total power loss is shown in blue. At 13.5V in at 100uA you will get still over 320uA usable output current at 3.3V. The graph in Figure 1 and 2 shows the output current.

[image-2-Figure 2. LT8610 Efficiency 13.5V to 3.3V]

The graph in Figure 3 is rearranged and shows the input current at 13.5V. At 100uA input current the LT8610 shows over 81% efficiency. If you look for the effective input voltage at which you still can maintain 3.3V output voltage we find the drop out voltage in the LT8610 data sheet.

[image-3-Figure 3. LT8610 Efficiency]

The graph in Figure 4 says nothing at which voltage levels it is valid.

[image-4-Figure 4. LT8610 Dropout Voltage]

One limitation is the minimum input voltage at which the regulator core still is in operation. For this we find for the minimum input voltage in the data sheet:

[image-5-Figure 5. Minimum Input Voltage From LT8610 Datasheet]

It is specified with a red dot. This means that this parameter is specified over the full operating temperature range.

Figure 6 shows the typical temperature dependence of the UVLO (under voltage lock out) of Vin. We see that the worst case specified in Figure 5 is at cold temperatures. A test on the stock demo board at room temperature showed: Current Dropout voltage Iout for 3.3V out [mA] [mV] 0,1 26 1 37 10 33 100 43 200 58 500 100 1000 183 1500 277 2000 361 2500 468

[image-6-Figure 6. LT8610 UVLO]

The next limitation can come from the minimum and maximum duty cycle the regulator can manage. In the data sheet we find instead of a per cent value the minimum on and off times specified. This makes more sense for a regulator with such a wide frequency range from 200kHz to 2.2MHz.

[image-7-Figure 7. Minimum On-Time and Off-Time For LT8610]

For a step down in continuous mode Duty=Vout/Vin With Duty = 1 for 100% duty cycle. For a switching frequency at 1MHz you could assume that Duty max=(1000ns-110ns)/1000ns That would further increase the drop out voltage. However in the case of the LT8610 family the part will start skipping off time cycles so that the minimum off time will not further increase the drop out voltage.

[image-8-Figure 8.]

The effective frequency of the LT8610 in drop out will go down from its set 700khz a lot, because it starts skipping off time cycles in drop out. That will keep the output voltage up as long as possible. With 2.5uA operating quiescent current, excellent drop out behaviour, high efficiency, low UVLO and high operating frequency range the LT8610 family sets a new standard in the industry.

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