In my last entry, I boiled down the design of automotive electronic modules or ECUs to the ability to meet three essential requirements:
- Can handle high voltage situations like load dumps or double-battery jumps.
- Can ride-through low voltage cold crank conditions
- Draw minimal current in standby conditions, a.k.a. low quiescent current.
In this entry, I’ll explain how to achieve all three by choosing the right regulator for the ECU, module or board.
Let’s start with the type of regulator. Switch mode regulators offer the highest efficiency and greatest output power availability (over linear regulators), especially when inputs are limited to 12V at 100µA. A synchronous step-down configuration produces the highest efficiency.
For example, lets look the LT8610 synchronous step-down regulator. It satisfies the input requirement by taking up to 42V at its input. It satisfies the quiescent current requirement: 2.5µA in shutdown. The question is will it satisfy efficiency and dropout (cold crank) requirements.
For the dropout test, I’ll assume a 3.3V output configuration. This is valid for most applications, as for other than USB, ECUs do not use much 5V circuitry. For a 3.3V output, cold crank pulses pose no problem to a straightforward step-down topology, so if the part can meet dropout requirements at cold crank, no other fancy circuitry will be needed. I’ll show that the LT8610 can satisfy the dropout requirements below, but first, what about efficiency?
The LT8610 data sheet shows a 12V input to 3.3V output efficiency curve (Figure 1). This curve covers a very wide load current range, over six decades of load, or about 128dB of current dynamic.
To better illustrate automobile-world efficiency, I took some measurements using the stock LT8610 demo board DC1749A at 700kHz, and plotted only six decades of load current to show a bit more resolution in Figure 2. I changed the input voltage to a more realistic 13.5V to fit the auto world. Total power loss is shown in blue.
At 13.5V/100µA input you can still get over 320µA usable output current at 3.3V output. While the graphs in Figure 1 and 2 show efficiency against output current, the graph in Figure 3 is rearranged and shows the input current at 13.5V. At 100µA 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 dropout voltage in the LT8610 data sheet.
The graph in Figure 4 says nothing about at which voltage levels it is valid. One limitation is the minimum input voltage at which the regulator core still operates.
For this we find for the minimum input voltage in the data sheet:
The red dot in the table means that this parameter is specified over the full operating temperature range.
Figure 6 shows the typical temperature dependence of the UVLO (undervoltage lockout) of VIN. We see that the worst case specified in Figure 6 is at cold temperatures. A test on the stock demo board at room temperature showed:
All the values in Table 1 are consistently below the data sheet values shown in the graph in Figure 4.
The next limitation can come from the minimum and maximum duty cycle the regulator can manage. In the data sheet instead of duty cycle limits, we find percent values of the minimum on and off times. This makes sense for a regulator with such a wide frequency range, from 200kHz to 2.2MHz.
For a step-down regulator 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 = .89.
That would usually further increase the dropout voltage, but in the case of the LT8610 family, the part will start skipping off time cycles so that the minimum off time does not further increase the dropout voltage.
The LT8610 in dropout will significantly lower its effective operating frequency from its set 700khz as it skips off time cycles. This keeps the output voltage up as long as possible.
So, it is possible to use a single step-down regulator such as the LT8610 to meet the three major requirements of most ECUs. It can accept 42V inputs, allowing it to operate directly from the battery bus. Its dropout behaviour and low UVLO make it possible to handle cold crank conditions directly. Finally, its 2.5µA operating quiescent current is certainly low enough and its high efficiency preserves battery life.