Automotive Linear Regulators M
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We must, therefore, optimize the power supply block (top left corner) in terms of minimizing quiescent current. Aside from low quiescent current, requirements on the internal linear regulator are not very stringent. It must simply provide an input, an output, and a ground pin. Shutdown and Enable functions are not required because the receiver is always powered. But, we must look closely at the way it sets the output voltage. Many linear regulators set the output voltage using an external resistor-divider, but that is probably not a good idea for this application. Consider the following scenario.
The MAX1470 RF receiver operates on a 3.3V supply. To ensure its low quiescent current we cannot allow a significant bias current in the resistor-divider. For a maximum current of 2μA, the divider resistance must be no less than 1.65MΩ. That value is available in a chip resistor, but high values have other drawbacks. They make the divider sensitive to distortion that could influence the linear regulator's output voltage.
Another major objection to the use of external resistor-dividers is that car manufacturers anticipate the long-term deposit of a parasitic dirt layer formed by oil, dirt, dust, evaporated plastic, and other particles on the PCB. Thus, the high impedance of the divider tends to decrease over time, as the parasitic layer forms parallel resistances. Direct consequences of this contamination are a slow but continuous change in the output voltage, and a steadily increasing quiescent current. Fixed-output linear regulators containing an internal divider are therefore preferred.
Given these considerations, the MAX15006 linear regulator is a good choice (Figure 4). Its small 6-pin TDFN package is capable of dissipating 1.5W, and it comes with a fixed output voltage of either 3.3V or 5V (other voltages are available upon request). The MAX15006's input voltage range (to 40V, max) allows a direct connection to the car battery, but the main advantage is an ultra-low no-load quiescent current of 9.5μA typical. For a 1mA load, the maximum quiescent current over the automotive temperature range is only 19μA, increasing to 110μA at the maximum load current of 50mA. This performance is acceptable, because the maximum supply current for the MAX1470 receiver in our example is well below 10mA.
Figure 4. This automotive linear regulator requires a minimum number of pins and external components, minimal board space, and a minimum (typical) no-load supply current of 9.5μA.
Power-Dissipation Problems
To save board space and cost, designers sometimes avoid the use of a switch-mode power supply, even when the application is pushing the linear regulator to the edge of its power-dissipation capability. Consider, for example, an automotive electrical control unit (ECU) that requires a 5V supply voltage. The ECU normally includes a microcontroller, a sensor, and a CAN driver or other bus-interface device with minor analog circuitry connected to it. It might draw 150mA of supply current in full operation.
A linear regulator with 13V nominal input voltage can, therefore, dissipate 1.2W, and that level can rise briefly to 4W during a load dump. To handle the nominal 26V found in a truck, the regulator must dissipate more than 3W continuously. For these applications, another automotive linear regulator (MAX5087) has been designed for output currents as large as 400mA. It also comes in a larger 8mm x 8mm, 56-pin QFN package that boosts the allowable power dissipation to 3.8W (Figure 5).
Figure 5. One package option for the MAX5087 linear regulator is a 56-pin QFN, which increases its maximum allowable power dissipation to 3.8W.
For applications that require "housekeeping" features, the best choice is a MAX6791 linear regulator (Figure 6). Its 20-pin QFN package can dissipate 2.7W, and one of its dual linear regulators can generate a second supply voltage: 3.3V for a microprocessor and 5V for a CAN transceiver, for example. Other features include a watchdog with programmable trigger window, a power-on reset, a power-fail comparator for monitoring an additional supply voltage, and driver logic for an external power MOSFET (intended to replace a bulky and power-dissipating reverse-current protection diode).
Figure 6. This automotive linear regulator contains two linear regulators plus a set of "housekeeping" functions. It can dissipate 2.7W at 70°C.
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