Living and working in Asia has brought me a lot of interesting support issues. For example, someone recently asked me if TI has a cross-device for a low-drop controller. The controller is housed in a small outline transistor (SOT)-236 package that occupies an area of ​​3mm x 3mm on a printed circuit board (PCB). Figure 1 shows the recommended controller schematic.
Figure 1: Simple low-drop controller
On the surface, using a high-current regulator seems like a good choice. Knowing that the design engineer wants to support a maximum of 4A at 1.35VIN to 1.0VOUT, I recommend using the TPS7A85. The TPS7A85 is not a controller, but it can be fully integrated into a 3.5mm × 3.5mm, 20-pin, quad flat no-lead (QFN) packaged 4A LDO voltage regulator. Obviously, this package is slightly larger than SOT-236.
The immediate response I got was, "The TPS7A85 is too complicated." Sometimes, the more pins it means, the more complicated it is; however, in the TPS7A85, more pins are actually converted to fewer components. Looking at the TPS7A85 equivalent schematic in Figure 2, you can see that the number of external components has been reduced from 9 to 5.
Input power
Bias power supply
To the load
Figure 2: TPS7A85 Low Dropout Regulator for 4A
Why are you missing four components? The TPS7A85 features TI's adjustable output function, which allows the user to set VOUT dynamically using the voltage setting pins. As long as one of these pins is grounded, the corresponding voltage is applied with an internal 800mV reference voltage. Therefore, by grounding the 200mV pin, VOUT immediately becomes 1.0V.
With this feature, you can create the desired output voltage by simply grounding the appropriate voltage setting pin and adjusting the voltage from 800mV to 3.95V. The quantifiable benefits of this method are:
Ensure 1% precision output adjustment.
There is no need to purchase a precision feedback resistor to set VOUT (of course, if you wish, you can still use the FB pin and an external resistor).
VOUT can be set dynamically in the application.
With a low dropout controller, the total efficiency is only 1/1.35V, or 74%. The worst total power consumption in the power FET is 4A × 4350mV, or 1.4W. The results show that this is the same efficiency that you get with a low dropout voltage regulator.
To manage heat, the controller uses two external FETs to help dissipate heat, as shown in Figure 3.
Figure 3: Low Dropout Controller Drives Dual Channel FETs
If you wish to use a controller, it is recommended that you add an RGATE to help ensure that the gate drive layout is as symmetrical as possible so that Q1 and Q2 can share the current properly. In this application, the FET is located in a 5mm x 6mm package, which occupies more than six times more space than the controller itself.
The TPS7A85 package size is only 3.5mm × 3.5mm, so its thermal performance may not be as good as a low dropout controller. Let's compare it. With the controller, the junction-to-ambient thermal resistance (TJA) of the FET is 25°C/W. Therefore, the power consumption is 1.4 W at the peak current, and the temperature rise should be about 1.4 W × 25°C/W, or 35°C. The temperature rises to 17.5°C/FET - assuming the same temperature rise for both FETs. This seems great. What is the effect compared to the TPS7A85?
The TJA of the TPS7A85 is 35.4°C/W, so the peak power rises to 1.4W × 35.4°C/W, or 49.6°C. On the surface, it seems that it is not as good as a controller, but is it true? Let's take a look at the practical advantages of an integrated low-dropout regulator compared to a low-dropout controller:
Thermal Shutdown - The low dropout controller does not have the ability to sense FET temperature. The TPS7A85 there.
Current Limit - The only duty of the low dropout controller is to regulate VOUT. When the load current is too high, it does not limit the current or shutdown function. The TPS7A85 has.
Stability - If you want to ensure that the low dropout controller works stably with FETs, parasitic capacitors, and inductors in your application, additional components must be added to measure loop stability. With the TPS7A85, there is no need to bother.
Dimensions - Low dropout controllers with external FETs therefore take up more space. The TPS7A85 is different.
Accuracy - In this application, the overall accuracy of the low dropout controller is 2.5%. The worst case external resistor is 4.5%. The total accuracy of TPS7A85 is higher than 1%.
Noise - The low dropout controller does not mention this on the data sheet. The noise of the TPS7A85 at 1VOUT is ~5μVRMS10-100KHz.
The last two advantages in this list are especially noteworthy, as most applications abandon DC/DC converters and choose low dropout controllers or low dropout regulators to power the precision VCORE rail or precision ADC/DAC of an FPGA or DSP. .
In summary, the low dropout voltage regulator can be said to be the simplest solution to this problem. In your next design, consider TI's TPS7A85 high-current, high-precision, low-noise LDO regulator.
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