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Biasing An Adjustable SMPS

The PEGMA project uses two types of adjustable switched mode power supplies (SMPS) in multiple configurations. The LM2623 is a boost SMPS; five are used as input power supplies and three are used for peripherals. The TPS62240 is a buck SMPS; one provides power to the STM32F205VG microcontroller and therefore needs to be adjusted between 1.8v and 3.3v. There are 7 other adjustable domains powering various peripherals around the board.

The system controls the output of both these buck and boost converters by modulating the adjustable feedback pin on the SMPS shown here on the boost converter:

boostAdj1

 

and here on the buck converter:

buckAdj

 

From the data sheet we are given the following equations for each SMPS:

Boost: boostEq

Buck: buckEq

Where Rf1 is the top of the voltage divider (R69) and Rf2 is the bottom of the voltage divider (R70) in the boost SMPS pictured above. R1 is the top of the voltage divider (R118) and R2 is the bottom of the voltage divider (R119) in the buck SMPS pictured above.

Solving for Vout in the boost equation yields the same form as the buck equation for varying Vref:

modBoost1

So then how can we determine the impact of the resistor, diode and op-amp that we have added above? First, the diode is necessary because the op-amp may be turned off and therefore would result in a low impedance path to ground in that case. Other than that, the diode is negligible because the op-amp compensates for the voltage drop. So we need to look at some corner cases: High-Z, 0V(gnd) and 1.8V(Vmax).

In the High-Z case, the equations above of course continue to hold. Now lets modify them to acknowledge the additional complexity.

Lets begin by modifying the above equation in terms of currents:

modEq1

 

Now all that remains is to incorporate the I3 term provided by the adjustable feedback circuit:

boostAdj2

 

With this equation in hand, we can now begin to set minimum and maximums for the various circuits. Our intention is to provided the widest, safe dynamic range while also maximizing the contribution of the feedback circuit to provide the finest resolution possible.

Here is an ideal configuration for the boost converters:

BoostConfig

Note that Vf is capable of swinging Vout to each the acceptable minimum and maximum input values for the following buck converters. Here is one particular configuration for the microcontroller domain:

CPUBuckConfig

Note that Vout cannot exceed the threshold of the part and that Vref = Vf (High-Z condition) provides the minimum voltage to run. You can tryout the spreadsheet yourself PEGMABook.

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