How does this +/-15 V power supply from the LT3467 datasheet work?

Question

I came across a problem where I need to generate a +/-15 V power supply. I searched and found this reference design in the LT3467 datasheet:

How does it work?

Answer 1

Here's a simulation of the positive voltage booster:

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The equivalent circuit of the +15 V output.

Figure 2. V_A, V_B and V_C when V1 is a 0 - 15 V squarewave.

• C1 decouples the output from the switched DC from L1.
• D1 charges up C2.
• D2 clamps the right hand side of C1 at 0 V when V1 goes negative.
• In the LT schematic R1 and R2 provide a feedback voltage proportional to the output voltage. They're in the ratio 147:13.3 and since the output voltage is 15 V I would guess that the chip regulates so that it gets \$ \frac {13.3}{147} \times 15 = 1.33 \ \text V \$ back on its FB input. (I didn't check the datasheet.)

The -15 V rail works in the same way but without feedback. This simplifies the design and since C5 = C4 and C3 = C2 the negative voltage should mirror the positive voltage well enough for typical applications.

Answer 2

The upper part is almost a classic step-up converter, except they add AC coupling capacitor (C4). So you can look it up on the internet.

The lower part of a circuit (negative part) is also a classic circuit "capacitive inverter/ voltage doubler". I do not know the exact name for this circuit.

When the SW pin (internal MOSFET N-chanel connected between SW and GND) is open the voltage at this pin is high (15V + 0.3V or more).

The C5 will be charged to arond 15V.

And when the SW pin is LOW (0V)now the previous charged C5 capacitor will start discharge phase and charge the output capacitor (C3).

And after a couple of cycle charging/discharging the negative voltage will reach -15V.

Also I found this simulation of lower part of a circuit:

https://www.multisim.com/content/d2e88qRjgcLEh6SRT3PAa3/switched-diode-capacitor-positive-dc-to-negative-dc-voltage-inverter/open/