I received an evaluation board for the TPS54A20 DC/DC converter from TI. This switcher is specific for low voltage designs. The output range is 0.5 - 2 V. That's a very narrow range. In that range it can deliver 10 A, with a typical input of 12 V. Efficiency is in the lower-to-mid 80%. With a switch frequency of 2 MHz (more on that later) it allows for small passive components and a condensed footprint of the whole stepdown module.
The reason that I want to review this particular converter is because it's a two-phase type. TI specifies it's in essence a 4 MHz design because of this. I'll validate if that's sales lingo or truth. |
There's a new regulation in the US that stipulates that I have to disclose that I received this board from Texas Instruments. I don't like that. It's none of your business, and anyways, it's not a product review or endorsement. It's the comments and explanation of an interested person. I don't like to be told by a gouvernment what legal clauses I should put in an amateur blog. I don't put this disclosure here because I have the intention to comply. Only to avoid that TI gets sanctioned. |
Basic Specs
The full specs of the converter are available on the TPS54A20 landing page
The parameters are all focused on getting a small-footprint (both pcb real-estate and height) and reasonable efficient conversion from 12 V in to maximum 2 V out.
- Output Voltage Range: 0.5 - 2 V
- Output Current: 10 A
- Typical switching frequency: 2 MHz (per phase)
- Input Voltage Range: 8 - 14V
- Max. efficiency is just above 90% in the IC datasheet. The evaluation board has a maximum efficiency of 84.7%
Because the passives required by the converter are really small, the converter and it's surrounding components can easily be mounted on the underside of a PCB.
On the evaluation board, the tallest components are the two coils. They are MLA-FY12NR22N-M3-RU. They have a height of 1.2 mm.
Two-Phase? Series Capacitor?
This is the reason why I'm interested in this converter. It's a particular design that splits into two converters that are phase shifted.
Image from the product landing page
The name TI gives to this design is "two-phase, synchronous series capacitor buck converter".
In this design you have one inductor per phase. You can find an explanation of the concept here.
The image below is taken from that link. Check the topology section of the datasheet for a per-phase breakdown of the circuit.
I can't explain it better than what Paul Pickering tells us in that link.
In essence, the capacitor Ct (the Series Capacitor"), because of where it's positioned in the circuit, has half VIN over it in steady state.
So one component deals with stepping down the input voltage by half, with theoretically no energy loss.
What remains abstracts to two out-of-phase buck converters.
Leave your better explanation of the design in the comments.
Evaluation Board Configuration
The board is configured for 1.2 V output and 2 MHz switching frequency.
Input voltage is between 9.2 and 14 V, although 9.4 V is required initially to prime the converter.
Below is the schematic of the switching core.
I've left out the output filters and the transient load circuit (this merits a separate blog).
This is virtually the same as the typical application from the datasheet.
The frequency is set by the resistor R1 connected to SS/F SEL. See this table to find out how to translate frequency, soft start time and hiccup (related to overcurrent recovery) time against resistor value.
On the evaluation moule, R1 is not populated, so these are the parameters applicable for that setup:
| RSS/FSEL (kΩ) | FOSC (MHz) | FSW (MHz) | Soft Start Time (µs) | Hiccup Time (ms) |
|---|---|---|---|---|
| Open | 4 | 2 | 512 | 32.8 |
This is a light touch on the subject. In the next posts I'll go deeper into the topology and the design choices made on the EVM board.
And maybe the first measurements...
| Related Blog |
|---|
| Low Voltage Step-Down Converter TPS54A20 - First Check |
| Low Voltage Step-Down Converter TPS54A20 - Series Capacitor |
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