What is Power Good of DC-DC converter and how to use it?
Introduction
Many DC-DC converters have PG (Power Good) pin, which says that DC-DC converter's output voltage is stabilized and it's ready to use. Most likely you didn't use it (connect to ground of left float), but let's take a look how it works and why it can be useful 🤨
Example
Let's take a look at the TI's synchronous DC-DC converter TPS63020 with PG pin:
| 1 — good (HIGH due to external pull-up 1 MOhm) | 0 — failure (connected to ground) |
So, that means when DC-DC converter still not fully started (VOUT below nominal regulation) the PG=LOW, but as soon as the voltage becomes stable, the PG=HIGH (actually floating, but due to external pull-up it will be high).
Test
Let's take a look at the oscillogram at the startup, it's pretty clear that when voltage is low PG=0, but when VOUT is OK the PG=1.
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*PG pin connected to another voltage source through converter for demonstration | |
For this converter startup time is approximately 0.215 ms = 215 us
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Datasheet
As you can see according to the tests from datasheet the startup time is approximately 0.15-0.25ms = 150-250 us, which is very close to the measurements above.
Use Cases
Status
You can connect is to the any GPIO pin and use as a status of the DC-DC (ready to use or not).
CPU/FPGA/MCU Reset
If you power IC you want to enable it only when supply voltage is stable and voltage converter can handle high output current, so the easiest way to improve stability is connect PG from DC-DC to EN (NRST) of the microcontroller of other IC.
Sequencing
The main way is to use it for sequentially start several DC-DC converter's using EN (Enable pin). So, that means you need to connect PG of first DC-DC to the EN of the second DC-DC, which give a chance to start second DC-DC only after the first one is ready to use:
| Some IC's need multiple power rails with a certain voltage start sequence. |
Conclusions
- PG can be used for sequencing DC-DC converters, which critical for some IC's with several power rails
- PG could be used for more stable powering of the device, so the second DC-DC will start only when first DC-DC is fully ready to operate
- PG could be used for more stable powering of the ICs (CPU/FPGA/MCU), so the IC will start only when DC-DC is fully ready to use