FAQ
TL;DR: Need -3V from +10V? A simple charge‑pump like ICL7660 delivers up to 40 mA; "IOUT is very low." Use buck‑boost or isolated DC‑DC for higher current. [Elektroda, Mark Harrington, post #21665317]
Why it matters: This FAQ helps PCB designers quickly pick a safe, space‑efficient way to create a -3V rail without adding a full negative supply.
Quick Facts
- ICL7660/charge‑pump inverters suit light loads; expect approx. 40 mA max before regulation sags noticeably. [Elektroda, Mark Harrington, post #21665317]
- MC34063A supports inverting topologies; example schematics show -V generation from +10V. [Elektroda, Eugene Lisovy, post #21665316]
- Buck‑boost (positive‑to‑negative) modules are inexpensive and about ~1 sq in, making them a compact fix. [Elektroda, Earl Albin, post #21665307]
- Isolated DC‑DC + diode/LDO can be referenced to ground to realize a regulated -3V rail. [Elektroda, Earl Albin, post #21665310]
- Define load current and ripple early; missing specs block reliable solutions and can cause rework. [Elektroda, Earl Albin, post #21665312]
What’s the simplest way to get -3V from a +10V rail on my PCB?
Use a charge‑pump inverter such as the ICL7660. It needs only capacitors and can generate about −3V at light loads. Its practical output current is around 40 mA, beyond which voltage droop increases. Keep traces short and add low‑ESR caps near the IC. [Elektroda, Mark Harrington, post #21665317]
How do I build a quick -3V inverter with ICL7660 (3 steps)?
- Drop or regulate a +3V node if needed, then feed the ICL7660 VIN.
- Wire the ICL7660 in voltage‑inverter mode with the recommended flying and reservoir capacitors.
- Take −VOUT as your −3V rail and tie circuit ground to the original board ground.
“Refer the datasheet and feed it to ICL7660.” [Elektroda, gurpreet Bhui, post #21665308]
I need more than 40 mA. What should I use instead of ICL7660?
Pick a buck‑boost (positive‑to‑negative) DC‑DC module or controller. These regulate better at higher current and waste less power. As one expert put it: “A very simple slam dunk solution is to buy a buck‑boost.” Size around one square inch is typical for small modules. [Elektroda, Earl Albin, post #21665307]
Can I just shift ground with a linear regulator to get -3V?
Beware: when you stack regulators to move ground, the reference regulator must sink current. Many LDOs source only, so voltage rises and heat increases. Without proper sink/shunt capability, the scheme fails under load. This approach needs detailed current‑path analysis and thermal design. [Elektroda, Earl Albin, post #21665314]
Will two 3V LDOs in series give me -3V with respect to ground?
No, not reliably with standard LDOs. LDOs behave as current sources; driving current back into them forces their node upward unless they can sink. That creates instability and heat. Use a genuine inverting converter or an isolated supply referenced as negative. [Elektroda, Earl Albin, post #21665314]
Can I derive -3V from AC using rectifiers and filters?
Yes, but it’s bulky and unregulated. A simple AC/bridge/RC design needs heavy 120 Hz filtering and large capacitors. Regulation will vary with load. For compact, stable rails, a buck‑boost or inverting converter is the better fit on modern PCBs. [Elektroda, Earl Albin, post #21665307]
How do I use an isolated DC‑DC to create a clean -3V?
Use a small isolated 10–12V to 5V module. Reference its + output to your board ground through diode drops or an LDO to land near −3V at the board. Isolation lets you define the negative rail cleanly and regulate it if needed. [Elektroda, Earl Albin, post #21665310]
What frequency and noise should I expect from an ICL7660 inverter?
The internal oscillator runs at about 10 kHz. You can lower it to ~1 kHz by adding a 100 pF capacitor from pin 7 to supply. Lower frequency eases switching loss but increases ripple amplitude, so size capacitors accordingly. [Elektroda, Mark Harrington, post #21665317]
What is a charge‑pump inverter?
It’s a switched‑capacitor circuit that flips a positive input to a negative output using flying capacitors. The ICL7660 and MAX232 families use this method. It’s efficient for low current rails and requires few external parts. [Elektroda, Mark Harrington, post #21665317]
What is a buck‑boost (positive‑to‑negative) converter?
It’s a switching regulator topology that inverts voltage and regulates the negative output. It handles higher current loads than simple charge pumps and maintains regulation across varying loads. Many off‑the‑shelf modules implement this compactly. [Elektroda, Earl Albin, post #21665307]
Which design details must I specify before choosing a method?
State load current, allowable ripple, noise sensitivity, isolation needs, and available PCB area. Without these, proposals miss the mark and can fail under real‑world conditions. Document exact operating modes and duty cycle for worst‑case sizing. [Elektroda, Earl Albin, post #21665312]
Can MC34063A generate -3V from +10V?
Yes. Configure MC34063A in an inverting topology per shared schematics to produce a regulated negative rail. Select inductor, diode, and feedback resistors for −3.0 V and your load current. Validate ripple and stability on your PCB layout. [Elektroda, Eugene Lisovy, post #21665316]
Could I just use a 555 timer to drive a charge pump?
You can. A 555 can clock a discrete charge pump similarly to RS‑232 style inverters. Ensure the driver frequency, capacitor size, and diode selection meet your load current and ripple targets. Watch thermal limits at higher loads. [Elektroda, Mark Harrington, post #21665317]
