Test a negative 5V power supply using the LM2662 Switched Capacitor Voltage Converter
The LM2662 is a single-chip CMOS charge pump voltage converter:
- 1.5V to 5.5V positive voltage input
- up to 200 mA of output current
- operating efficiency greater than 90% at most loads
There are three basic application topologies:
- voltage inverter (Vout = - Vin)
- voltage divider (Vout = 1/2 Vin)
- voltage doubler (Vout = 2 Vin)
This circuit is a basic test of the voltage inverter configuration, with a few simplifications:
- 5V regulated input voltage
- frequency control is left open, so operates at 20kHz (as opposed to 150kHz)
- I'm using 47µF electrolytic capacitors since they are what I have on hand. The datasheet recommends low-ESR ceramic or tantalum for better performance.
I have some LM2662MX chips in SOP-8 packaging, so I mounted one on an adapter module for easy breadboarding.
The circuit includes a simple LED/resistor fixed load.
| Item | Value | Notes |
|---|---|---|
| Vin | 4.97V | from LM7805 regulator powered by 9V battery |
| Vout | -4.96V | with no load |
| Vout | -4.83V | with LED/resistor load |
Here's Vin (CH1) and Vout (CH2) DC-coupled on a scope, with load attached:
Zooming in on Vout (CH1) AC-coupled to see the ripple:
I'm seeing 200-400mV dropouts every few seconds as captured above. I'm not sure of the source or if that is particularly normal. The electrolytic capacitors may play a role, so I might measure that again if/when I can get my hands on some sufficiently large low-ESR ceramic capacitors.
As it is, it seems the inverted voltage output would need further filtering/smoothing and perhaps regulation for applications requiring a good clean negative voltage supply.
NB: Fritzing diagrams use the LM2662 Switched Capacitor Voltage Converter - DIP8 module custom part
- LM2662M datasheet
- LM7805 datasheet
- SO16 SSOP16 TSSOP16 SOIC16 To DIP Adapter PCB Board Converter - modules I used from an aliexpres seller to mount the SOP-8 chip
- LM2662 Switched Capacitor Voltage Converter - DIP8 module custom part