#165 LM2662 Inverter

Test a negative 5V power supply using the LM2662 Switched Capacitor Voltage Converter

Notes

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.

Some Measurements

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.

Construction

NB: Fritzing diagrams use the LM2662 Switched Capacitor Voltage Converter - DIP8 module custom part

Credits and References

• LM2662M datasheet
• LM7805 datasheet
• SO16 SSOP16 TSSOP16 SOIC16 To DIP Adapter PCB Board Converter - modules I used from an aliexpress seller to mount the SOP-8 chip
• LM2662 Switched Capacitor Voltage Converter - DIP8 module custom part