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Background

I am using a TI LM2776 Switched Capacitor Inverter (datasheet here) to obtain a -3.3v bias voltage for an LCD (Newhaven Display, NHD-12864MZ-FSW-GBW-L, datasheet here).

The LM2776 is connected exactly as shown in this digram from its datasheet:

LM2776 Typical Application

Vin is 3.3v, EN is connected to Vin, and I'm using the same capacitor values.

Question

When I turn on the circuit with no load, the output of the LM2776 is about -0.9v. When I connect it to the LCD bias pin to load it, the output is about -0.7v. Why am I not seeing about -3.3v?

Other Notes

LCD Load

When I connect the LCD bias voltage pin to a DC power supply (at -3.3v) instead of the LM2776, the LCD draws about 20mA, so the LCD should be an appropriate load for the LM2776. According to the datasheet, I shouldn't be seeing any significant voltage drop at this current draw:

LM2776 Vout vs Iout

Capacitors

All capacitors are ceramic X7R radial capacitors.

Layout

The circuit is currently on a breadboard, and the LM2776 is connected to it through a SOT23-6 -> DIP adapter. I know these devices can be sensitive to layout considerations, but the same configuration worked a couple of weeks ago, and I have no reason to suspect there was damage of any kind done to it since then.

LM2776 Breadboard

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  • \$\begingroup\$ Pictures of your circuit layout and details about the capacitors would be appropriate, as switching converters are sensitive to these things. \$\endgroup\$
    – uint128_t
    Mar 13, 2016 at 7:09
  • \$\begingroup\$ @uint128_t Added. \$\endgroup\$
    – skrrgwasme
    Mar 13, 2016 at 7:29
  • \$\begingroup\$ @uint128_t I know they're sensitive, but since I've gotten it to work before, I thought I was set. Can you think of any reason they may have worked before when they probably shouldn't have? \$\endgroup\$
    – skrrgwasme
    Mar 13, 2016 at 7:48
  • \$\begingroup\$ What kind of power supply are you using for the 3.3V? Can you confirm it fulfills the current supply specifications for LM2776 (EN=1) and for the LCD? \$\endgroup\$
    – Alf
    Mar 13, 2016 at 11:38
  • \$\begingroup\$ Can you get a scope plot at the C1+ pin (or anywhere on that net)? I would be suspicious that the device is not switching. \$\endgroup\$ Mar 13, 2016 at 13:38

2 Answers 2

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The breadboard prototype of this circuit did work a couple of times, (maybe 3 out of 50 tries), but those may have just been flukes that mislead me into thinking that something must have changed to make it not work anymore.

When I assembled the circuit on a PCB with all surface-mount capacitors that adhere to the layout recommendations given in the LM2776 datasheet, it works 100% of the time.

I suspect uin28_t's comment was on the right track - the through-hole capacitor leads and DIP adaptor for the IC introduced enough impedance for the LM2776 to be unable to maintain the correct voltage, and I just got lucky the rare times it did work.

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  • \$\begingroup\$ "When I assembled the circuit on a PCB with all surface-mount capacitors that adhere to the layout recommendations given in the LM2776 datasheet, it works 100% of the time." This, exactly. Just worked on a similar problem with the LM2776; having all three capacitors too far away lowered the output voltage to -2.0v when +5.0v was used as a supply and no load on output. Moving them directly next to the LM2776 using 0805 SMTs worked. \$\endgroup\$
    – dgreene
    Mar 12, 2020 at 17:34
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The suggested layout at section 10.2 of the datasheet does not match the part pinout in section 5! I have been through much grief finding this out. http://www.ti.com/lit/ds/symlink/lm2776.pdf

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  • \$\begingroup\$ Do you mean sections 5 and 8.2? I remember being confused by that, but 10.2 and 5 match. \$\endgroup\$
    – skrrgwasme
    Aug 23, 2018 at 22:48

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