-2.5V Voltage Regulator suggestion?

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I was hoping for some advice on selecting a VReg or a family of VRegs to supply -2.5V to a PCB - and ideally with variants that could also supply +5, -5, +2.5.

I've been using the LM336-2.5 and LM336-5 wired in both positive and negative orientation with an op-amp buffer output as a VRef and occasionally as a VReg/Supply for low current applications...

…but now I need to source something capable of supplying a higher current. I recently used the LM1117-5 which also has a +2.5 variant - but no negative equivalent.

I tried searching on Farnell here in the UK - but it seems like a -2.5V LDO VReg is a very rare thing?

Input supply voltage of approx. +/-10 to +/-18V and current supply of approx. >100mA. SOIC, SOT-23 or TO252 case style or similar is all good. I'd rather avoid an adjustable Vreg but if I have no choice...

Suggestions? Thanks!

 

[asdf44]

Neither of those packages is going to be able to handle the wattage which ranges from 0.75W to 1.55W at 0.1A (The SOT-23 won't come close). More current will just mean more losses.

You need a bigger package like TO-220 and probably a heatsink if you really want to operate at -18Vin and >100mA. A quick search turned up this:

LT3015IT-2.5#PBF

Otherwise various forms of switchers provide the opportunity for more efficiency and probably smaller size, although with noise and complexity trade-offs. A quick search turned up this for an inverting charge pump:

MAX889RESA+

 

[BradtheRad]

A zener diode and transistor (PNP type) make a simple negative voltage regulator.



With a PNP type, the regulated output is referenced to 0V ground (the same ground for your positive supplies).

The supply ranges from -10 to -18V.

The load receives stable -2.5V at 100mA.

 

[asdf44]

That's a good circuit but the wattage issue is the same and it lacks the features of a dedicated regulator (over temp, over current). The PNP is still on the hook for up to 1.5W which just won't work with the packages the OP suggested.

 

[BradtheRad]

That's a good circuit but the wattage issue is the same and it lacks the features of a dedicated regulator (over temp, over current). The PNP is still on the hook for up to 1.5W which just won't work with the packages the OP suggested.

Right, in these modern times we have come to expect high efficiency.

You mention switching type converters. A buck converter is suitable.

Here is how a 555 timer IC might drive a buck converter.
From another standpoint, the coil smoothes pulses from the 555.

The 555 has a 'control' pin, which allows us to adjust duty cycle longer or shorter.
This simulation shows how to use that pin for the feedback function. It allows automatic regulation of load voltage.
A transistor responds to a rising output by pulling down voltage on the control pin. (And vice-versa.)
The output maintains 2.5V (give or take a tenth of a volt), over the spec 10 to 18V input range.

The 555 can provide 200 mA. If this circuit is operating properly, then it should be possible to obtain the spec 100 mA at the load.



It's hard to be sure what the 555 will do in this circuit, whether it will permit current flow through the inductor during the last half of the cycle. A freewheeling diode may be necessary.

The frequency must be tailored to the inductor. Odd things can happen if the frequency is slow. During one half of the cycle, current might flow in the wrong direction through the coil.