When you need to step down a negative voltage in a lowcurrent application, a non-inductor configuration offers two advantages: ease of use and a low number of external components. Such step-down converters can be implemented with two chargepump devices. The first produces a positive output by doubling and inverting the negative input voltage, and the second acts as an inverter to produce the desired negative output (Fig. 1). Input and output capabilities for the circuit depend on the input/output voltages allowed by the IC components chosen.
U1, a switched-capacitor voltage doubler, accepts the -5-V input and produces a +5-V output. A regulated voltage inverter (U2) then accepts the +5 V and produces a -3-V output. A voltage divider at U2’s output (R1/R2) provides feedback for regulating U2’s output voltage to the desired level (VOUT). The threshold voltage at U2’s FB input is factory-set to zero.
You can choose the values for R1 and R2 using:
R2 × (5 V/R1) = -VOUT
plus the condition that their sum should allow a minimum current flow of 50 µA. The accuracy of VOUT depends largely on the accuracy of the -5-V input. Figure 2 depicts the circuit efficiency and output regulation for inputs of -4.75 V, -5 V, and -5.25 V.
This idea isn't so bad considering the advantage it has over the proposed PNP transistor solution (tempco) and 3-terminal regulator solution (the potential for higher efficiency than linear regulator).
robert W. -July 28, 2009
Maxim should be ashamed to publish this circuit, it makes them look incompetent. The damage to their reputation in the eyes of competent engineers can not be worth the meager (if any) sales they would get from incompetent one that think this is a good idea.
I like Ron K's idea. Not only would it be lower cost, but lower noise, higher efficiency, and have better regulation to boot!
Bill W -June 03, 2009
I suspect a PNP transistor and a two resistor divider would be adequate and probably the lowest cost.
Ron K -May 18, 2009
I agree with Andy - Looking at the efficiency I would think a 3 terminal regulator would be simpler and cheaper. Or even a transistor and a zener. It looks like a way to sell Maxim parts. Maxim makes good parts, but they need to be properly used.
Pat B -May 18, 2009
The link to fig 2 was not included in the text. http://electronicdesign.com/files/29/21106/fig_02.gif
The efficiency of this circuit is no better than a linear 3-terminal regulator. What is the advantage?
Andy Alexander -May 08, 2009
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