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.
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