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Antony Pierre Carvajales

Dec 27, 2016

LDO

LM27761

DCDC

LM27762

TPS65133

TLE2426

LP5907

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Related

The top three ways to split a voltage rail to a bipolar
supply

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Audio applications, data-signal acquisition and analog sensors benefit from a
bipolar bias power supply. A bipolar supply provides the best use of the
analog-to-digital converter’s (ADC) dynamic range, enables rail-to-rail
amplification, isolates the analog signal from ground noise and offers many
other benefits. In this post, I’ll describe three ways to split a single supply
rail into a bipolar voltage rail. Table 1 shows the advantages and limitations
of each of the three most common methods of splitting a single positive-polarity
voltage rail into a bipolar rail.

Table 1: Split-rail method comparison
table

The first (and simplest) method is to create a virtual ground by implementing
a resistor divider; unfortunately, this configuration is susceptible to becoming
unbalanced at very low loads. The TLE2426, shown in
Figure 1, creates a common ground buffered from the source; the buffer creates a
more stable center point between the rails under load condition. The
disadvantage is that it can only handle a few milliamps.

Figure 1: Simplified schematic of the TLE2426 virtual ground driver in a split-rail
configuration

Switching topologies offer higher efficiency, accuracy and stability, as well
as more features than a discrete approach. There are two categories of switching
regulators: inductive and capacitive (that is, a charge pump or switched
capacitor). Inductive regulators are the most efficient devices, but are more
complex and require an inductor, increasing bill of materials (BOM) cost and
total solution size. Figure 2 shows a typical schematic for the TPS65133, a boost converter with a bipolar output capable of
supplying a ±250mA output current with 90% efficiency for most input and output
voltage configurations.

Figure
2: TPS65133 boost converter with dual output
voltage

Another popular inductive split-rail solution is a fly-buck topology, which
is a step-down synchronous buck converter with a coupled inductor in the
feedback loop; this topology offers an isolated voltage supply. The fly-buck
method is a more complex technique: for step-by-step details, see the
application report, “Creating a Split-Rail Power Supply with a Wide Input Voltage
Buck Regulator.”

Charge pumps are a popular and easy way of inverting positive rails. You can
create a split rail by combining a charge pump with LDOs or DC/DC boost
converters, as shown in Figure 3.The LM27761 inverting charge pump in Figure 3 has an integrated
adjustable negative LDO capable of supplying 250mA; you can adjust the output
voltage using external resistors.

Figure 3: Split-rail solutions combine an
inverting charge pump and a positive voltage regulator

TI also offers integrated solutions like the TPS65133, which is a boost converter with an integrated LDO
and an inverting charge pump capable of supplying ±5V at 90% efficiency. For
step-down split-rail applications where you must minimize noise, ripple, cost
and space, the LM27762 charge pump
has integrated positive and negative LDOs. Again, you can adjust the output
voltage with external resistors. The LM27762 requires
only a few compensating elements, as shown in Figure 4.

Figure 4: The LM27762 split-rail
charge pump with integrated LDOs

In this post, I covered the top three ways to split a voltage rail into a
bipolar supply. The discrete solution is a simple solution but is limited on
output current, voltage regulation and stability. Inductive switchers have great
efficiency, high output current and are capable of isolating the output supply,
but the total BOM cost and total solution size are high. Combining the charge
pump and LDOs offers low noise, high power supply rejection ratio (PSRR) supply,
low cost and small size, but cannot provide currents as high as the inductive
solution. Regardless of your application requirements, TI offers a variety of
bipolar supply reference designs to get you started on your next
design.

Additional resources

Get to know “The forgotten converter.”

Read more about virtual ground circuits.

Sign
in to Comment

Vincent Rheaume

The article states that the TL2426 "can only handle a few tenths of
milliamps"; is that a typo? The product page says 20mA source/sink (unless I
miss something).

I like generic circuits that can be implemented with generic parts; the
"floating ground" topology can also be implemented with a resistor divider +
an op-amp set up as a voltage buffer (+ power transistor(s) if more current is
needed, although power dissipation rises quickly). The "capacitive" topology
can also be implemented with an opamp set up as a square-wave oscillator +
some diodes and capacitors for the charge pump action (+ power transistors if
more current is needed).

As for the inductive topology: if you already have a switching regulator in
the design for the positive rail, it is possible to use a "coupled inductor"
instead of a standard inductor to generate a crudely-regulated negative rail.
See also EDN article "Buck regulator and two inductors make dual-polarity
converter".

Dec 30, 2016

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