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Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 4405
Maxim > Design support >
App notes
> Power-Supply
Circuits > APP 4405

Jun 05, 2009

Keywords: voltage regulation, op amps, pnp
transistors, linear regulators
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parts

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APPLICATION NOTE 4405

Spare Op Amp Generates Its Own Regulated Negative
Supply

By: Gert
N. Helles, Ben Wolde

Abstract: This
application note shows how to use a spare op amp to generate a
regulated negative voltage supply, used to power the op amp
itself as well as other low current circuitry. The regulated
negative supply is generated from a regulated positive voltage
and an unregulated negative voltage.

This
design idea appeared in the April 14, 2006 issue of EE
Times.

For systems that require low current
(100mA or less) and include a spare op
amp along with unregulated positive and negative voltages,
the circuit of Figure 1 generates a regulated positive
and negative supply voltage. The op amp then operates from the
supply rails that it has helped to generate (±5V in this
case). Figure 2 shows the power-up response. The
circuit achieves regulation regardless of which unregulated
voltage is applied first.

Figure
1. This circuit derives a regulated ± voltage from ±
unregulated input voltages.

Figure
2. The power-up response for the Figure 1 circuit.
CH1 =
regulated -5V output
CH2 = negative supply voltage
CH3 =
regulated +5V output
CH4 = positive supply
voltage

By controlling the base drive to Q1, the op
amp maintains equilibrium at its inverting input, and thereby
ensures that VN =
-VP(R3/R1). Resistor R5 provides the initial
base current to Q1 until OP1 takes over. When that happens,
OP1, Q1, and R1, R3, R4, and R5 form a negative feedback
network that regulates VN to -VP(R3/R1):

VOP1+ =
VOP1-
VOP1+ =
0V

Using Superposition:

VOP1-' =
VP(R3/(R3+R1)
VOP1-'' =
(VN(R1/(R1+R3)
VOP1- =
VOP1-' + VOP1-''
VOP1- =
VP(R3/(R3+R1)) +
VN(R1/(R1+R3))
VN =
-VP(R3/(R3+R1))((R1+R3)/R1)

Therefore:

VN =
-VP(R3/R1)

Alternatively:

IR1 =
VP/R1

Therefore:

VN = -IR1 × R3 =
(-VP/R1)R3

The allowable range
of the unregulated negative supply voltage is -6V to -24V. For
a wider input-supply range, either replace Q1 with a higher
gain transistor
or lower the resistance
of R4 and R5. VN can deliver up to 100mA without distortion,
but VP is limited by the reference of IC1 to a
maximum of 30mA, with a VP reduction at that level
of ~0.3V. (For higher positive load currents you can
substitute a different positive regulator.)

IBIAS
is negligible for the op amp shown. To minimize the
IBIAS error for other op amps,
R2 should have the value R1||R3.

Related parts

MAX1615

High-Voltage, Low-Power
Linear Regulators for Notebook Computers

Free
samples

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Download,
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APP 4405: Jun 05, 2009

AN4405, AN 4405, APP4405,
Appnote4405, Appnote 4405

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