DN21 - Floating Input Extends Regulator Capabilities

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Floating Input Extends Regulator Capabilities – Design Note 21
Brian Huffman
however, if the secondary windings are isolated from
one another, a low dropout positive voltage regulator
can be used for negative regulation (Figure 1).
Many applications require circuit performance that is
unachievable with conventional regulator design. This
results in added complexity to the circuit. However, some
problems can easily be solved by floating the input to
the regulator. A floating input can either be a battery, or
a secondary winding that is galvanically isolated from
all other windings. With this method high efficiency
negative voltage regulation, high voltage regulation, and
low saturation loss positive buck switching regulator
can all be achieved easily.
In this circuit the LT®1086 servos the voltage between
the output and the adjust pin to 1.25V. The positive
regulation is accomplished by conventional regulator
design. Negative voltage regulation is achieved by
connecting the output of the positive voltage regulator
to ground. The VIN pin floats to 1.5V or greater, above
ground. This technique can be used with any positive
voltage regulator, although highest efficiency occurs
with low dropout types.
Low dropout negative voltage regulators are not currently available. This would seem to preclude high efficiency negative linear regulators. Such regulation is
frequently desired in switching supply post regulators;
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FEEDBACK PATH
MUR410
5V OUTPUT
(TYPICAL)
+
470μF
MUR410
VIN
LT1086
ADJ
+
470μF
+VIN
12V
1.5A
VOUT
124Ω*
+
10μF
+
10μF
1N4002
10μF
1N4002
1.07k*
MUR410
VIN
SWITCHING
REGULATOR
LT1086
VOUT
ADJ
+
470μF
124Ω*
+
10μF
+
1.07k*
* = 1% FILM RESISTORS
DN021 F01
Figure 1. High Efficiency Negative Voltage Regulation
04/89/21_conv
–12V
1.5A
Another example where floating a linear regulator can
be useful is shown in Figure 2. In this case high voltage
regulation can be handled if split secondary windings
are available. This allows the regulators to be connected
in series. Neither regulator exceeds its maximum differential voltage even under short circuit conditions.
High current positive buck switching regulators can
have excessive saturation losses since most switches
are Darlingtons. As much as 2V can be dropped across
a Darlington or composite PNP switching transistor.
However, efficiency can be increased and power dissipation requirements greatly reduced if the input is
allowed to float (Figure 3).
~
18VAC
+
VIN
LT1086
R1
100Ω
1000μF
115VAC
~
–
~
+
VOUT
5V–45V
VOUT
ADJ
+
MDA201
The circuit in Figure 3 uses an LT1070 to perform
a buck conversion. The LT1070 is a current mode
switching regulator. The VSW pin output is a collector
of a common emitter NPN, so current flows through it
when it is low. The 40kHz repetition rate is set by the
LT1070’s internal oscillator. When the VSW pins “on,”
current flows through the load, the inductor, and into
the VSW pin. During this time a magnetic field is built
up in the inductor. When the switch is turned “off,” the
magnetic field collapses dumping energy into the load
through D1. The input of the switching regulator floats
to a potential set by the output.
R2
2k
R3
2k
1W
+
10μF
1N4002
10μF
1N4002
STANCOR
P-8685
18VAC
VIN
VOUT
ADJ
+
MDA201
LT1086
+
1000μF
~
–
360Ω
5W
R4
2k
1W
VOUT = 2[1.25V(R2/R1 + 2)]
IF R3 = R4, IADJ = 0
DN021 F02
Figure 2. High Voltage Regulation
~
115VAC
MDA801
~
VOUT
5V
4A
+
VIN 12V
(10V-40V)
–
VIN
RL
4700μF
3.9k*
VSW
+
4700μF
* = 1% FILM RESISTORS
MBR360 = MOTOROLA
L1 = PULSE ENGINEERING #PE-92113
+
D1
MBR745
L1
170μH
LT1070
2N5401
VFB
GND
VC
1k
1.1k*
1μF
DN021 F03
Figure 3. Floating Input Low Saturation Loss Buck Regulator
Data Sheet Download
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