AN28

Application Note 28
Issue 1 June 1996
The ZLDO series of Low Dropout Voltage
Regulators
David Bradbury
Introduction
The ZLDO series of low dropout linear
regulators operate with an exceptionally
low dropout voltage, typically only
30mV with a load current of 100mA. The
series feature output voltages in the
range 3 to 18V, supply loads up to
300 mA, and yet consumes typical
quiescent currents of only 630µA. The
parts include a logic level shutdown
control and provide a low battery
warning flag. Designed for where power
losses must be minimised for efficiency
or thermal reasons, the ZLDO Series
regulators have many potential uses.
dropout regulators can consume high
quiescent currents, sometimes
approaching as much as a tenth of their
maximum load current specification
when approaching dropout conditions.
Despite its 300mA output rating, when
enabled the ZLDO series of devices
consume typically only 600µA when
regulating normally, and 3mA when the
input falls too low for regulation.
ZLDO500
Operation From A Low Voltage
Battery Pack
LBF
Spg
SC
Gnd
Vin
D/C
Figure 1 shows the ZLDO500 regulator
being used to stabilise the output of a 6V
battery pack. The ultra low dropout
voltage of only 100mV at full load
(300mA) given by the regulator, allows
the minimum number of cells to be used
in the pack, and also maximises the
energy that can be removed from the
battery before the output of the regulator
starts to fail. At a load current of 100mA
the dropout voltage falls to around
30mV. The endurance of the battery pack
is not only dependent on dropout
voltage. When operating, some low
N/C Vout
6V
C3
100nF
C1
10pF
+5V
C2
1uF
Output
0V
Figure 1
Basic ZLDO500 Based Battery Powered
Supply.
AN 28 - 1
Applications Note 28
Issue 1 June 1996
Distributed Power Supplies
Logic Controlled Power Supply
A common problem with large multiple
board logic systems is that the total
supply current taken at 5V can become
excessive, causing voltage drops and
noise in the power supply wiring unless
heavy cables and large decoupling
capacitors are used. A convenient
solution to this problem is to provide
power using a higher voltage supply
locally regulated to 5V. Voltage drops
and noise are now eliminated by the
regulators but they introduce a new
problem of significantly increased
power losses if standard regulators are
used. By employing the ZLDO500 for the
local regulators in a circuit similar to
Figure 1 but repeated on each logic
board, the power supply to the logic
boards can be distributed at a voltage
close to 5V. This will largely eliminate
the added losses of a distributed power
supply system, whilst minimising the
supply voltage errors and noise.
Figure 2 shows all that is necessary to
allow a microprocessor to control a
power supply based on the ZLDO500.
The Shutdown Control pin (pin 2), is a
logic compatible input that disables the
regulator when a voltage in excess of
1.5V is applied. The current required to
drive this input is less than 10µA. When
the regulator is shutdown in this way,
the quiescent current of the ZLDO500
falls to around 10µA. This makes the
regulator suitable for a wide range of
battery powered applications where
intermittent operation occurs. The
shutdown control pin should not be
taken to a voltage higher than Vin if low
quiescent supply current is important.
The shutdown control is a high
impedance input and so if not required,
should be wired to the ground pin (pin
7).
+6.7V
to 20V
Vin
IC1
ZSR500
IC2
ZLDO500
Vout
Gnd
Microproc.
System
Supply Input
LBF
Spg
SC
Gnd
Vin
D/C
C1
10pF
N/C Vout
C3
100nF
+5V
C2
1uF
Switched
Output
Applications Note 28
Issue 1 June 1996
ZLDO500
LBF
Spg
SC
Gnd
Vin
D/C
C1
10pF
+5V
N/C Vout
6V
C3
100nF
R1
100k
C2
1uF
Microproc.
System
Interrupt
Input
0V
Figure 3
Use of the Low Battery Flag (LBF) to
Low Battery Flag
The ZLDO500 series provides an output
called Low Battery Flag (LBF). Unlike
many regulators that only signal that
they are falling out of regulation, the LBF
output of the ZLDO series indicates that
the voltage drop across the regulator
has fallen to less than 400mV and so
supply failure is imminent. This
improved warning gives both more time
for the system to shutdown gracefully,
and maintains regulation while this
happens. This could be a vital point for
instance if measurements are under way
and must be completed accurately. The
LBF output is driven by an open collector
NPN transistor which pulls low when the
supply to the regulator is failing. Figure
3 shows this output being used. Note
that resistor R1 is necessary only if the
interrupt logic does not include a pull-up
resistor.
guarantee correct operation. Although
this approach is not particularly energy
efficient, if the load taken at 3.3V is not
too large, then the added complexity and
cost of a 3.3V switching converter may
not be justifiable and so this linear
solution can be preferable. This circuit
will also give far less noise than a
sw itching regulator w hich can be
important when handling low level
analogue signals or low voltage
measurements.
Post Converter Regulation
A common problem with multiple output
switch mode converters is that only one
output can be used in the feedback
control loop of the switching regulator.
Thus only one output is fully regulated.
All other outputs are prone to tracking
errors that occur if the load on any
output change significantly. By ensuring
c l o s e c ou pl i n g o f a l l t ra n s fo rme r
w i n d i n g s a n d m i n i m i s i n g th e
impedance of all outputs, these errors
can be reduced but never eliminated. A
simple solution to this problem is to
wind the switching regulator
+5V In
+5V Out
IC1
ZLDO330
LBF
Spg
SC
Gnd
Vin
D/C
C1
10pF
+3.3V Out
N/C Vout
C3
100nF
C2
1uF
Simple 3.3V Supply
0V In
0V
0V
Figure 2
Employing the Shutdown Pin to Conserve Battery Capacity.
AN 28 - 2
Using a circuit such as Figure 4, the
ZLDO330 can easily provide a 3.3V logic
supply from an available 5V rail where
most standard regulators could not
0V Out
Figure 4
ZLDO330 Used to Regulate 5V to 3.3V.
AN 28 - 3
Applications Note 28
Issue 1 June 1996
Distributed Power Supplies
Logic Controlled Power Supply
A common problem with large multiple
board logic systems is that the total
supply current taken at 5V can become
excessive, causing voltage drops and
noise in the power supply wiring unless
heavy cables and large decoupling
capacitors are used. A convenient
solution to this problem is to provide
power using a higher voltage supply
locally regulated to 5V. Voltage drops
and noise are now eliminated by the
regulators but they introduce a new
problem of significantly increased
power losses if standard regulators are
used. By employing the ZLDO500 for the
local regulators in a circuit similar to
Figure 1 but repeated on each logic
board, the power supply to the logic
boards can be distributed at a voltage
close to 5V. This will largely eliminate
the added losses of a distributed power
supply system, whilst minimising the
supply voltage errors and noise.
Figure 2 shows all that is necessary to
allow a microprocessor to control a
power supply based on the ZLDO500.
The Shutdown Control pin (pin 2), is a
logic compatible input that disables the
regulator when a voltage in excess of
1.5V is applied. The current required to
drive this input is less than 10µA. When
the regulator is shutdown in this way,
the quiescent current of the ZLDO500
falls to around 10µA. This makes the
regulator suitable for a wide range of
battery powered applications where
intermittent operation occurs. The
shutdown control pin should not be
taken to a voltage higher than Vin if low
quiescent supply current is important.
The shutdown control is a high
impedance input and so if not required,
should be wired to the ground pin (pin
7).
+6.7V
to 20V
Vin
IC1
ZSR500
IC2
ZLDO500
Vout
Gnd
Microproc.
System
Supply Input
LBF
Spg
SC
Gnd
Vin
D/C
C1
10pF
N/C Vout
C3
100nF
+5V
C2
1uF
Switched
Output
Applications Note 28
Issue 1 June 1996
ZLDO500
LBF
Spg
SC
Gnd
Vin
D/C
C1
10pF
+5V
N/C Vout
6V
C3
100nF
R1
100k
C2
1uF
Microproc.
System
Interrupt
Input
0V
Figure 3
Use of the Low Battery Flag (LBF) to
Low Battery Flag
The ZLDO500 series provides an output
called Low Battery Flag (LBF). Unlike
many regulators that only signal that
they are falling out of regulation, the LBF
output of the ZLDO series indicates that
the voltage drop across the regulator
has fallen to less than 400mV and so
supply failure is imminent. This
improved warning gives both more time
for the system to shutdown gracefully,
and maintains regulation while this
happens. This could be a vital point for
instance if measurements are under way
and must be completed accurately. The
LBF output is driven by an open collector
NPN transistor which pulls low when the
supply to the regulator is failing. Figure
3 shows this output being used. Note
that resistor R1 is necessary only if the
interrupt logic does not include a pull-up
resistor.
guarantee correct operation. Although
this approach is not particularly energy
efficient, if the load taken at 3.3V is not
too large, then the added complexity and
cost of a 3.3V switching converter may
not be justifiable and so this linear
solution can be preferable. This circuit
will also give far less noise than a
sw itching regulator w hich can be
important when handling low level
analogue signals or low voltage
measurements.
Post Converter Regulation
A common problem with multiple output
switch mode converters is that only one
output can be used in the feedback
control loop of the switching regulator.
Thus only one output is fully regulated.
All other outputs are prone to tracking
errors that occur if the load on any
output change significantly. By ensuring
c l o s e c ou pl i n g o f a l l t ra n s fo rme r
w i n d i n g s a n d m i n i m i s i n g th e
impedance of all outputs, these errors
can be reduced but never eliminated. A
simple solution to this problem is to
wind the switching regulator
+5V In
+5V Out
IC1
ZLDO330
LBF
Spg
SC
Gnd
Vin
D/C
C1
10pF
+3.3V Out
N/C Vout
C3
100nF
C2
1uF
Simple 3.3V Supply
0V In
0V
0V
Figure 2
Employing the Shutdown Pin to Conserve Battery Capacity.
AN 28 - 2
Using a circuit such as Figure 4, the
ZLDO330 can easily provide a 3.3V logic
supply from an available 5V rail where
most standard regulators could not
0V Out
Figure 4
ZLDO330 Used to Regulate 5V to 3.3V.
AN 28 - 3
Applications Note 28
Issue 1 June 1996
Voltage
Feedback
D1
+5V Out
TR1
Switching
Regulator
ZLDO330
C4
220uF
LBF Spg
D2
SC
Gnd
Vin
D/C
C1
10pF
+3.3V Out
N/C Vout
C5
220uF
C2
1uF
0V Out
Figure 5
ZLDO Switched Mode Supply Post-Regulator to Improve Output Impedance and Noise
Performance.
transformer to give a slightly higher
voltage than required and regulate down
from this to the desired voltage with a
linear regulator. To keep losses low and
so maintain the advantages of a switch
mode supply, it is important that the
voltage drop across this regulator is kept
as low as possible, i.e. just high enough
to compensate for the poor output
impedance of the switching power
supply but no higher. The low dropout
voltage of the ZLDO330 allows this
circuit technique to be implemented
very effectively, giving a highly stable
and accurate low noise supply. Figure 5
shows
this
technique
being
implemented.
Over Temperature Shutdown
The ZLDO regulator series include an
over temperature shutdown circuit that
d i s a b l e s t h e r e g u l a t o r i f i ts c hi p
temperature should exceed 125°C for
any re ason. Although intended to
provide a limited guard against
excessive internal power dissipation,
this circuit will shut down the regulator
if the ambient rises above 125°C. Thus,
the regulator could be used to disable a
circuit in the event of the ambient
temperature within which the circuit is
mounted becoming too high. Any
internal power dissipation due to load
current will reduce the ambient
temperature at which shutdown occurs
to some extent. A consequence of
achieving the extremely low dropout
voltage and high current performance
provided by the ZLDO series, is that the
parts can be damaged by sustained
output shorts or excessive loads when
c omb ined w ith hig h input s upply
voltages. To ensure reliable operation,
keep loads within the SOA graph
boundaries provided on the data sheet
for the respective part.
AN 28 - 4
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