AN-D14

Supertex inc.
AN-D14
Application Note
Low Dropout 3.0 Volt Linear Regulator
by Jimes Lei, Applications Engineering Manager
Introduction
to the open-loop gain of A2. The output of A2 regulates the
gate of LP07 for a VOUT of 0.2V x [R1/(R1 + R2) x (R4/R3 +
1)]. The resistor values are chosen (explained in detail in
the design considerations section of this application note)
and R1 adjusted for an output voltage of 3.0V. C3 is in parallel with R4 to reject external noise. C1 and C2 are bypass
capacitors.
Low dropout regulators are becoming increasingly important
as more and more equipment utilizes 3.0 and 5.0V analog
and digital circuits.
The main advantage of low dropout 3.0V linear regulators
is full utilization of battery life which makes them desirable
for battery-powered applications. The low dropout feature
will allow for output regulation even when the input battery
voltage is discharged close to its output regulated voltage.
This will extend the operating input voltage range and allow
circuits to operate at a lower battery voltage.
Any small decrease in VOUT due to a load applied to the output is sensed by R3 and R4 which is fed back to the noninverting input of A2. The output of A2 will drive the gate of the
LP07 to a lower potential thereby increasing the gate drive
adequately to source current to the output load and maintain
a constant output voltage.
This application note discusses the advantages of using
Supertex part number LP0701N3, which is a very low gate
threshold voltage P-Channel MOSFET. This part has a guaranteed maximum threshold of -1.0V and a maximum RDS(ON)
of 2.0Ω at -3.0V drive. This performance is essential for designing an ultralow dropout, low voltage linear regulator.
Design Considerations
The objective is to implement a 3.0V linear regulator with
the lowest possible voltage drop from input to output. The
output transistor for a linear regulator can be designed with
N-Channel or P-Channel MOSFETs or bipolar NPN or PNP
transistors. Figures 2A to 2D show the four possibilities.
Circuit Description
The low dropout 3.0V linear regulator shown on Figure 1
utilizes an LP07, an LM10, 4 resistors, and 3 capacitors. The
LP07 is a 16.5V, 2.0Ω, P-Channel MOSFET with a maximum
threshold of -1.0V. The LM10 is a dual op-amp with a 0.2V
reference. R1 is a potentiometer. R2, R3, and R4 are 5%, 1/4
watt resistors. C1, C2, and C3 can be either ceramic or electrolytic capacitors.
In Figure 2A, the dropout voltage using an N-Channel
MOSFET is too large since it cannot be better than the
threshold voltage of the MOSFET, which is 1.0 to 4.0V, depending on the type of device used. In figure 2b, the dropout voltage using an NPN is lower but still fairly large. The
dropout voltage is typically 0.7V, which is the VBE rating of
the transistor.
A1 is configured as a unity gain buffer for the 0.2V reference.
The output of A1 is attenuated by R1 and R2 and is connected
to the inverting input of A2. A2 is configured as a noninverting
amplifier with a closed-loop gain of (R4/R3 + 1). The LP07 is
configured as a common source amplifier, which functions
as a series pass transistor while contributing additional gain
8
+
VREF +
0.2V
1
A1
1/2
LM10
R2
22kΩ
1/2
In Figure 2C, the dropout voltage using a PNP transistor is
limited by the VCE(sat) rating of the transistor, which is typically
-200mV at low collector current. This approach also requires
the output of the op-amp to operate 0.7V below its most positive rail at all times.
LM10
2
R1
2.0kΩ
3
+
7
A2
6
4
LP0701N3
R4
150k
R3
500Ω
C3
0.01µF
C1
1.0µF
VOUT = 3.0V
C2
10µF
Figure 1: Low Dropout 3.0V Linear Regulator
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A040213
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AN-D14
+
+
-
VOUT = 3.0V
R4
NPN
R4
Fig 2A:
N-Channel MOSFET
VOUT = 3.0V
Typ
Max
Units
Conditions
VGS(th)
-0.5
-0.7
-0.1
V
VGS = VDS,
ID = -1.0mA
-
2.0
4.0
Ω
-
1.7
2.0
Ω
-
1.3
1.5
Ω
The LP07 acts as an additional gain stage to the open-loop
gain of A2. The increase in open-loop gain causes the loop
gain to be greater than 1 at low closed-loop gain conditions,
which causes oscillation. Oscillation can be eliminated by
setting the loop-gain to be less than 1. This can be achieved
by setting ß(negative feedback) < 1 / gain contributed by the
LP07.
The gain contributed by the LP07 is a function of the load
and the transconductance, GFS, of the LP07. Figure 3 shows
an equivalent circuit of the open-loop gain of the LP07.
GFS =
LP07
Vg
VOUT
δ Id
δ Vg
(R3 + R4)(RLOAD)
R3 + R4 + RLOAD
VOUT = Id
R4
VOUT
= GFS
Vg
RLOAD
R3
(R3 + R4)(RLOAD)
R3 + R4 + RLOAD
Figure 3: LP07 Open-Loop
VGS = -2.0V,
ID = -50mA
The GFS of the LP07 varies with ID, which is also the load
current. Typical GFS versus ID for low and high currents of the
LP07 is shown on figure 4a and 4b respectively.
VGS = -3.0V,
ID = -150mA
VGS = -5.0V,
ID = -300mA
For the no load condition, ID = 3.0V/(R3 + R4). It is desirable have R3 + R4 large to minimize the amount of biasing
current. The sum of R3 + R4 is chosen to be approximately
150K. From figure 4a, GFS is 0.62m for an ID of 20µA. VOUT/
VG is calculated as (0.62m )(150K) = 93.
At -3.0V, the on-resistance is 1.7Ω typical and 2.0Ω maximum, which helps achieve a low drain-to-source voltage
drop. Since the LM10 can swing very close to ground i.e.,
0V, the dropout voltage can be estimated to be 2.0Ω x (ILOAD).
For a 50mA load, the dropout voltage is 0.1V which means
the battery voltage can be 3.1V with the output still regulated
at 3.0V.
Ω
Ω
For a load current of 100mA, RLOAD = 3.0V/100mA. Using
figure 4b, VOUT/VG is calculated as (310m )(30Ω) = 9.3. The
open-loop gain varies with load and is at its maximum during
Ω
Doc.# DSAN-AN-D14
A040213
Fig 2D:
P-Channel MOSFET
Preventing Unwanted Oscillation
The Supertex LP07 has a guaranteed maximum threshold
of -1.0V and guaranteed on-resistance at -2.0V, -3.0V, and
-5.0V drives. The specifications are shown on the following
table:
Min
VOUT = 3.0V
R4
R3
Figure 2C:
PNP Transistor
Conventional P-Channel MOSFETs have guaranteed maximum thresholds of -4.0V, which would require the supply
voltage to be greater than 4.0V for adequate turn on. A low
threshold, low on-resistance P-Channel MOSFET is ideal for
this approach.
Parameter
P-Channel
+
R3
Figure 2B:
NPN Transistor
-
VOUT = 3.0V
R4
In Figure 2D, the dropout voltage for the P-Channel
MOSFET approach is determined by the on-resistance of
the device times the load current. The device is driven by the
battery voltage minus the minimum output voltage of the opamp. Similar to the PNP approach, the op-amp is required to
operate one threshold below the battery voltage during the
no load condition. When the battery voltage is discharged
close to 3.0V, the MOSFET chosen should have a very low
threshold and a very low on-resistance at low VGS ratings to
achieve low dropout.
RDS(ON)
PNP
+
R3
R3
-
≈
N-Channel
VBATT
VBATT
≈
-
VBATT
≈
≈
VBATT
2
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300
1.00
250
GFS (m )
1.20
0.80
Ω
Ω
GFS (m )
AN-D14
0.60
200
150
0.40
100
0.20
50
0
0
10
20
30
40
50
60
0
70
Figure 4a: GFS vs. ID at Low Currents
To determine the range of R1, the range of VI needs to be
determined under the worst case conditions. Using superposition, VOUT is calculated as:
R3
R2
80
100
120
140
157.5k
30nA (2k) (
+ 1)
475
3.0V = 3332.6Vi + 1.330V + 4.725mV + 19.95mV
The offset voltage, VOS, input biasing current, Ib+ and Ib-,
and tolerances of the external resistors will affect the output
voltage. R1 is used to adjust VOUT to 3.0V. Figure 5 is an
equivalent circuit showing VOS, Ib+ , and Ib-.
(R1R2 )
60
157.5k
3.0V = (Vi + 4mV) (
+ 1) + 30nA(157.5k) +
475
Calculations
+ 1) + (Ib+) R4 + (Ib-)
40
The LM10 guarantees VOS = 4.0mV max and Ib = 30nA max.
(R1 • R2) / (R1 + R2) is set at 2K.
For minimum Vi:
It is desirable to set ß <<1 / 93 since 1 / 93 is a typical value.
R3 and R4 are chosen to be 500Ω and 150k respectively for
a ß of 1/301, providing an adequate safety margin.
R4
20
Figure 4b: GFS vs. ID at High Currents
the no load condition. The negative feedback, ß, is R3 / (R3 +
R4) and should be set less than or equal to 1/(VOUT/VG).
VOUT = (VOS + Vi ) (
0
ID (mA)
ID (mA)
(
Vi(min) = 4.947mV
R4
+1)
(R1 + R2 ) R3
VBATT
Vi
Ib-
- +
-
VOS
+
R1
A2
R4
LP07
VOUT
R3
Ib+
Figure 5: Offset Voltage and Input Biasing Current
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200
3.0
RLOAD = 50Ω
VOUT (Volts)
VOUT - VIN (mV)
150
VOUT = 3.0V
100
1.0
50
0
2.0
0
25
50
75
100
0
IL (mA)
0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
VIN (Volts)
Figure 6: Dropout Voltage
Figure 7: VOUT vs VIN
Measurements
For maximum Vi:
Actual measurements were recorded and are shown on figures 6 and 7. Figure 6 shows the dropout voltage at different
load currents. Figure 7 shows the output voltage regulation
versus the decrease in battery voltage with a fixed load.
142.5k
3.0V = (Vi - 4mV) (
+ 1) - 30nA (142.5k) 525
142.5k
30nA (2k) (
+ 1)
525
3.0V = 272.4Vi - 1.090V - 4.275mV - 16.35mV
5.0V Regulators
The low dropout 3.0V regulator in figure 1 can be easily
modified to a 5.0V or adjustable low dropout regulator by
changing R1 to a 5.0k potentiometer. Using a voltage controlled resistor for R1 will allow for a programmable low dropout regulator.
Vi(max) = 15.09mV
The range for R1 is:
R1
Vi =
R1 =
R1 + R2
R2
39.40
Conclusion
(200mV)
to
Low dropout 3.0V linear voltage regulators are ideal for
portable battery operated applications to help extend battery life. The low dropout voltage allows the battery powered
equipment to operate at a lower battery voltage. In addition to the other advantages discussed, MOSFETs increase
the efficiency of the circuit because of the current required
to drive the gate is virtually zero as it is usually in the sub
nanoampere area. Bipolars need base current and this is
undesirable especially when battery energy is at a budget.
LP07 is ideal for linear applications requiring high efficiency
because of its low threshold voltage and low guaranteed onresistances at 2V, 3V and 5V drives.
R2
12.25
Choosing R1 to be a 2k potentiometer, R2 = (2k)(12.25) =
24.5k. R2 should be less than 24.5k so under the worst case
conditions, R1 would not operate at its maximum value of
2k. R2 is chosen to be 22k. The range of R1 is calculated as:
R1 = 22k(0.95) / 39.4 to 22k(1.05) / 12.25
R1 = 531Ω to 1.89kΩ
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com)
Supertex inc.
©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited.
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Tel: 408-222-8888
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