DN44 - A Simple Ultra-Low Dropout Regulator

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A Simple Ultra-Low Dropout Regulator – Design Note 44
Jim Williams
Single Devices Now Provide This Function
Switching regulator post regulators, battery powered
apparatus, and other applications frequently require low
drop-out linear regulators. Often, battery life is significantly affected by the regulator’s dropout performance.
Figure 1’s simple circuit offers lower dropout voltage
than any monolithic regulator. Dropout is below 50mV
at 1A, increasing to only 450mV at 5A. Line and load
regulation are within 5mV, and initial output accuracy
is inside 1%. Additionally, the regulator is fully short
circuit protected, and has a no load quiescent current
of 600μA.
Circuit operation is straightforward. The 3-pin LT®1123
regulator (TO-92 package) servo controls Q1’s base to
maintain its feedback pin (FB) at 5V. The 10μF output
capacitor provides frequency compensation. If the
circuit is located more than six inches from the input
source the optional 10μF capacitor should bypass the
+
100
+5VOUT
10μF
20Ω
DRIVE
LT1123 FB
GND
* = OPTIONAL (SEE TEXT)
MJE1123 = MOTOROLA
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
110
+
10μF* 600Ω
Normally, configurations of this type offer unpredictable
short circuit protection. Here, the MJE1123 transistor
shown has been specially designed for use with the
LT1123. Because of this, beta based current limiting is
practical. Excessive output current causes the LT1123
to pull down harder on Q1 until beta limiting occurs.
Under these conditions the controlled pull down current combines with Q1’s beta and safe operating area
characteristics to provide reliable short circuit limiting.
Figure 3 details current limit characteristics for 30
randomly selected transistors.
DN044 F01
POWER LIMITING RESISTOR
INPUT
Q1
MJE1123
input. The optional 20Ω resistor limits LT1123 power
dissipation and is selected based upon the maximum
expected input voltage (see Figure 2).
90
80
70
60
50
ASSUME: VBE = 0.9V
VSAT, LT1123 = 2V
IDRIVE MAX = 150mA
40
30
20
10
5
Figure 1. The Ultra-Low Dropout Regulator. LT1123
Combines with Specially Designed Transistor for
Lowest Dropout and Short Circuit Protection
02/91/44_conv
15
10
INPUT VOLTAGE (V)
20
DN044 F02
Figure 2. LT1123 Power Dissipation Limiting
Resistor Value vs Input Voltage
0.75
0.60
DROPOUT VOLTAGE (V)
NUMBER OF UNITS
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
4.00 4.25 4.50 4.75 5.00 5.25 5.50 5.75 6.00
OUTPUT CURRENT (A)
0.45
0.30
0.15
0
0
1
3
4
2
OUTPUT CURRENT (A)
DN044 F03
Figure 4 shows dropout characteristics. Even at 5A,
dropout is about 450mV, decreasing to only 50mV
at 1A. Monolithic regulators cannot approach these
figures, primarily because monolithic power transistors do not offer Q1’s combination of high beta and
excellent saturation. For comparison, Figure 5 compares the circuits performance against some popular
monolithic regulators. Dropout is ten times better than
138 types, and significantly better than the other types
shown. Because of Q1’s high beta, base drive loss is
only 1%-2% of output current, even at full 5A output.
This maintains high efficiency under the low VIN – VOUT
conditions the circuit will typically operate at. As an
exercise, the MJE1123 was replaced with a 2N4276,
a Germanium device. This combination provided even
lower dropout performance, although current limit
characteristics cannot be guaranteed.
Data Sheet Download
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Linear Technology Corporation
DN044 F04
Figure 4. Dropout Voltage vs Output Current
3.0
2.5
DROPOUT VOLTAGE (V)
Figure 3. Short Circuit Current for 30 Randomly
Selected MJE1123 Transistors at VIN = 7V
5
LT138
2.0
1.5
LT1084
1.0
LT1185
LT1123/2N4276
0.5
LT1123/MJE1123
0
0
1
2
3
4
OUTPUT CURRENT (A)
5
DN044 F05
Figure 5. Dropout Voltage vs Output Current for
Various Regulators
For applications help,
call (408) 432-1900
dn44f_conv IM/GP 0291 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1991
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