DN32 - A Simple Ultra-Low Dropout Regulator

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A Simple Ultra-Low Dropout Regulator – Design Note 32
Jim Williams
Linear voltage regulators with low dropout characteristics are a frequent requirement, particularly in battery powered applications. It is desirable to maintain
regulation until the battery is almost entirely depleted.
Regulator dropout limits significantly impact useful
battery life, and as such should be minimized. Figure 1
shows dropout characteristics for a monolithic regulator,
the LT®1085. The <1.5V dropout performance is about
twice as good as standard monolithic regulators. In
many cases this device will serve nicely, but applications
requiring lower dropout mandate a different approach.
Figure 2’s simple regulator has only 85mV dropout at
2.5A – a 13x improvement. At lower currents dropout
decreases to vanishingly small values. This circuit is
particularly applicable in battery driven lap top computers, where multi-output power supplies are used.
In operation, the LT1431 shunt regulator adjusts its
output (“collector”) to whatever value is required to
force circuit output to 5V. The LT1431’s internal trimming
eliminates the usual feedback resistors and trimpots. Q1,
6VIN
(TYP. 5.04V – 7.2V)
the pass element, runs as a voltage overdriven source
follower. This configuration offers the lowest possible
dropout voltage,* although it does require a +12V bias
source for Q1’s gate. This +12V source is commonly
present in lap top computers and similar devices because of disc drive and peripheral power requirements.
Power drain on the +12V supply is a few milliamperes.
Providing short circuit protection without introducing significant loss requires care. A1 achieves this by
sensing across a 0.002Ω shunt (1.5" of #23 wire). This
introduces only 6mV of drop at the circuits 3A current
limit threshold. A 6mV current limit trip point is derived
by grounding A1’s offset pin 5. The 6mV input offset
generated at A1 by doing this is stable over time, tem*A detailed discussion of various methods for achieving low
dropout appears as Appendix A (“Achieving Low Dropout”) in LTC
Application Note 32, “High Efficiency Linear Regulators.”
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.
Q1
MTP50N05EL
D
0.002Ω (1.5" #23 WIRE)
+
S
5VOUT
1.5k
–
1N4148
EOS = 6mV
SEE TEXT
A1
LT1006
12V
5
+
MINIMUM INPUT/OUTPUT DIFFERENTIAL (V)
47μF
12V
2
LT1085
1
COLLECTOR
V+
FIGURE 2
GNDF
0
0
3
2
1
OUTPUT CURRENT (A)
REF
RMID
GNDS
DN032 F02
4
DN032 F01
Figure 1. Dropout Performance for a Low
Dropout Monolithic Regulator vs Figure 2
03/90/32_conv
LT1431
RTOP
CONNECT ALL LABELED RETURNS TO A
SINGLE POINT AT THE 6VIN SOURCE
Figure 2. Ultra-Low Dropout Regulator
perature and unit-unit variation, and substitutions for
A1 are not advisable. Currents beyond 3A cause A1 to
pull low, stealing Q1’s gate drive and shutting off the
regulators output. Under overload conditions A1 and Q1
from a well controlled linear current control loop with
smooth limiting. Figure 3 details dropout characteristics.
Results for the MTP50N05EL MOSFET specified for Q1
show only 85mV dropout, decreasing to just 8mV at
0.25A. For comparison, data for some higher resistance
transistors also appears.
Q1’s source follower connection makes regulator
dynamics quite good compared to common source/
emitter approaches. Figure 4 shows no load (Trace
A low) to full load (Trace A high) response. Regulator
output (Trace B) dips only 200mV and recovers quickly
with clean damping. The positive slew recovery time is
due to the 1.5kΩ bias resistor acting against Q1’s input
capacitance (Trace C is Q1’s gate). Quicker response is
300
275
Q1 = MTP15N08L
(0.135Ω)
DROPOUT VOLTAGE (mV)
250
225
Q1 = MTP25N05L
(0.08Ω)
200
2.5A = 338mV
DROPOUT
175 Q1 = MTP40N06EL
(0.06Ω)
150
125
100
possible by a reduction in this value, although current
drain from the +12V supply will increase. The value used
represents a good compromise. Transient recovery for
load removal is also well controlled.
This regulator offers a simple solution to applications
requiring extremely low dropout over a range of output
currents. The performance, low parts count and lack
of trimming make it an attractive alternative to other
approaches. For reference, pertinent information on
construction of wire shunts appears in Figures 5 and 6.
WIRE GAUGE
μΩ/INCH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
83
100
130
160
210
265
335
421
530
670
890
1000
1300
1700
2100
2700
75
50
0
Figure 5. Resistance vs Size for Various Copper Wire
Types
Q1 = MTP50N05EL
(0.032Ω)
25
0 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50
OUTPUT CURRENT (A)
DN032 F03
Figure 3. Dropout Characteristics for Figure 2. Q1’s
Saturation Directly Influences Performance.
LENGTH DETERMINES
SHUNT VALUE
MAJOR
CURRENT
FLOW
MAJOR
CURRENT
FLOW
LOW RESISTANCE WIRE
A = 5V/DIV
SENSE
POINT
WIRE
B = 100mV/DIV
C = 0.5V
(AC COUPLED)
PRINTED
CIRCUIT
VERSION
HORIZ = 1μs/DIV
DN032 F04
Figure 4. Transient Response for a Full Load Step.
Follower Connection Provides Clean Dynamics.
Data Sheet Download
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Linear Technology Corporation
SENSE
POINT
WIRE
SENSE
POINT
TRACE
MAJOR CURRENT FLOW
SENSE
POINT
TRACE
DN032 F06
Figure 6. Detail of a Low Resistance Current Shunt
For applications help,
call (408) 432-1900
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