DN292 - Very Low Dropout (VLDO) Linear Regulators Supply Low Voltage Outputs

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Very Low Dropout (VLDO) Linear Regulators Supply
Low Voltage Outputs
Design Note 292
Tom Gross
Introduction
With each new generation of computing systems, total
power continues to increase while system voltages
fall. CPU core voltages and logic supplies below 1.8V
are now commonplace. Power supplies must not only
regulate low output voltages but they must also operate
from low input voltages. A low voltage, low dropout
linear regulator is an attractive conversion option for
applications where output currents are in the several
ampere range. Component count and cost are low in
comparison to switching regulator solutions, and with
low input-to-output voltage differentials, efficiencies
are comparable.
MOSFET controller/driver are devices well suited to
deliver lower output voltages from a supply of 1.8V
or lower. Each device offers excellent line/load regulation, temperature performance and transient load
step response.
Figure 1 illustrates the LT1580 delivering 1.3V at 3A
(maximum), from a 1.8V input supply. This particular
configuration requires a higher voltage supply to bias
the control circuitry. Specifically, the control voltage
must be 1V above the output voltage for proper operation or 2.3V minimum in this case. In current systems,
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property of their respective owners.
VLDO Circuit Descriptions
The LT®1580 monolithic low dropout linear regulator, the LT1573 LDO PNP driver and the LT1575 LDO
VIN
1.8V
VCONT
2.5V
+
CIN
100μF
TANT
AVX TPSD
+
CCONT
10μF
TANT
AVX TAJT
VIN
VOUT
LT1580
SENSE
VCONT ADJ
R1
402Ω
1%
+
R2
16.2Ω
1%
COUT
100μF
×3
TANT
AVX TPSD
VOUT
50mV/DIV
VOUT
1.3V
3A
CBYP
10μF
X5R
CER
IOUT
1A/DIV
DN292 F01
Figure 1. LT1580 Fast Transient Response
Low Dropout Linear Regulator
+
VIN
DRIVE
LT1573
LATCH COMP
FB
SHDN
GND
VOUT
CPWR
100μF
TANT
AVX TPSD
RD
12.4Ω
1%
RB 100Ω
+
RB
100Ω
VOUT
50mV/DIV
R1
499Ω
1%
Q1
D45H11
CC 470pF
COUT1
22μF
X5R CER
1210 CASE
+
R2
53.6Ω
1%
COUT2
100μF ×2
TANT
AVX TPSD
R3
499Ω
VOUT
1.2V
3A
COUT3
2.2μF ×2
X5R CER
0805 CASE
Figure 3. LT1573 Low Dropout Linear Regulator
with Low Output Voltage
08/02/292_conv
DN292 F02
Figure 2. Load Transient Response for
Circuit in Figure 1
VPWR
1.8V
VIN
CIN
3.3μF
TANT
AVX TAJA
20μs/DIV
DN292 F03
IOUT
1A/DIV
5μs/DIV
DN292 F04
Figure 4. 3A Load Transient Response
for Circuit in Figure 3
a 2.5V supply is typically available and is used here as
the control supply voltage. The dropout voltage from
input-to-output is 300mV. The load step transient
response is shown in Figure 2. With a 3A load step,
the output voltage deviation is less than 50mV and
the output voltage recovers within 20 microseconds.
Figure 3 shows a circuit where the regulated output
voltage is less than the feedback reference voltage. The
circuit consists of an LT1573 linear regulator generating 1.2V at 3A from a 1.8V supply. As in the previous
circuit, a second 3.3V input voltage is required for the
control circuitry. A resistor divider connected from the
1.8V supply to the output biases the feedback pin above
the regulated output voltage by 65mV. This allows the
feedback pin to regulate at 1.265V with a 1.2V output.
R3’s value is chosen such that Q1 must be biased in
order for the feedback pin to reach its regulated voltage. This method of generating the feedback voltage
is acceptable when the input voltage is regulated. If
necessary, an external voltage reference can be used
to acquire a tighter output voltage tolerance. Figure 4
shows the transient response for a 3A load step. Like
the previous circuit, the minimum input voltage is 1.6V.
A linear regulator based on the LT1575 MOSFET driver
can handle higher output power and very low dropout
requirements. Figure 5 shows the LT1575 controller
driving an external N-channel MOSFET. The regulator
converts 1.8V to 1.5V, capable of delivering 4A maxiD1
MBR0520
R2
61.9k
1%
L1
10μH
SW
VIN
LT1613
FB
SHDN
R1
13.7k
1%
+
+
GND
SHDN
VGATE
6.8V
VIN
1.8V
COUT
4.7μF
TANT
AVX TAJA
FB
RFB1
243Ω
1%
RFB2
1.02k
1%
Figure 6 depicts the LT1575 load step transient response of less than 50mV output voltage deviation
and under 100μs response. The efficiency of this
linear regulator is 83%, primarily due to the low inputto-output voltage differential. An external MOSFET
with lower RDS(ON) can make for even lower dropout
performance and potentially higher efficiency. The
LT1575 linear regulator controller offers the lowest
dropout voltage performance of any commercially
available linear regulator. For instance, the dropout
voltage for this circuit is less than 100mV.
Conclusion
Low output voltage, low dropout voltage linear regulators are practical alternatives to switching regulators
in the current and future generations of computer
systems. All the circuits described above offer viable
solutions for the power supply designer. For systems
with bus voltages less than 2.5V, the LT1580 linear
regulator circuit provides the necessary power conversion with the fewest external components. The
LT1573 driver allows the use of a high current PNP
pass transistor. The LT1575 linear regulator combines
very low dropout voltage performance with high output
current capability.
CIN: PANASONIC SP
SERIES EEFUE0E221R
L1: MURATA LQH3C100K24
IPOS
INEG
VIN
LT1575
GND GATE
CIN
220μF
w2
mum, using a logic-level Siliconix Si4410 MOSFET as
the pass element. The LT1613 boost converter, which is
able to operate with input voltages down to 1.1V, generates the appropriate gate drive for the MOSFET. Note
that the input capacitors used, Panasonic SP capacitors
(part number EEFUE0E221R), were chosen because
they represent a typical output capacitor network of
microprocessor power supplies. High frequency, low
ESR tantalum capacitors, such as AVX TPS capacitors
can be substituted for these capacitors.
RG 1.5Ω
RC 1.5k
VOUT
50mV/DIV
Q1
Si4410
COMP
CC2
50pF
IOUT
2A/DIV
CC1
0.022μF
VOUT
1.5V
COUT
4A (MAX)
2.2μF w10
X5R CER
DN292 F05
0805 CASE
Figure 5. LT1575/LT1613 Low Dropout, High Current Linear Regulator
Data Sheet Download
www.linear.com
Linear Technology Corporation
50μs/DIV
DN292 F06
Figure 6. Transient Response for a 4A
Step in the Circuit of Figure 5
For applications help,
call (408) 432-1900
dn292f_conv LT/TP 0802 316.5K • PRINTED IN THE USA
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