DN119 - LT1580 Fast Response Low Dropout Regulator Achieves 0.4 Dropout at 4 Amps

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Fast Response Low Dropout Regulator Achieves 0.4 Dropout
at 4 Amps – Design Note 119
Craig Varga
Low dropout regulators have become more common
in desktop computer systems as microprocessor
manufacturers have moved away from 5V only CPUs.
A wide range of supply requirements exists today with
new voltages just over the horizon. In many cases, the
input-output differential is very small, effectively disqualifying many of the low-dropout regulators on the
market today. Several manufacturers have chosen to
achieve lower dropout by using PNP-based regulators.
The drawbacks of this approach include much larger die
size, inferior line rejection and poor transient response.
Enter the LT®1580
The new LT1580 NPN regulator is designed to make use
of the higher supply voltages already present in most
systems. The higher voltage source is used to provide
power for the control circuitry and supply the drive
current to the NPN output transistor. This allows the
NPN to be driven into saturation, thereby reducing the
dropout voltage by a VBE compared to a conventional
design. Applications for the LT1580 include 3.3V to
2.5V conversion with a 5V control supply, 5V to 4.2V
conversion with a 12V control supply, or 5V to 3.6V
conversion with a 12V control supply. It is easy to obtain dropout voltages as low as 0.4V at 4A, along with
excellent static and dynamic specifications.
The LT1580 is capable of 7A maximum with approximately 0.8V input-to-output differential. The
current requirement for the control voltage source
is approximately 1/100 of the output load current or
about 70mA for a 7A load. The LT1580 presents no
supply-sequencing issues. If the control voltage comes
up first, the regulator will not try to supply the full-load
demand from this source. The control voltage must be
at least 1V greater than the output to obtain optimum
performance. For adjustable regulators, the adjust-pin
current is approximately 60μA and varies directly with
absolute temperature. In fixed regulators, the ground pin
current is about 10mA and stays essentially constant
as a function of load. Transient response performance
12/95/119_conv
is similar to that of the LT1584 fast-transient-response
regulator. Maximum input voltage from the main power
source is 7V and the absolute maximum control voltage is 13V. The part is fully protected from overcurrent
and over-temperature conditions. Both fixed voltage
and adjustable voltage versions are available. The
adjustables are packaged in 5-pin TO-220s, whereas
the fixed-voltage parts are 7-pin TO-220s.
The LT1580 Brings Many New Features
Why so many pins? The LT1580 includes several innovative features that require additional pins. Both the
fixed and adjustable versions have remote-sense pins,
permitting very accurate regulation of output voltage at
the load, where it counts, rather than at the regulator.
As a result, the typical load regulation over a range of
100mA to 7A with a 2.5V output is approximately ±1mV.
The Sense pin and the VCONT pin, plus the conventional
three pins of an LDO regulator, give a pin count of five
for the adjustable design. The fixed voltage part adds a
GND pin for the bottom of the internal feedback divider,
bringing the pin count to six. Pin 7 is a no connect.
Note that the Adjust pin is brought out even on the fixed
voltage parts. This allows the user to greatly improve
the dynamic response of the regulator by bypassing the
feedback divider with a capacitor. In the past, using a
fixed regulator meant suffering a loss of performance
due to the lack of such a bypass. A capacitor value of
0.1μF to approximately 1μF will generally provide optimum transient response. The value chosen depends
on the amount of output capacitance in the system.
In addition to the enhancements already mentioned,
the reference accuracy has been improved by a factor
of two, with a guaranteed 0.5% tolerance. Temperature
drift is also very well controlled. When combined with
ratiometrically accurate internal divider resistors,
the part can easily hold 1% output accuracy over
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.
temperature, guaranteed, while operating with an input/
output differential of well under 1V.
Circuit Example
Figure 1 shows a circuit designed to deliver 2.5V from
a 3.3V source with 5V available for the control voltage. Figure 2 shows the response to a load step of
200mA to 4.0A. The circuit is configured with a 0.33μF
adjust-pin bypass capacitor. The performance without
this capacitor is shown in Figure 3. This difference in
performance is the reason for providing the Adjust pin
on the fixed-voltage devices. A substantial savings in
expensive output decoupling capacitance may be realized by adding a small ceramic capacitor at this pin.
In some cases, a higher supply voltage for the control
voltage will not be available. If the Control pin is tied
to the main supply, the regulator will still function as a
conventional LDO and offer a dropout specification approximately 70mV better than conventional NPN-based
LDOs. This is the result of eliminating the voltage drop of
the on-die connection to the control circuit that exists in
older designs. This connection is now made externally,
on the PC board, using much larger conductors than
are possible on the die.
5V
5
3.3V
VCONT
VIN
U1 SENSE
LT1580
ADJ
VOUT
4
1
VOUT = 2.5V
3
VCC
2
C3
22μF
25V
+
+
C2
220μF
10V
C4
0.33μF
R1
110Ω
1%
C1
R2
110Ω 100μF
10V
1%
+
100μF
10V
w2
+
1μF
25V
w10
MICROPROCESSOR
SOCKET
VSS
RTN
1580_1.eps
Figure 1. LT1580 Delivers 2.5V from 3.3V at Up to 6A
VO
50mV/DIV
VO
50mV/DIV
IO
2A/DIV
IO
2A/DIV
50μs/DIV
Figure 2. Transient Response of Figure 1’s Circuit with
Adjust-Pin Bypass Capacitor. Load Step Is from 200mA
to 4 Amps
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50μs/DIV
Figure 3. Transient Response Without Adjust-Pin Bypass
Capacitor. Otherwise, Conditions Are the Same as in
Figure 2
For applications help,
call (408) 432-1900
dn119f_conv LT/GP 1295 155K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1995
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