LP3891
0.8A Fast-Response Ultra Low Dropout Linear
Regulators
General Description
The LP3891 is a high current, fast response regulator which
can maintain output voltage regulation with minimum input to
output voltage drop. Fabricated on a CMOS process, the
device operates from two input voltages: Vbias provides
voltage to drive the gate of the N-MOS power transistor,
while Vin is the input voltage which supplies power to the
load. The use of an external bias rail allows the part to
operate from ultra low Vin voltages. Unlike bipolar regula-
tors, the CMOS architectutre consumes extremely low qui-
escent current at any output load current. The use of an
N-MOS power transistor results in wide bandwidth, yet mini-
mum external capacitance is required to maintain loop sta-
bility.
The fast transient response of these devices makes them
suitable for use in powering DSP, Microcontroller Core volt-
ages and Switch Mode Power Supply post regulators. The
parts are available in TO-220 and TO-263 packages.
Dropout Voltage: 100 mV (typ)
@
0.8A load current.
Ground Pin Current: 3 mA (typ) at full load.
Shutdown Current: 60 nA (typ) when S/D pin is low.
Precision Output Voltage: 1.5% room temperature accu-
racy.
Features
n
Ultra low dropout voltage (100mV
@
0.8A typ)
n
Low ground pin current
n
Load regulation of 0.04%/A
n
60 nA typical quiescent current in shutdown
n
1.5% output accuracy (25C)
n
TO-220, TO-263 packages
n
Over temperature/over current protection
n
-40C to +125C junction temperature range
Applications
n
DSP Power Supplies
n
Server Core and I/O Supplies
n
PC Add-in-Cards
n
Local Regulators in Set-Top Boxes
n
Microcontroller Power Supplies
n
High Efficiency Power Supplies
n
SMPS Post-Regulators
Typical Application Circuit
20069501
At least 10 F of input and output capacitance is required for stability.
*Tantalum capacitors are recommended. Aluminum electrolytic capacitors may be used for restricted temperature range. See application hints.
Connection Diagrams
20069502
TO-220, Top View
20069503
TO-263, Top View
September 2003
LP3891
0.8A
Fast-Response
Ultra
Low
Dropout
Linear
Regulators
2003 National Semiconductor Corporation
DS200695
www.national.com
Ordering Information
Order Number
Package Type
Package Drawing
Supplied As
LP3891ES-1.2
TO263-5
TS5B
Rail
LP3891ESX-1.2
TO263-5
TS5B
Tape and Reel
LP3891ET-1.2
TO220-5
T05D
Rail
LP3891ES-1.5
TO263-5
TS5B
Rail
LP3891ESX-1.5
TO263-5
TS5B
Tape and Reel
LP3891ET-1.5
TO220-5
T05D
Rail
LP3891ES-1.8
TO263-5
TS5B
Rail
LP3891ESX-1.8
TO263-5
TS5B
Tape and Reel
LP3891ET-1.8
TO220-5
T05D
Rail
Block Diagram
20069524
LP3891
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2
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Storage Temperature Range
-65C to +150C
Lead Temp. (Soldering, 5 seconds)
260C
ESD Rating
Human Body Model (Note 3)
Machine Model (Note 10)
2 kV
200V
Power Dissipation (Note 2)
Internally Limited
V
IN
Supply Voltage (Survival)
-0.3V to +6V
V
BIAS
Supply Voltage (Survival)
-0.3V to +7V
Shutdown Input Voltage (Survival)
-0.3V to +7V
I
OUT
(Survival)
Internally Limited
Output Voltage (Survival)
-0.3V to +6V
Junction Temperature
-40C to +150C
Operating Ratings
V
IN
(V
OUT
+ V
DO
) to 5.5V
Shutdown
0 to +6V
I
OUT
0.8A
Junction Temperature
-40C to +125C
V
BIAS
4.5V to 6V
Electrical Characteristics
Limits in standard typeface are for T
J
= 25C, and limits in boldface type apply
over the full operating temperature range. Unless otherwise specified: V
IN
= V
O
(NOM) + 1V, V
BIAS
= 4.5V, I
L
= 10 mA, C
IN
=
C
OUT
= 10 F, V
S/D
= V
BIAS
.
Symbol
Parameter
Conditions
Typical
(Note 4)
MIN
(Note
5)
MAX
(Note
5)
Units
V
O
Output Voltage Tolerance
10 mA
I
L
0.8A
V
O
(NOM) + 1V
V
IN
5.5V
4.5V
V
BIAS
6V
1.216
1.198
1.186
1.234
1.246
V
1.5
1.478
1.455
1.522
1.545
1.8
1.773
1.746
1.827
1.854
V
O
/
V
IN
Output Voltage Line Regulation
(Note 7)
V
O
(NOM) + 1V
V
IN
5.5V
0.01
%/V
V
O
/
I
L
Output Voltage Load Regulation
(Note 8)
10 mA
I
L
0.8A
0.04
0.06
%/A
V
DO
Dropout Voltage (Note 9)
I
L
= 0.8A
100
300
400
mV
I
Q
(V
IN
)
Quiescent Current Drawn from
V
IN
Supply
10 mA
I
L
0.8A
3
7
8
mA
V
S/D
0.3V
0.03
1
30
A
I
Q
(V
BIAS
)
Quiescent Current Drawn from
V
BIAS
Supply
10 mA
L
0.8A
1
2
3
mA
V
S/D
0.3V
0.03
1
30
A
I
SC
Short-Circuit Current
V
OUT
= 0V
1.8
A
Shutdown Input
V
SDT
Output Turn-off Threshold
Output = ON
0.7
1.3
V
Output = OFF
0.7
0.3
Td (OFF)
Turn-OFF Delay
R
LOAD
X C
OUT
<<
Td (OFF)
20
s
Td (ON)
Turn-ON Delay
R
LOAD
X C
OUT
<<
Td (ON)
15
I
S/D
S/D Input Current
V
S/D
=1.3V
1
A
V
S/D
0.3V
-1
LP3891
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3
Electrical Characteristics
Limits in standard typeface are for T
J
= 25C, and limits in boldface type apply
over the full operating temperature range. Unless otherwise specified: V
IN
= V
O
(NOM) + 1V, V
BIAS
= 4.5V, I
L
= 10 mA, C
IN
=
C
OUT
= 10 F, V
S/D
= V
BIAS
. (Continued)
Symbol
Parameter
Conditions
Typical
(Note 4)
MIN
(Note
5)
MAX
(Note
5)
Units
AC Parameters
PSRR (V
IN
)
Ripple Rejection for V
IN
Input
Voltage
V
IN
= V
OUT
+1V, f = 120 Hz
80
dB
V
IN
= V
OUT
+ 1V, f = 1 kHz
65
PSRR
(V
BIAS
)
Ripple Rejection for V
BIAS
Voltage
V
BIAS
= V
OUT
+ 3V, f = 120 Hz
70
V
BIAS
= V
OUT
+ 3V, f = 1 kHz
65
Output Noise Density
f = 120 Hz
1
V/root-Hz
e
n
Output Noise Voltage
BW = 10 Hz - 100 kHz, V
OUT
=
1.8V
150
V (rms)
BW = 300 Hz - 300 kHz, V
OUT
= 1.8V
90
Note 1: Absolute maximum ratings indicate limits beyond which damage to the component may occur. Operating ratings indicate conditions for which the device
is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications, see Electrical Characteristics. Specifications do not
apply when operating the device outside of its rated operating conditions.
Note 2: At elevated temperatures, device power dissipation must be derated based on package thermal resistance and heatsink thermal values.
J-A
for TO-220
devices is 65C/W if no heatsink is used. If the TO-220 device is attached to a heatsink, a
J-S
value of 4C/W can be assumed.
J-A
for TO-263 devices is
approximately 40C/W if soldered down to a copper plane which is at least 1.5 square inches in area. If power dissipation causes the junction temperature to exceed
specified limits, the device will go into thermal shutdown.
Note 3: The human body model is a 100 pF capacitor discharged through a 1.5k resistor into each pin.
Note 4: Typical numbers represent the most likely parametric norm for 25C operation.
Note 5: Limits are guaranteed through testing, statistical correlation, or design.
Note 6: If used in a dual-supply system where the regulator load is returned to a negative supply, the output pin must be diode clamped to ground.
Note 7: Output voltage line regulation is defined as the change in output voltage from nominal value resulting from a change in input voltage.
Note 8: Output voltage load regulation is defined as the change in output voltage from nominal value as the load current increases from no load to full load.
Note 9: Dropout voltage is defined as the minimum input to output differential required to maintain the output with 2% of nominal value.
Note 10: The machine model is a 220 pF capacitor discharged directly into each pin. The machine model ESD rating of pin 5 is 100V.
LP3891
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4
Typical Performance Characteristics
Unless otherwise specified: T
J
= 25C, C
OUT
= 10 F, Cin =
10F, S/D pin is tied to V
BIAS
, V
IN
= 2.2V, V
OUT
= 1.8V
I
GND
vs VSD
V
OUT
vs Temperature
20069505
20069506
DC Load Regulation
Line Regulation vs V
IN
20069507
20069508
Line Regulation vs V
BIAS
I
BIAS
vs I
L
20069509
20069510
LP3891
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5
Typical Performance Characteristics
Unless otherwise specified: T
J
= 25C, C
OUT
= 10 F, Cin =
10F, S/D pin is tied to V
BIAS
, V
IN
= 2.2V, V
OUT
= 1.8V (Continued)
I
GND
vs VSD
Noise Measurement
20069512
20069514
V
OUT
Startup Waveform
Line Regulation vs V
BIAS
20069515
20069518
Line Regulation vs V
BIAS
V
IN
PSRR
20069519
20069520
LP3891
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6
Typical Performance Characteristics
Unless otherwise specified: T
J
= 25C, C
OUT
= 10 F, Cin =
10F, S/D pin is tied to V
BIAS
, V
IN
= 2.2V, V
OUT
= 1.8V (Continued)
V
IN
PSRR
V
BIAS
PSRR
20069523
20069522
LP3891
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7
Application Hints
V
BIAS
RESTRICTIONS FOR PROPER START-UP
To prevent misoperation, ensure that V
BIAS
is below 50mV
before start-up is initiated. This scenario can occur in sys-
tems with a backup battery using reverse-biased "blocking"
diodes which may allow enough leakage current to flow into
the V
BIAS
node to raise it's voltage slightly above ground
when the main power is removed. Using low leakage diodes
or a resistive pull down can prevent the voltage at V
BIAS
from
rising above 50mV. Large bulk capacitors connected to
V
BIAS
may also cause a start-up problem if they do not
discharge fully before re-start is initiated (but only if V
BIAS
is
allowed to fall below 1V). A resistor connected across the
capacitor will allow it to discharge more quickly. It should be
noted that the probability of a "false start" caused by incor-
rect logic states is extremely low.
EXTERNAL CAPACITORS
To assure regulator stability, input and output capacitors are
required as shown in the Typical Application Circuit.
OUTPUT CAPACITOR
At least 10F of output capacitance is required for stability
(the amount of capacitance can be increased without limit).
The output capacitor must be located less than 1cm from the
output pin of the IC and returned to a clean analog ground.
The ESR (equivalent series resistance) of the output capaci-
tor must be within the "stable" range as shown in the graph
below over the full operating temperature range for stable
operation.
20069531
Minimum ESR vs Output Load Current
Tantalum capacitors are recommended for the output as
their ESR is ideally suited to the part's requirements and the
ESR is very stable over temperature. Aluminum electrolytics
are not recommended because their ESR increases very
rapidly at temperatures below 10C. Aluminum caps can only
be used in applications where lower temperature operation
is not required.
A second problem with Al caps is that many have ESR's
which are only specified at low frequencies. The typical loop
bandwidth of a linear regulator is a few hundred kHz to
several MHz. If an Al cap is used for the output cap, it must
be one whose ESR is specified at a frequency of 100 kHz or
more.
Because the ESR of ceramic capacitors is only a few milli
Ohms, they are not suitable for use as output capacitors on
LP389X devices. The regulator output can tolerate ceramic
capacitance totaling up to 15% of the amount of Tantalum
capacitance connected from the output to ground.
INPUT CAPACITOR
The input capacitor must be at least 10 F, but can be
increased without limit. It's purpose is to provide a low
source impedance for the regulator input. Ceramic capaci-
tors work best for this, but Tantalums are also very good.
There is no ESR limitation on the input capacitor (the lower,
the better). Aluminum electrolytics can be used, but their
ESR increase very quickly at cold temperatures. They are
not recommended for any application where temperatures
go below about 10C.
BIAS CAPACITOR
The 0.1F capacitor on the bias line can be any good quality
capacitor (ceramic is recommended).
BIAS VOLTAGE
The bias voltage is an external voltage rail required to get
gate drive for the N-FET pass transistor. Bias voltage must
be in the range of 4.5 - 6V to assure proper operation of the
part.
UNDER VOLTAGE LOCKOUT
The bias voltage is monitored by a circuit which prevents the
regulator output from turning on if the bias voltage is below
approximately 4V.
SHUTDOWN OPERATION
Pulling down the shutdown (S/D) pin will turn-off the regula-
tor. Pin S/D must be actively terminated through a pull-up
resistor (10 k
to 100 k) for a proper operation. If this pin
is driven from a source that actively pulls high and low (such
as a CMOS rail to rail comparator), the pull-up resistor is not
required. This pin must be tied to Vin if not used.
POWER DISSIPATION/HEATSINKING
A heatsink may be required depending on the maximum
power dissipation and maximum ambient temperature of the
application. Under all possible conditions, the junction tem-
perature must be within the range specified under operating
conditions. The total power dissipation of the device is given
by:
P
D
= (V
IN
-V
OUT
)I
OUT
+ (V
IN
)I
GND
where I
GND
is the operating ground current of the device.
The maximum allowable temperature rise (T
Rmax
) depends
on the maximum ambient temperature (T
Amax
) of the appli-
cation, and the maximum allowable junction temperature
(T
Jmax
):
T
Rmax
= T
Jmax
- T
Amax
The maximum allowable value for junction to ambient Ther-
mal Resistance,
JA
, can be calculated using the formula:
JA
= T
Rmax
/ P
D
These parts are available in TO-220 and TO-263 packages.
The thermal resistance depends on amount of copper area
or heat sink, and on air flow. If the maximum allowable value
of
JA
calculated above is
60 C/W for TO-220 package
and
60 C/W for TO-263 package no heatsink is needed
since the package can dissipate enough heat to satisfy these
requirements. If the value for allowable
JA
falls below these
limits, a heat sink is required.
LP3891
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8
Application Hints
(Continued)
HEATSINKING TO-220 PACKAGE
The thermal resistance of a TO220 package can be reduced
by attaching it to a heat sink or a copper plane on a PC
board. If a copper plane is to be used, the values of
JA
will
be same as shown in next section for TO263 package.
The heatsink to be used in the application should have a
heatsink to ambient thermal resistance,
HA
JA
-
CH
-
JC
.
In this equation,
CH
is the thermal resistance from the case
to the surface of the heat sink and
JC
is the thermal resis-
tance from the junction to the surface of the case.
JC
is
about 3C/W for a TO220 package. The value for
CH
de-
pends on method of attachment, insulator, etc.
CH
varies
between 1.5C/W to 2.5C/W. If the exact value is unknown,
2C/W can be assumed.
HEATSINKING TO-263 PACKAGE
The TO-263 package uses the copper plane on the PCB as
a heatsink. The tab of these packages are soldered to the
copper plane for heat sinking. The graph below shows a
curve for the
JA
of TO-263 package for different copper area
sizes, using a typical PCB with 1 ounce copper and no solder
mask over the copper area for heat sinking.
20069525
FIGURE 1.
JA
vs Copper (1 Ounce) Area for TO-263
package
LP3891
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9
Application Hints
(Continued)
As shown in the graph below, increasing the copper area
beyond 1 square inch produces very little improvement. The
minimum value for
JA
for the TO-263 package mounted to a
PCB is 32C/W.
Figure 2 shows the maximum allowable power dissipation
for TO-263 packages for different ambient temperatures,
assuming
JA
is 35C/W and the maximum junction tempera-
ture is 125C.
20069526
FIGURE 2. Maximum power dissipation vs ambient
temperature for TO-263 package
LP3891
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10
Physical Dimensions
inches (millimeters) unless otherwise noted
TO220 5-lead, Molded, Stagger Bend Package (TO220-5)
NS Package Number T05D
LP3891
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11
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
TO263 5-Lead, Molded, Surface Mount Package (TO263-5)
NS Package Number TS5B
LIFE SUPPORT POLICY
NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:
1. Life support devices or systems are devices or
systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.
2. A critical component is any component of a life
support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.
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www.national.com
LP3891
0.8A
Fast-Response
Ultra
Low
Dropout
Linear
Regulators
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.
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