LP3875ES-1.8 [NSC]
1.5A Fast Ultra Low Dropout Linear Regulators; 1.5A快速超低压降线性稳压器
| 型号： | LP3875ES-1.8 |
| 厂家： | 
National Semiconductor |
| 描述： | 1.5A Fast Ultra Low Dropout Linear Regulators |
| 文件： | 总18页 (文件大小：742K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
July 2005
LP3872/LP3875
1.5A Fast Ultra Low Dropout Linear Regulators
General Description
Features
n Ultra low dropout voltage
n Low ground pin current
The LP3872/LP3875 series of fast ultra low-dropout linear
regulators operate from a +2.5V to +7.0V input supply. Wide
range of preset output voltage options are available. These
ultra low dropout linear regulators respond very quickly to
step changes in load, which makes them suitable for low
voltage microprocessor applications. The LP3872/LP3875
are developed on a CMOS process which allows low quies-
cent current operation independent of output load current.
This CMOS process also allows the LP3872/LP3875 to op-
erate under extremely low dropout conditions.
n Load regulation of 0.06%
n 10nA quiescent current in shutdown mode
n Guaranteed output current of 1.5A DC
n Available in TO-263, TO-220 and SOT-223 packages
n Output voltage accuracy 1.5%
n Error flag indicates output status
n Sense option improves load regulation
n Minimum output capacitor requirements
n Overtemperature/overcurrent protection
n −40˚C to +125˚C junction temperature range
Dropout Voltage: Ultra low dropout voltage; typically 38mV
at 150mA load current and 380mV at 1.5A load current.
Ground Pin Current: Typically 6mA at 1.5A load current.
Shutdown Mode: Typically 10nA quiescent current when
the shutdown pin is pulled low.
Applications
n Microprocessor power supplies
n GTL, GTL+, BTL, and SSTL bus terminators
n Power supplies for DSPs
n SCSI terminator
Error Flag: Error flag goes low when the output voltage
drops 10% below nominal value.
SENSE: Sense pin improves regulation at remote loads.
Precision Output Voltage: Multiple output voltage options
are available ranging from 1.8V to 5.0V with a guaranteed
accuracy of 1.5% at room temperature, and 3.0% over all
conditions (varying line, load, and temperature).
n Post regulators
n High efficiency linear regulators
n Battery chargers
n Other battery powered applications
Typical Application Circuits
20063301
*SD and ERROR pins must be pulled high through a 10kΩ pull-up resistor. Connect the ERROR pin to ground if this function is not used. See application hints
for more information.
© 2005 National Semiconductor Corporation
DS200633
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Typical Application Circuits (Continued)
20063345
*SD must be pulled high through a 10kΩ pull-up resistor. See application hints for more information.
Connection Diagrams
20063306
20063305
Top View
Top View
TO263-5 Package
TO220-5 Package
Bent, Staggered Leads
20063386
Top View
SOT223-5 Package
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2
Pin Description for TO220-5 and TO263-5 Packages
LP3872
LP3875
#
Pin
Name
SD
Function
Shutdown
Name
SD
Function
Shutdown
1
2
3
4
5
VIN
Input Supply
Ground
VIN
Input Supply
Ground
GND
VOUT
ERROR
GND
VOUT
SENSE
Output Voltage
ERROR Flag
Output Voltage
Remote Sense Pin
Pin Description for SOT223-5 Package
LP3872
LP3875
#
Pin
Name
SD
Function
Shutdown
Name
SD
Function
Shutdown
1
2
3
4
5
VIN
Input Supply
Output Voltage
ERROR Flag
Ground
VIN
Input Supply
Output Voltage
Remote Sense Pin
Ground
VOUT
ERROR
GND
VOUT
SENSE
GND
3
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Ordering Information
20063331
Package Type Designator is "T" for TO220 package, "S" for TO263 package, and "MP" for SOT223-5 package.
TABLE 1. Package Marking and Ordering Information
Output
Voltage
5.0
Description
(Current, Option)
Order Number
Package Type
Package Marking
Supplied As:
LP3872EMP-5.0
1.5A, Error Flag
SOT223-5
LHDB
1000 Units on Tape and
Reel
5.0
LP3872EMPX-5.0
1.5A, Error Flag
SOT223-5
LHDB
2000 Units on Tape and
Reel
5.0
5.0
3.3
LP3872ES-5.0
LP3872ESX-5.0
LP3872EMP-3.3
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
TO263-5
TO263-5
SOT223-5
LP3872ES-5.0
LP3872ES-5.0
LHCB
Rail
Tape and Reel
1000 Units on Tape and
Reel
3.3
LP3872EMPX-3.3
1.5A, Error Flag
SOT223-5
LHCB
2000 Units on Tape and
Reel
3.3
3.3
2.5
LP3872ES-3.3
LP3872ESX-3.3
LP3872EMP-2.5
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
TO263-5
TO263-5
SOT223-5
LP3872ES-3.3
LP3872ES-3.3
LHBB
Rail
Tape and Reel
1000 Units on Tape and
Reel
2.5
LP3872EMPX-2.5
1.5A, Error Flag
SOT223-5
LHBB
2000 Units on Tape and
Reel
2.5
2.5
1.8
1.8
1.8
LP3872ES-2.5
LP3872ESX-2.5
LP3872ES-1.8
LP3872ESX-1.8
LP3872EMP-1.8
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
TO263-5
TO263-5
TO263-5
TO263-5
SOT223-5
LP3872ES-2.5
LP3872ES-2.5
LP3872ES-1.8
LP3872ES-1.8
LHAB
Rail
Tape and Reel
Rail
Tape and Reel
1000 Units on Tape and
Reel
1.8
5.0
5.0
LP3872EMPX-1.8
LP3875EMP-5.0
LP3875EMPX-5.0
1.5A, Error Flag
1.5A, SENSE
1.5A, SENSE
SOT223-5
SOT223-5
SOT223-5
LHAB
LHRB
LHRB
2000 Units on Tape and
Reel
1000 Units on Tape and
Reel
2000 Units on Tape and
Reel
5.0
5.0
3.3
LP3875ES-5.0
LP3875ESX-5.0
LP3875EMP-3.3
1.5A, SENSE
1.5A, SENSE
1.5A, SENSE
TO263-5
TO263-5
SOT223-5
LP3875ES-5.0
LP3875ES-5.0
LHPB
Rail
Tape and Reel
1000 Units on Tape and
Reel
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4
Ordering Information (Continued)
TABLE 1. Package Marking and Ordering Information (Continued)
Output
Voltage
3.3
Description
(Current, Option)
Order Number
Package Type
Package Marking
LHPB
Supplied As:
LP3875EMPX-3.3
1.5A, SENSE
SOT223-5
2000 Units on Tape and
Ree
3.3
3.3
2.5
LP3875ES-3.3
LP3875ESX-3.3
LP3875EMP-2.5
1.5A, SENSE
1.5A, SENSE
1.5A, SENSE
TO263-5
TO263-5
SOT223-5
LP3875ES-3.3
LP3875ES-3.3
LHNB
Rail
Tape and Reel
1000 Units on Tape and
Reel
2.5
LP3875EMPX-2.5
1.5A, SENSE
SOT223-5
LHNB
2000 Units on Tape and
Ree
2.5
2.5
1.8
LP3875ES-2.5
LP3875ESX-2.5
LP3875EMP-1.8
1.5A, SENSE
1.5A, SENSE
1.5A, SENSE
TO263-5
TO263-5
SOT223-5
LP3875ES-2.5
LP3875ES-2.5
LHLB
Rail
Tape and Reel
1000 Units on Tape and
Reel
1.8
1.8
1.8
5.0
3.3
2.5
1.8
5.0
3.3
2.5
1.8
LP3875EMPX-1.8
LP3875ES-1.8
LP3875ESX-1.8
LP3872ET-5.0
LP3872ET-3.3
LP3872ET-2.5
LP3872ET-1.8
LP3875ET-5.0
LP3875ET-3.3
LP3875ET-2.5
LP3875ET-1.8
1.5A, SENSE
1.5A, SENSE
1.5A, SENSE
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
1.5A, Error Flag
1.5A, SENSE
1.5A, SENSE
1.5A, SENSE
1.5A, SENSE
SOT223-5
TO263-5
TO263-5
TO220-5
TO220-5
TO220-5
TO220-5
TO220-5
TO220-5
TO220-5
TO220-5
LHLB
LP3875ES-1.8
LP3875ES-1.8
LP3872ET-5.0
LP3872ET-3.3
LP3872ET-2.5
LP3872ET-1.8
LP3875ET-5.0
LP3875ET-3.3
LP3875ET-2.5
LP3875ET-1.8
Rail
Tape and Reel
Rail
Rail
Rail
Rail
Rail
Rail
Rail
Rail
5
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Block Diagrams
LP3872
20063303
LP3875
20063329
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6
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
IOUT (Survival)
Short Circuit Protected
Maximum Voltage for ERROR
Pin
VIN
Maximum Voltage for SENSE Pin
VOUT
Storage Temperature Range
Lead Temperature
−65˚C to +150˚C
Operating Ratings
Input Supply Voltage (Note 11)
(Soldering, 5 sec.)
260˚C
2 kV
2.5V to 7.0V
−0.3V to 7.0V
1.5A
ESD Rating (Note 3)
Power Dissipation (Note 2)
Input Supply Voltage (Survival)
Shutdown Input Voltage
(Survival)
Shutdown Input Voltage
Internally Limited
−0.3V to +7.5V
Maximum Operating Current (DC)
Junction Temperature
−40˚C to +125˚C
−0.3V to +7.5V
−0.3V to +6.0V
Output Voltage (Survival), (Note
6), (Note 7)
Electrical Characteristics
LP3872/LP3875
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = VO(NOM) + 1V, IL = 10 mA, COUT = 10µF, VSD = 2V.
Symbol
Parameter
Output Voltage
Conditions
Typ
(Note 4)
LP3872/5 (Note 5)
Units
Min
Max
VOUT +1V ≤ VIN ≤ 7.0V
10 mA ≤ IL ≤ 1.5A
-1.5
+1.5
VO
Tolerance
0
%
%
%
-3.0
+3.0
(Note 8)
∆V OL
Output Voltage Line
Regulation (Note 8)
Output Voltage Load
Regulation
VOUT + 1V ≤ VIN ≤ 7.0V
10 mA ≤ IL ≤ 1.5A
0.02
0.06
0.06
0.12
∆VO/ ∆IOUT
(Note 8)
VIN - VOUT
IL = 150 mA
IL = 1.5A
38
380
5
50
60
450
550
9
Dropout Voltage
(Note 10)
mV
mA
IL = 150 mA
IL = 1.5A
10
14
15
10
50
Ground Pin Current In
Normal Operation Mode
IGND
6
IGND
Ground Pin Current In
Shutdown Mode
VSD ≤ 0.3V
0.01
1.8
3.2
µA
A
-40˚C ≤ TJ ≤ 85˚C
VO ≥ VO(NOM) - 4%
IO(PK)
Peak Output Current
Short Circuit Protection
ISC Short Circuit Current
A
7
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Electrical Characteristics
LP3872/LP3875 (Continued)
Limits in standard typeface are for TJ = 25˚C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = VO(NOM) + 1V, IL = 10 mA, COUT = 10µF, VSD = 2V.
Symbol
Parameter
Conditions
Typ
LP3872/5 (Note 5)
Units
(Note 4)
Min
Max
0.3
Shutdown Input
Output = High
VIN
0
2
VSDT
Shutdown Threshold
V
Output = Low
IL = 1.5A
TdOFF
TdON
ISD
Turn-off delay
Turn-on delay
SD Input Current
20
25
1
µs
µs
nA
IL = 1.5A
VSD = VIN
Error Flag
VT
Threshold
(Note 9)
10
5
5
2
16
8
%
%
VTH
Threshold Hysteresis
Error Flag Saturation
Flag Reset Delay
Error Flag Pin Leakage
Current
(Note 9)
VEF(Sat)
Td
Isink = 100µA
0.02
1
0.1
V
µs
nA
Ilk
1
Imax
Error Flag Pin Sink
Current
VError = 0.5V
1
mA
dB
AC Parameters
VIN = VOUT + 1V
COUT = 10uF
73
57
VOUT = 3.3V, f = 120Hz
VIN = VOUT + 0.5V
COUT = 10uF
PSRR
Ripple Rejection
VOUT = 3.3V, f = 120Hz
f = 120Hz
ρn(l/f)
Output Noise Density
Output Noise Voltage
0.8
µV
BW = 10Hz – 100kHz
VOUT = 2.5V
150
en
µV (rms)
BW = 300Hz – 300kHz
VOUT = 2.5V
100
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions for which the device is
intended to be functional, but does not guarantee specific performance limits. For guaranteed specifications and test conditions, see Electrical Characteristics. The
guaranteed specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed
test conditions.
Note 2: At elevated temperatures, devices must be derated based on package thermal resistance. The devices in TO220 package must be derated at θ = 50˚C/W
jA
2
(with 0.5in , 1oz. copper area), junction-to-ambient (with no heat sink). The devices in the TO263 surface-mount package must be derated at θ = 60˚C/W (with
0.5in , 1oz. copper area), junction-to-ambient. The SOT-223 package must be derated at θ = 90˚C/W (with 0.5in , 1oz. copper area), junction-to-ambient.
jA
2
2
jA
Note 3: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 4: Typical numbers are at 25˚C and represent the most likely parametric norm.
Note 5: Limits are guaranteed by testing, design, or statistical correlation.
Note 6: If used in a dual-supply system where the regulator load is returned to a negative supply, the output must be diode-clamped to ground.
Note 7: The output PMOS structure contains a diode between the V and V
terminals. This diode is normally reverse biased. This diode will get forward biased
OUT
IN
if the voltage at the output terminal is forced to be higher than the voltage at the input terminal. This diode can typically withstand 200mA of DC current and 1Amp
of peak current.
Note 8: Output voltage line regulation is defined as the change in output voltage from the nominal value due to change in the input line voltage. Output voltage load
regulation is defined as the change in output voltage from the nominal value due to change in load current. The line and load regulation specification contains only
the typical number. However, the limits for line and load regulation are included in the output voltage tolerance specification.
Note 9: Error Flag threshold and hysteresis are specified as percentage of regulated output voltage. See Application Hints.
Note 10: Dropout voltage is defined as the minimum input to output differential voltage at which the output drops 2% below the nominal value. Dropout voltage
specification applies only to output voltages of 2.5V and above. For output voltages below 2.5V, the drop-out voltage is nothing but the input to output differential,
since the minimum input voltage is 2.5V.
Note 11: The minimum operating value for V is equal to either [V
+ V
] or 2.5V, whichever is greater.
DROPOUT
IN
OUT(NOM)
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8
Typical Performance Characteristics Unless otherwise specified: TJ = 25˚C, COUT = 10µF,
CIN = 10µF, S/D pin is tied to VIN, VOUT = 2.5V, VIN = VO(NOM) + 1V, IL = 10mA
Ground Current vs Output Voltage
Dropout Voltage vs Output Load Current
IL = 1.5A
20063360
20063354
Shutdown IQ vs Junction Temperature
Errorflag Threshold vs Junction Temperature
20063357
20063355
DC Load Reg. vs Junction Temperature
DC Line Regulation vs Temperature
20063358
20063359
9
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Typical Performance Characteristics Unless otherwise specified: TJ = 25˚C, COUT = 10µF,
CIN = 10µF, S/D pin is tied to VIN, VOUT = 2.5V, VIN = VO(NOM) + 1V, IL = 10mA (Continued)
Load Transient Response
Noise vs Frequency
CIN = COUT = 10µF, OSCON
20063381
20063361
Load Transient Response
CIN =COUT = 100µF, OSCON
Load Transient Response
CIN =COUT = 100µF, POSCAP
20063382
20063383
Load Transient Response
Load Transient Response
CIN =COUT = 10µF, TANTALUM
CIN =COUT = 100µF, TANTALUM
20063384
20063385
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10
Application Hints
EXTERNAL CAPACITORS
than 10 mΩ). However, some dielectric types do not have
good capacitance characteristics as a function of voltage
and temperature.
Like any low-dropout regulator, external capacitors are re-
quired to assure stability. These capacitors must be correctly
selected for proper performance.
Z5U and Y5V dielectric ceramics have capacitance that
drops severely with applied voltage. A typical Z5U or Y5V
capacitor can lose 60% of its rated capacitance with half of
the rated voltage applied to it. The Z5U and Y5V also exhibit
a severe temperature effect, losing more than 50% of nomi-
nal capacitance at high and low limits of the temperature
range.
INPUT CAPACITOR: An input capacitor of at least 10µF is
required. Ceramic Ceramic, Tantalum, or Electrolytic capaci-
tors may be used, and capacitance may be increased with-
out limit
OUTPUT CAPACITOR: An output capacitor is required for
loop stability. It must be located less than 1 cm from the
device and connected directly to the output and ground pins
using traces which have no other currents flowing through
them (see PCB Layout section).
X7R and X5R dielectric ceramic capacitors are strongly rec-
ommended if ceramics are used, as they typically maintain a
capacitance range within 20% of nominal over full operat-
ing ratings of temperature and voltage. Of course, they are
typically larger and more costly than Z5U/Y5U types for a
given voltage and capacitance.
The minimum value of output capacitance that can be used
for stable full-load operation is 10µF, but it may be increased
without limit. The output capacitor must have an ESR value
as shown in the stable region of the curve below. Tantalum
capacitors are recommended for the output capacitor.
TANTALUM: Solid Tantalum capacitors are recommended
for use on the output because their typical ESR is very close
to the ideal value required for loop compensation. They also
work well as input capacitors if selected to meet the ESR
requirements previously listed.
ESR Curve
Tantalums also have good temperature stability: a good
quality Tantalum will typically show a capacitance value that
varies less than 10-15% across the full temperature range of
125˚C to −40˚C. ESR will vary only about 2X going from the
high to low temperature limits.
The increasing ESR at lower temperatures can cause oscil-
lations when marginal quality capacitors are used (if the ESR
of the capacitor is near the upper limit of the stability range at
room temperature).
ALUMINUM: This capacitor type offers the most capaci-
tance for the money. The disadvantages are that they are
larger in physical size, not widely available in surface mount,
and have poor AC performance (especially at higher fre-
quencies) due to higher ESR and ESL.
Compared by size, the ESR of an aluminum electrolytic is
higher than either Tantalum or ceramic, and it also varies
greatly with temperature. A typical aluminum electrolytic can
exhibit an ESR increase of as much as 50X when going from
25˚C down to −40˚C.
20063370
SELECTING A CAPACITOR
It is important to note that capacitance tolerance and varia-
tion with temperature must be taken into consideration when
selecting a capacitor so that the minimum required amount
of capacitance is provided over the full operating tempera-
ture range. In general, a good Tantalum capacitor will show
very little capacitance variation with temperature, but a ce-
ramic may not be as good (depending on dielectric type).
Aluminum electrolytics also typically have large temperature
variation of capacitance value.
It should also be noted that many aluminum electrolytics only
specify impedance at a frequency of 120 Hz, which indicates
they have poor high frequency performance. Only aluminum
electrolytics that have an impedance specified at a higher
frequency (between 20 kHz and 100 kHz) should be used for
the LP387X. Derating must be applied to the manufacturer’s
ESR specification, since it is typically only valid at room
temperature.
Any applications using aluminum electrolytics should be
thoroughly tested at the lowest ambient operating tempera-
ture where ESR is maximum.
Equally important to consider is a capacitor’s ESR change
with temperature: this is not an issue with ceramics, as their
ESR is extremely low. However, it is very important in Tan-
talum and aluminum electrolytic capacitors. Both show in-
creasing ESR at colder temperatures, but the increase in
aluminum electrolytic capacitors is so severe they may not
be feasible for some applications (see Capacitor Character-
istics Section).
PCB LAYOUT
Good PC layout practices must be used or instability can be
induced because of ground loops and voltage drops. The
input and output capacitors must be directly connected to the
input, output, and ground pins of the regulator using traces
which do not have other currents flowing in them (Kelvin
connect).
CAPACITOR CHARACTERISTICS
CERAMIC: For values of capacitance in the 10 to 100 µF
range, ceramics are usually larger and more costly than
tantalums but give superior AC performance for bypassing
high frequency noise because of very low ESR (typically less
The best way to do this is to lay out CIN and COUT near the
device with short traces to the VIN, VOUT, and ground pins.
11
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cific frequency (measured with a 1Hz bandwidth). This type
of noise is usually plotted on a curve as a function of fre-
quency.
Application Hints (Continued)
The regulator ground pin should be connected to the exter-
nal circuit ground so that the regulator and its capacitors
have a "single point ground".
Total output Noise or Broad-band noise is the RMS sum
of spot noise over a specified bandwidth, usually several
decades of frequencies.
It should be noted that stability problems have been seen in
applications where "vias" to an internal ground plane were
used at the ground points of the IC and the input and output
capacitors. This was caused by varying ground potentials at
these nodes resulting from current flowing through the
ground plane. Using a single point ground technique for the
regulator and it’s capacitors fixed the problem.
Attention should be paid to the units of measurement. Spot
√
√
noise is measured in units µV/ Hz or nV/ Hz and total output
noise is measured in µV(rms).
The primary source of noise in low-dropout regulators is the
internal reference. In CMOS regulators, noise has a low
frequency component and a high frequency component,
which depend strongly on the silicon area and quiescent
current. Noise can be reduced in two ways: by increasing the
transistor area or by increasing the current drawn by the
internal reference. Increasing the area will decrease the
chance of fitting the die into a smaller package. Increasing
the current drawn by the internal reference increases the
total supply current (ground pin current). Using an optimized
trade-off of ground pin current and die size, LP3872/LP3875
achieves low noise performance and low quiescent current
operation.
Since high current flows through the traces going into VIN
and coming from VOUT, Kelvin connect the capacitor leads to
these pins so there is no voltage drop in series with the input
and output capacitors.
RFI/EMI SUSCEPTIBILITY
RFI (radio frequency interference) and EMI (electromagnetic
interference) can degrade any integrated circuit’s perfor-
mance because of the small dimensions of the geometries
inside the device. In applications where circuit sources are
present which generate signals with significant high fre-
The total output noise specification for LP3872/LP3875 is
presented in the Electrical Characteristics table. The Output
noise density at different frequencies is represented by a
curve under typical performance characteristics.
>
quency energy content ( 1 MHz), care must be taken to
ensure that this does not affect the IC regulator.
If RFI/EMI noise is present on the input side of the regulator
(such as applications where the input source comes from the
output of a switching regulator), good ceramic bypass ca-
pacitors must be used at the input pin of the IC.
SHORT-CIRCUIT PROTECTION
The LP3872 and LP3875 are short circuit protected and in
the event of a peak over-current condition, the short-circuit
control loop will rapidly drive the output PMOS pass element
off. Once the power pass element shuts down, the control
loop will rapidly cycle the output on and off until the average
power dissipation causes the thermal shutdown circuit to
respond to servo the on/off cycling to a lower frequency.
Please refer to the section on thermal information for power
dissipation calculations.
If a load is connected to the IC output which switches at high
speed (such as a clock), the high-frequency current pulses
required by the load must be supplied by the capacitors on
the IC output. Since the bandwidth of the regulator loop is
less than 100 kHz, the control circuitry cannot respond to
load changes above that frequency. This means the effective
output impedance of the IC at frequencies above 100 kHz is
determined only by the output capacitor(s).
In applications where the load is switching at high speed, the
output of the IC may need RF isolation from the load. It is
recommended that some inductance be placed between the
output capacitor and the load, and good RF bypass capaci-
tors be placed directly across the load.
ERROR FLAG OPERATION
The LP3872/LP3875 produces a logic low signal at the Error
Flag pin when the output drops out of regulation due to low
input voltage, current limiting, or thermal limiting. This flag
has a built in hysteresis. The timing diagram in Figure 1
shows the relationship between the ERROR flag and the
output voltage. In this example, the input voltage is changed
to demonstrate the functionality of the Error Flag.
PCB layout is also critical in high noise environments, since
RFI/EMI is easily radiated directly into PC traces. Noisy
circuitry should be isolated from "clean" circuits where pos-
sible, and grounded through a separate path. At MHz fre-
quencies, ground planes begin to look inductive and RFI/
EMI can cause ground bounce across the ground plane.
The internal Error flag comparator has an open drain output
stage. Hence, the ERROR pin should be pulled high through
a pull up resistor. Although the ERROR flag pin can sink
current of 1mA, this current is energy drain from the input
supply. Hence, the value of the pull up resistor should be in
the range of 10kΩ to 1MΩ. The ERROR pin must be
connected to ground if this function is not used. It should
also be noted that when the shutdown pin is pulled low, the
ERROR pin is forced to be invalid for reasons of saving
power in shutdown mode.
In multi-layer PCB applications, care should be taken in
layout so that noisy power and ground planes do not radiate
directly into adjacent layers which carry analog power and
ground.
OUTPUT NOISE
Noise is specified in two ways-
Spot Noise or Output noise density is the RMS sum of all
noise sources, measured at the regulator output, at a spe-
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12
Application Hints (Continued)
20063307
FIGURE 1. Error Flag Operation
SENSE PIN
tance. For example, in the case of a 3.3V output, if the trace
resistance is 100mΩ, the voltage at the remote load will be
3.15V with 1.5A of load current, ILOAD. The LP3875 regulates
the voltage at the sense pin. Connecting the sense pin to the
remote load will provide regulation at the remote load, as
shown in Figure 2. If the sense option pin is not required, the
sense pin must be connected to the VOUT pin.
In applications where the regulator output is not very close to
the load, LP3875 can provide better remote load regulation
using the SENSE pin. Figure 2 depicts the advantage of the
SENSE option. LP3872 regulates the voltage at the output
pin. Hence, the voltage at the remote load will be the regu-
lator output voltage minus the drop across the trace resis-
20063308
FIGURE 2. Improving remote load regulation using LP3875
SHUTDOWN OPERATION
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.
A CMOS Logic level signal at the shutdown ( SD) pin will
turn-off the regulator. Pin SD must be actively terminated
through a 10kΩ pull-up resistor for a proper operation. If this
13
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HEATSINKING TO-263 PACKAGE
Application Hints (Continued)
DROPOUT VOLTAGE
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. Figure 3 shows a curve for the
The dropout voltage of a regulator is defined as the minimum
input-to-output differential required to stay within 2% of the
nominal output voltage. For CMOS LDOs, the dropout volt-
age is the product of the load current and the Rds(on) of the
internal MOSFET.
θJA 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.
REVERSE CURRENT PATH
The internal MOSFET in LP3872 and LP3875 has an inher-
ent parasitic diode. During normal operation, the input volt-
age is higher than the output voltage and the parasitic diode
is reverse biased. However, if the output is pulled above the
input in an application, then current flows from the output to
the input as the parasitic diode gets forward biased. The
output can be pulled above the input as long as the current
in the parasitic diode is limited to 200mA continuous and 1A
peak.
POWER DISSIPATION/HEATSINKING
20063332
LP3872 and LP3875 can deliver a continuous current of
1.5A over the full operating temperature range. A heatsink
may be required depending on the maximum power dissipa-
tion and maximum ambient temperature of the application.
Under all possible conditions, the junction temperature must
be within the range specified under operating conditions.
The total power dissipation of the device is given by:
FIGURE 3. θJA vs Copper (1 Ounce) Area for TO-263
package
As shown in the figure, increasing the copper area beyond 1
square inch produces very little improvement. The minimum
value for θJA for the TO-263 package mounted to a PCB is
32˚C/W.
PD = (VIN−VOUT)IOUT+ (VIN)IGND
where IGND is the operating ground current of the device
(specified under Electrical Characteristics).
Figure 4 shows the maximum allowable power dissipation
for TO-263 packages for different ambient temperatures,
assuming θJA is 35˚C/W and the maximum junction tempera-
ture is 125˚C.
The maximum allowable temperature rise (TRmax) depends
on the maximum ambient temperature (TAmax) of the appli-
cation, and the maximum allowable junction temperature
(TJmax):
TRmax = TJmax− TAmax
The maximum allowable value for junction to ambient Ther-
mal Resistance, θJA, can be calculated using the formula:
θJA = TRmax / PD
LP3872 and LP3875 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 θJA 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
θJA falls below these limits, a heat sink is required.
20063333
HEATSINKING TO-220 PACKAGE
FIGURE 4. Maximum power dissipation vs ambient
temperature for TO-263 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 θJA will
be same as shown in next section for TO263 package.
HEATSINKING SOT223-5 PACKAGE
The heatsink to be used in the application should have a
heatsink to ambient thermal resistance,
Figure 5 shows a curve for the θJA of SOT-223 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.
θHA≤ θJA − θCH − θJC
.
In this equation, θCH is the thermal resistance from the case
to the surface of the heat sink and θJC is the thermal resis-
tance from the junction to the surface of the case. θJC is
about 3˚C/W for a TO220 package. The value for θCH de-
pends on method of attachment, insulator, etc. θCH varies
between 1.5˚C/W to 2.5˚C/W. If the exact value is unknown,
2˚C/W can be assumed.
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14
Application Hints (Continued)
20063388
FIGURE 6. SCENARIO A, θJA = 148˚C/W
20063387
FIGURE 5. θJA vs Copper(1 Ounce) Area for SOT-223
package
The following figures show different layout scenarios for
SOT-223 package.
20063389
FIGURE 7. SCENARIO B, θJA = 125˚C/W
15
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Physical Dimensions inches (millimeters) unless otherwise noted
TO220 5-lead, Molded, Stagger Bend Package (TO220-5)
NS Package Number T05D
For Order Numbers, refer to the “Ordering Information” section of this document.
www.national.com
16
Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
TO263 5-Lead, Molded, Surface Mount Package (TO263-5)
NS Package Number TS5B
For Order Numbers, refer to the “Ordering Information” section of this document.
17
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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)
SOT223, 5-Lead, Molded, Surface Mount Package (SOT223-5)
NS Package Number MP05A
For Order Numbers, refer to the “Ordering Information” section of this document.
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.
For the most current product information visit us at www.national.com.
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