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AN765
Using Microchip’s Micropower LDOs
Author:
Paul Paglia,
Microchip Technology Inc.
EQUATION 1:
V OUT = V REF [ ( R 1 ⁄ R 2 ) + 1 ]
VREF
INTRODUCTION
Microchip Technology, Inc.’s family of micropower
LDOs utilizes low-voltage CMOS process technology.
These LDOs provide similar ripple rejection and dropout characteristics as their bipolar equivalents, but are
significantly more efficient. A typical bipolar regulator
has base current equal to 1-2% of the output load,
whereas Microchip’s LDOs have approximately 60 µA
resulting in total operating current orders of magnitude
lower than their bipolar counterparts. In addition,
Microchip’s LDOs can be placed in a shutdown mode,
further enhancing their effectiveness in low-power
applications.
This low-power operation makes Microchip’s family of
LDOs ideal for upgrading the LP2980 and MIC5205
bipolar LDOs in cellular phones, pagers, PDAs,
laptops, hand-held meters, and other portable
applications.
Microchip’s micropower LDOs are available with fixed
and adjustable outputs, supporting load currents up to
50 mA, 100 mA, 150 mA and 300 mA. SOT-23-5, SOT23-6, SOT-223, and MSOP-8 packaging require
minimal board space. Shutdown capability, thermal
protection, and current limiting are standard in every
device. Adjustable output, error flag, and noise bypass
capability are provided on select devices (see Table 3).
APPLICATIONS
Optimizing Output Voltage Accuracy of
TC1070/TC1071 Adjustable LDOs
=
VIN
CIN +
1 µF –
SHDN
1.20V
VOUT
VIN
TC1070
GND TC1071
(SOT-23-5)
SHDN
VOUT
+
–
R1
COUT
1 µF
ADJ
R2
FIGURE 1:
Circuit.
Adjustable LDO Feedback
The ADJ pin is a high impedance CMOS input.
Consequently, resistor values can be between 300 kΩ
and 1 MΩ to minimize the current through R1 and R2.
Inspection of Equation 1 reveals the following:
1.
2.
When VOUT is made equal to VREF (i.e., R1 is
zero), the tolerance of VOUT will be
approximately that of VREF.
The tolerance of VOUT is a function of both the
tolerance of VREF and the tolerance of the R1/R2
ratio when VOUT is greater than VREF (i.e., when
R1/R2 > 0).
For the purposes of worst case analysis, the tolerances
of R1 and R2 are additive. For example, if R1 and R2 are
both 1% resistors, the maximum tolerance of the R1/R2
ratio is 2%.
Microchip’s LDOs are available in both adjustable and
fixed output voltage options. The accuracy of the output
depends on the initial accuracy, stability, and
temperature coefficient of the internal bandgap
reference and the feedback resistors.
Re-examining the effect of tolerances on Equation 1
reveals that the tolerance of VOUT worsens proportionally as the VOUT setting departs the value of VREF.
Stated another way:
Rather than specifying VOUT accuracy on adjustable
regulators, the initial accuracy and temperature
coefficient of the internal reference is specified. VOUT
accuracy is not specified because it depends on the
external feedback resistors. Figure 1 shows a typical
adjustable LDO feedback circuit in which resistors R1
and R2 set the output voltage per the following formula:
EQUATION 2:
© 2007 Microchip Technology Inc.
ERROR VOUT α ( V OUT – V REF )
Table 1 shows that percentage of total output voltage
error contributed by the tolerances of VREF and R1/R2
for various values of VOUT.
DS00765B-page 1
AN765
TABLE 1:
Power-Saving Shutdown Mode
OUTPUT ERROR
CONTRIBUTORS
VOUT
(V)
Reference
Tolerance
(%)
Resistor
Tolerance
(%)
Resistor
Error
(%)
Total
Output
Error (%)
1.23
2
1
0
2
1.23
2
2
0
2
2.0
2
1
0.77
2.77
2.0
2
2
1.54
3.54
2.46
2
1
1.0
3.0
2.46
2
2
2.0
4.0
3.0
2
1
1.2
3.2
3.0
2
2
2.4
4.2
4.0
2
1
1.38
3.38
4.0
2
2
2.76
4.76
5.0
2
1
1.50
3.5
5.0
2
2
3.0
5.0
The output voltage accuracy of the adjustable regulator
improves with tighter tolerance resistors. However,
accuracy will be limited to ±2% due to the accuracy of
the reference. Table 2 shows output voltage accuracy
for the adjustable LDO using 1%, 0.5%, and 0.1%
tolerance resistors.
TABLE 2:
RESISTOR TOLERANCE
EFFECT ON VOUT ERROR
VOUT Error
VOUT
1%
0.5%
0.1%
Resistor Tol. Resistor Tol. Resistor Tol.
5.0V
3.5%
2.75%
2.15%
4.0V
3.38%
2.69%
2.14%
3.0V
3.2%
2.6%
2.12%
2.46V
3.0%
2.5%
2.10%
2.0V
2.77%
2.39%
2.08%
1.23V
2.0%
2.0%
2.0%
All of Microchip’s micropower LDOs have a shutdown
input that allows the user to digitally disconnect the
load from the power source and send the regulator into
a low-power “sleep” mode. The supply current is
reduced from 50 µA, during normal operation, to
0.05 µA in shutdown.
The SHDN pin input current is guaranteed to be no
greater than 1 µA (an order of magnitude lower than
bipolar counterparts).
Shutdown mode is activated when SHDN is below
0.2 x VIN. In this mode, the pass transistor is turned
OFF, disconnecting the load from the power source.
Shutdown mode is disabled, allowing normal device
operation, when the input is above 0.4 x VIN. This VIN
is low enough to ensure that a control output from a
3.3V microcontroller, operating from four fully-charged
NiCad/NiMH cells (6V), can enable the LDO. If not
used, SHDN should not be left floating, but rather
connected to VIN.
Out-of-Regulation (ERROR) Flag
The TC1070/1/2/3 and TC1054/5 each have Error Flag
outputs that are asserted when the LDO falls out of
regulation by approximately –5%.
The ERROR pin is an N-channel open-drain output that
can sink up to 1 mA. However, larger value pull-up
resistors should be selected so that energy loss
through ERROR is kept to a minimum. ERROR must
be pulled to any supply voltage less than 7V through a
pull-up resistor.
ERROR output is valid for input voltages above 1V and
undefined for voltages below 1V. As the output is
transitioning between 0V and 1.0V during power up/
down, the Error output may float momentarily to 1.0V. If
1.0V is high enough to be interpreted as a logic ‘1’, the
two-resistor network shown in Figure 2 may be
used.This will ensure that ERROR never will rise above
0.5V during invalid states. Keep in mind the maximum
that Error output can bein its high state is VOUT/2.
VIN
CIN +
1 µF–
SHDN
TC1054/1055
(SOT-23-5)
VIN
VOUT
GND
SHDN
VOUT
+C
OUT
–
1 µF
R1
ERROR
ERROR
Note: R1 = R2
R2
FIGURE 2:
Ensuring Valid Error Output
for Low VIN Levels.
DS00765B-page 2
© 2007 Microchip Technology Inc.
AN765
By connecting an RC on ERROR output, it can be used
as a power on reset. During power up, the Error
comparator will go high as soon as the regulator output
is within tolerance. ERROR will be delayed by the RC
network before releasing the microcontroller from
reset.
VOUT
Hysteresis (VH)
VTH
ERROR
available in surface mount styles. Tantalums offer an
ESR similar to aluminum electrolytics. They also
provide a reasonable cost, high-volume efficiency
solution and are usually the capacitor of choice.
A 1 µF input capacitor should be installed from VCC to
GND (Figures 4 and 5) if the IC is powered from a
battery or if there is excessive (>1 ft) distance between
the regulator and the AC filter capacitor. A larger value
capacitor will provide better VCC noise rejection and
improved performance when the supply has a high AC
impedance. A 470 pf bypass capacitor can be tied to
the bypass pin on the TC1014/1015 and TC1072/1073
or the ADJ pin on the TC1070/1071 (see Figure 5) to
reduce the VREF noise.
VIH
Thermal Issues
VOL
FIGURE 3:
Flag.
Out-of-Regulation Error
ERROR also can be used as a power quality monitor.
If a low input voltage or an over-current condition
causes the output to fall out of regulation, ERROR will
pull low, signifying an unstable power condition. This
flags the microcontroller, which now can activate
proper shutdown sequencing, ensuring orderly system
operation.
The Error comparator has 50 mV of positive hysteresis
to provide some VIN noise immunity.
Input, Output and Bypass Capacitors
It is recommended that input, output, and bypass
capacitors be used for optimal device performance. To
ensure stability in the LDO’s feedback loop, a capacitor
is required from the output to ground (Figures 4 and 5).
Capacitors must be chosen that meet the ESR value
range and minimum capacitance identified in device
data sheets. In general, a 1 µF - 2.2 µF capacitor is
recommended to ensure stable operation under
maximum load conditions. Larger value capacitors
(4.7 µF to 10 µF) will increase transient load response
and ripple rejection performance.
Ceramic capacitors offer the lowest ESR followed by, in
order of increasing ESR, OS-CON, film, aluminum
electrolytic, and tantalum. Film capacitors provide good
performance, but usually are not a viable solution due
to excessive cost and size. Ceramics combine
excellent ESR with relatively small size. However, the
ESR of ceramic capacitors sometimes can be too low,
requiring a 1Ω series resistor to ensure stability. OSCON capacitors offer an ESR only slightly higher than
ceramics, but consume more volume. The OS-CON
capacitors exhibit rock-solid ESR from –55°C to 125°C.
Aluminum electrolytics are ideal for low-cost commercial temperature grade applications where board space
is not a concern. Like OS-CON capacitors, electrolytics
typically are offered in a radial lead package, but are
© 2007 Microchip Technology Inc.
The amount of power that the LDO dissipates is a
function of the bias supply current and the passthrough current. The pass-through current is the
current that flows from VCC through the pass transistor
of the LDO to the load. The following equation is used
to calculate power dissipation:
EQUATION 3:
P D = ( V CC × I S ) + [ ( V CC – V OUT )I LOAD ]
Maximum values of VCC and ILOAD and minimum
values for VOUT should be used when calculating PD to
ensure worst-case conditions are met.
The amount of power that the LDO can dissipate
depends on the ambient temperature (TA). A guardbanded maximum die temperature (TJMAX) of +125°C
is used to account for variations in thermal conductivity
of PC boards and variations in airflow.
EQUATION 4:
θ JA = ( T JMAX – T A ) ⁄ PD MAX
θ JA = θ JC + θ CA
θJC is the thermal resistance from the die surface to the
package body and leads. θCA is the thermal resistance
from the package body and leads to the surrounding
air, PC board dielectric, and traces.
The SOT-23-5 and SOT-23-6 packages have a worstcase θJA of 220°C/W when mounted on a single-layer
FR4 dielectric copper-clad PC board. This θJA can be
reduced by using a PC board made with a dielectric
that has a better heat transfer coefficient. Additionally,
adding a ground plane and large supply traces to the IC
will provide better thermal conductivity. The values for
θJA are for a system that uses natural convection. A
significant reduction in θCA can be induced with forced
airflow.
DS00765B-page 3
AN765
Excessive power dissipation will result in elevated die
temperatures that could activate the device’s thermal
shutdown. The LDOs have an integrated thermal
protection circuitry that disables the LDO when die
temperatures exceed approximately +160°C. Ten
degrees Celsius of hysteresis is built into the protection
circuitry, such that the LDO is not released from thermal
shutdown until the die temperature drops to +150°C. In
addition to thermal protection, an internal sense resistor in series with the pass element providesa short-circuit limit.
Given:
θJA = 220°C/W
∴PDMAX = (125°C - TA)/220°C/W
Ambient Temperature
PDMAX
+25°C
0.454W
+50°C
0.341W
+85°C
0.182W
VIN
CIN +
1 µF –
SHDN
VIN
CIN
1 µF
SHDN
VIN
VOUT
TC1014/1015
(SOT-23-5)
GND
VOUT
+
–
COUT
1 µF
VOUT
VIN
COUT +
1 µF –
TC1107
(SOIC8 & MSOP8)
ERROR
–
CIN
1 µF
GND
+CBYPASS
(optional)
470 pF
VIN
VOUT
TC1054/1055
+
(SOT-23-5)
–
GND
VIN
+
Bypass
SHDN
SHDN
VOUT
SHDN
VOUT
COUT +
1 µF –
VOUT
COUT
1 µF
VIN
ERROR
SHDN
VOUT
TC1108
(SOT-223)
GND
VIN
+
–
TC1072/1073
(SOT-23-6)
VIN
+
CIN
–
1 µF
SHDN
FIGURE 4:
DS00765B-page 4
VIN
VOUT
Bypass
+CBYPASS
(optional)
470 pF
ERROR
ERROR
GND
SHDN
VOUT
COUT
1 µF
Typical Application Circuit (Fixed Output)
© 2007 Microchip Technology Inc.
AN765
VOUT
VIN
VOUT
COUT +
1 µF –
VIN
CIN
1 µF
ADJ
TC1107-ADJ
(SOIC8 & MSOP8)
GND
+CBYPASS
0.01 µF
(optional)
VOUT
SHDN
1
C1 +
1 µF
SHDN
2
R1
VOUT
VIN
GND
NC
TC1174
3 NC
VIN
VIN
VOUT
TC1070/1071
(SOT-23-5)
GND
CIN +
1 µF –
SHDN
VOUT
R1
4
R2
COUT
1 µF
ADJ
SHDN
TABLE 3:
ADJ
VIN
7
NC
6
Shutdown
Control
(from Power
Control Logic)
+CBYPASS
470 pF
(optional)
Bypass 5
R
V OUT = V REF × ⎛ -----2- + 1⎞
⎝R
⎠
1
R2
FIGURE 5:
SHDN
8
Typical Application Circuit (Adjustable Output).
CMOS LDOS SELECTION GUIDE
ISS
Typ.
(µA)
X
X
X
50
50
85
X
X
X
X
50
100
180
X
X
X
X
50
50
85
X
X
X
50
100
180
VDROP
Typ.
(mV)
Bypass
X
X
IOUT
Max
(mA)
5.0V
X
X
Error
Flag
4.0V
X
X
SHDN
3.6V
X
X
ADJ
3.3V
X
3.15V
3.0V
X
X
2.84V
X
X
2.8V
SOT-23-5
SOT-23-5
2.7V
TC1014
TC1015
Package
2.5V
2.85V
Output Voltage †
Part No.
TC1054
SOT-23-5
X
X
X
X
X
X
TC1055
SOT-23-5
X
X
X
X
X
X
TC1070
SOT-23-5
X
X
50
50
85
TC1071
SOT-23-5
X
X
50
100
180
TC1072
SOT-23-6
X
X
X
X
X
X
X
X
X
X
50
50
85
TC1073
SOT-23-6
X
X
X
X
X
X
X
X
X
X
50
100
180
TC1107
MSOP-8, SOIC-8
X
X
X
X
X
X
TC1108
SOT-223
X
X
X
X
TC1173
MSOP-8, SOIC-8
X
X
X
X
TC1174
MSOP-8, SOIC-8
TC1185
SOT-23-5
X
X
X
X
X
X
X
TC1186
SOT-23-5
X
X
X
X
X
X
X
X
300
240
300
240
X
50
100
180
X
X
50
300
240
X
X
X
50
150
270
X
X
X
50
150
270
X
X
X
X
50
50
TC1187
SOT-23-5
50
150
270
TC1188*
SOT-23-5
X
X
X
X
50
100
55
TC1189*
SOT-23-5
X
X
X
X
50
100
55
TC1223
SOT-23-5
X
X
X
X
X
X
X
X
X
50
50
85
TC1224
SOT-23-5
X
X
X
X
X
X
X
X
X
50
100
180
*
†
X
Pin Compatible Replacement for MAX8863/8864.
Custom Output Voltages Available - Contact Microchip Technology.
© 2007 Microchip Technology Inc.
DS00765B-page 5
AN765
NOTES:
DS00765B-page 6
© 2007 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
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conveyed, implicitly or otherwise, under any Microchip
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Trademarks
The Microchip name and logo, the Microchip logo, Accuron,
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PICmicro, PICSTART, PRO MATE, PowerSmart, rfPIC, and
SmartShunt are registered trademarks of Microchip
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Memory, MXDEV, MXLAB, PS logo, SEEVAL, SmartSensor
and The Embedded Control Solutions Company are
registered trademarks of Microchip Technology Incorporated
in the U.S.A.
Analog-for-the-Digital Age, Application Maestro, CodeGuard,
dsPICDEM, dsPICDEM.net, dsPICworks, ECAN,
ECONOMONITOR, FanSense, FlexROM, fuzzyLAB,
In-Circuit Serial Programming, ICSP, ICEPIC, Mindi, MiWi,
MPASM, MPLAB Certified logo, MPLIB, MPLINK, PICkit,
PICDEM, PICDEM.net, PICLAB, PICtail, PowerCal,
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SQTP is a service mark of Microchip Technology Incorporated
in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2007, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received ISO/TS-16949:2002 certification for its worldwide
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Tempe, Arizona, Gresham, Oregon and Mountain View, California. The
Company’s quality system processes and procedures are for its PIC®
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EEPROMs, microperipherals, nonvolatile memory and analog
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manufacture of development systems is ISO 9001:2000 certified.
© 2007 Microchip Technology Inc.
DS00765B-page 7
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Tel: 44-118-921-5869
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China - Xian
Tel: 86-29-8833-7250
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12/08/06
DS00765B-page 8
© 2007 Microchip Technology Inc.
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