LT1761 Series
100mA, Low Noise,
LDO Micropower
Regulators in TSOT-23
FEATURES
DESCRIPTION
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The LT®1761 series are micropower, low noise, low
dropout regulators. With an external 0.01μF bypass
capacitor, output noise drops to 20μVRMS over a 10Hz to
100kHz bandwidth. Designed for use in battery-powered
systems, the low 20μA quiescent current makes them an
ideal choice. In shutdown, quiescent current drops to less
than 0.1μA. The devices are capable of operating over an
input voltage from 1.8V to 20V, and can supply 100mA of
output current with a dropout voltage of 300mV. Quiescent
current is well controlled, not rising in dropout as it does
with many other regulators.
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Low Noise: 20μVRMS (10Hz to 100kHz)
Low Quiescent Current: 20μA
Wide Input Voltage Range: 1.8V to 20V
Output Current: 100mA
Very Low Shutdown Current: 3300pF
1.5
1.0
0.5
CHANGE IN VALUE (%)
3.0
–20
–40
1
3
2
4 5 6 7 8 9 10
OUTPUT CAPACITANCE (μF)
1761 F02
Figure 2. Stability
Y5V
–60
–80
0
X5R
0
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
–100
–50 –25
50
25
75
0
TEMPERATURE (°C)
100
125
1761 F04
Figure 4. Ceramic Capacitor Temperature Characteristics
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16
LT1761 Series
APPLICATIONS INFORMATION
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress,
similar to the way a piezoelectric accelerometer or microphone works. For a ceramic capacitor the stress can be
induced by vibrations in the system or thermal transients.
The resulting voltages produced can cause appreciable
amounts of noise, especially when a ceramic capacitor is
used for noise bypassing. A ceramic capacitor produced
Figure 5’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
The ground pin current can be found by examining the
GND Pin Current curves in the Typical Performance Characteristics section. Power dissipation will be equal to the
sum of the two components listed above.
The LT1761 series regulators have internal thermal limiting
designed to protect the device during overload conditions.
For continuous normal conditions, the maximum junction
temperature rating of 125°C must not be exceeded. It is
important to give careful consideration to all sources of
thermal resistance from junction to ambient. Additional
heat sources mounted nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat generated by power devices.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
VOUT
500μV/DIV
Table 1. Measured Thermal Resistance
LT1761-5
COUT = 10μF
CBYP = 0.01μF
ILOAD = 100mA
100ms/DIV
1761 F05
Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125°C). The
power dissipated by the device will be made up of two
components:
1. Output current multiplied by the input/output voltage
differential: (IOUT)(VIN – VOUT), and
COPPER AREA
THERMAL RESISTANCE
BOARD AREA (JUNCTION-TO-AMBIENT)
TOPSIDE*
BACKSIDE
2500mm2
2500mm2
2500mm2
125°C/W
1000mm2
2500mm2
2500mm2
125°C/W
225mm2
2500mm2
2500mm2
130°C/W
100mm2
2500mm2
2500mm2
135°C/W
50mm2
2500mm2
2500mm2
150°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4V to 6V, an output current range of 0mA to 50mA
2. GND pin current multiplied by the input voltage:
(IGND)(VIN).
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17
LT1761 Series
APPLICATIONS INFORMATION
and a maximum ambient temperature of 50°C, what will
the maximum junction temperature be?
The power dissipated by the device will be equal to:
IOUT(MAX)(VIN(MAX) – VOUT) + IGND(VIN(MAX))
where,
IOUT(MAX) = 50mA
VIN(MAX) = 6V
IGND at (IOUT = 50mA, VIN = 6V) = 1mA
So,
P = 50mA(6V – 3.3V) + 1mA(6V) = 0.14W
The thermal resistance will be in the range of 125°C/W to
150°C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
0.14W(150°C/W) = 21.2°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
TJMAX = 50°C + 21.2°C = 71.2°C
Protection Features
The LT1761 regulators incorporate several protection
features which make them ideal for use in battery-powered circuits. In addition to the normal protection features
associated with monolithic regulators, such as current
limiting and thermal limiting, the devices are protected
against reverse input voltages, reverse output voltages
and reverse voltages from output to input.
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal operation,
the junction temperature should not exceed 125°C.
The input of the device will withstand reverse voltages
of 20V. Current flow into the device will be limited to less
than 1mA (typically less than 100μA) and no negative
voltage will appear at the output. The device will protect
both itself and the load. This provides protection against
batteries which can be plugged in backward.
The output of the LT1761-X can be pulled below ground
without damaging the device. If the input is left open circuit
or grounded, the output can be pulled below ground by
20V. For fixed voltage versions, the output will act like a
large resistor, typically 500k or higher, limiting current flow
to typically less than 100μA. For adjustable versions, the
output will act like an open circuit; no current will flow out
of the pin. If the input is powered by a voltage source, the
output will source the short-circuit current of the device
and will protect itself by thermal limiting. In this case,
grounding the SHDN pin will turn off the device and stop
the output from sourcing the short-circuit current.
The ADJ pin of the adjustable device can be pulled above
or below ground by as much as 7V without damaging the
device. If the input is left open circuit or grounded, the
ADJ pin will act like an open circuit when pulled below
ground and like a large resistor (typically 100k) in series
with a diode when pulled above ground.
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18
LT1761 Series
APPLICATIONS INFORMATION
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp voltage if the output is pulled high, the ADJ pin input current
must be limited to less than 5mA. For example, a resistor
divider is used to provide a regulated 1.5V output from the
1.22V reference when the output is forced to 20V. The top
resistor of the resistor divider must be chosen to limit the
current into the ADJ pin to less than 5mA when the ADJ
pin is at 7V. The 13V difference between output and ADJ
pin divided by the 5mA maximum current into the ADJ pin
yields a minimum top resistor value of 2.6k.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled
to ground, pulled to some intermediate voltage or is left
open circuit. Current flow back into the output will follow
the curve shown in Figure 6.
When the IN pin of the LT1761-X is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input current will typically drop to less than 2μA. This can happen
if the input of the device is connected to a discharged
(low voltage) battery and the output is held up by either
a backup battery or a second regulator circuit. The state
of the SHDN pin will have no effect on the reverse output
current when the output is pulled above the input.
REVERSE OUTPUT CURRENT (μA)
100
TJ = 25°C
LT1761-BYP
LT1761-SD
90 VIN = 0V
CURRENT FLOWS
80 INTO OUTPUT PIN
LT1761-1.2
70 VOUT = VADJ
(LT1761-BYP, -SD)
60
LT1761-1.5
LT1761-1.8
50
LT1761-2
LT1761-2.5
40
LT1761-2.8
30
LT1761-3
20
LT1761-3.3
10
LT1761-5
0
0
1
2
3 4 5 6 7 8
OUTPUT VOLTAGE (V)
9
10
1761 F06
Figure 6. Reverse Output Current
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19
LT1761 Series
PACKAGE DESCRIPTION
S5 Package
5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.62
MAX
0.95
REF
2.90 BSC
(NOTE 4)
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
0.95 BSC
0.80 – 0.90
0.20 BSC
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
0.09 – 0.20
(NOTE 3)
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
1.90 BSC
S5 TSOT-23 0302 REV B
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20
LT1761 Series
REVISION HISTORY
(Revision history begins at Rev F)
REV
DATE
DESCRIPTION
PAGE NUMBER
F
5/10
Added MP-grade
2, 3
Added Typical Application
22
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
21
LT1761 Series
TYPICAL APPLICATION
Noise Bypassing Provides Soft-Start
Startup Time
100
IN
VIN
5.4V TO 20V
5V
AT 100mA
OUT
LT1761-5
1μF
CBYP
10μF
OFF ON
STARTUP TIME (ms)
BYP
SHDN
GND
1761 TA02a
10
1
0.1
10
100
1000
10000
CBYP (pF)
1761 TA02b
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22 Linear Technology Corporation
LT 0510 REV F • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
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