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LT1962EMS8-3#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
LT1962 Series
13
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applicaTions inForMaTion
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used
are Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitance
in a small package, but exhibit strong voltage and tem
-
perature coefficients as shown in Figures 4 and 5. When
used with a 5V regulator, a 10µF Y5V capacitor can exhibit
an effective value as low as 1µF to 2µF over the operating
temperature range. The X5R and X7R dielectrics result in
more stable characteristics and are more suitable for use
as the output capacitor. The X7R type has better stability
across temperature, while the X5R is less expensive and
is available in higher values.
The LT1962-X is a micropower device and output tran
-
sient response
will be a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes. Bypass capacitors, used to
decouple individual components powered by the LT1962,
will increase the
effective output capacitor value. With
larger
capacitors used to bypass the reference (for low
noise operation), larger values of output capacitance are
needed. For 100pF of bypass capacitance, 4.7µF of output
capacitor is recommended. With a 1000pF bypass capaci
-
tor or larger, a 6.8µF output capacitor is recommended.
The
shaded region of Figure 3 defines the range over
which the LT1962 regulators are stable. The minimum ESR
needed is defined by the amount of bypass capacitance
used, while the maximum ESR is 3Ω.
Figure 3. Stability
OUTPUT CAPACITANCE (µF)
1
ESR (Ω)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
3
10
1962 F03
2 4 5
6
7 8
9
STABLE REGION
C
BYP
= 330pF
C
BYP
≥ 1000pF
C
BYP
= 100pF
C
BYP
= 0
Figure 5. Ceramic Capacitor Temperature Characteristics
Figure 4. Ceramic Capacitor DC Bias Characteristics
Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
1962 F04
20
0
–20
–40
–60
–80
–100
0
4
8
10
2 6
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
1962 F05
–25 0
50 100
125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
100ms/DIV
1962 F06
V
OUT
500µV/DIV
LT1962-5
C
OUT
= 10µF
C
BYP
= 0.01µF
I
LOAD
= 100mA
LT1962 Series
14
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For more information www.linear.com/LT1962
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 6’s trace in response to light tapping from a pencil.
Similar vibration induced behavior can masquerade as
increased output voltage noise.
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: (I
OUT
)(V
IN
– V
OUT
), and
2. GND pin current multiplied by the input voltage:
(I
GND
)(V
IN
).
The GND pin current can be found by examining the GND
Pin Current curves in the Typical Performance Character
-
istics section. Power dissipation will be equal to the sum
of the two components listed above.
The
LT1962 series
regulators have internal thermal
limiting designed to protect the device during overload
conditions. For continuous normal conditions, the maxi
-
mum 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 ambi-
ent. 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 gener
-
ated 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 1/16" FR-4 board with one ounce
copper.
Table 1. Measured Thermal Resistance
COPPER AREA
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)TOPSIDE* BACKSIDE
2500mm
2
2500mm
2
2500mm
2
110°C/W
1000mm
2
2500mm
2
2500mm
2
115°C/W
225mm
2
2500mm
2
2500mm
2
120°C/W
100mm
2
2500mm
2
2500mm
2
130°C/W
50mm
2
2500mm
2
2500mm
2
140°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input volt-
age range of 4V to 6V, an output current range of 0mA
to 100mA 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:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where,
I
OUT(MAX)
= 100mA
V
IN(MAX)
= 6V
I
GND
at (I
OUT
= 100mA, V
IN
= 6V) = 2mA
So,
P = 100mA(6V – 3.3V) + 2mA(6V) = 0.28W
The thermal resistance will be in the range of 110°C/W to
140°C/W depending on the copper area. So the junction
temperature rise above ambient will be approximately
equal to:
0.28W(125°C/W) = 35.3°C
The maximum junction temperature will then be equal to
the maximum junction temperature rise above ambient
plus the maximum ambient temperature or:
T
JMAX
= 50°C + 35.3°C = 85.3°C
LT1962 Series
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For more information www.linear.com/LT1962
applicaTions inForMaTion
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 OUT 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 7.
When the IN pin of the LT1962 is forced below the OUT
pin or the OUT pin is pulled above the IN pin, input cur
-
rent 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.
Protection Features
The LT1962 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 opera
-
tion, 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 LT1962 can be pulled below ground
without damaging the device. If the input is left open cir
-
cuit 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 less than 40µA. 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.
In situations where the ADJ pin is connected to a resistor
divider that would pull the ADJ pin above its 7V clamp volt
-
age if the output is pulled high, the ADJ pin input current
must
be limited to less than 5mA. For example, a resistor
Figure 7. Reverse Output Current
OUTPUT VOLTAGE (V)
0 1
REVERSE OUTPUT CURRENT (µA)
30
40
50
60
70
80
90
100
8 97
1962 F07
20
10
0
2 3
4
6
5
10
LT1962
LT1962-5
T
J
= 25°C
V
IN
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
V
OUT
= V
ADJ
(LT1962)
LT1962-1.5
LT1962-1.8
LT1962-2.5
LT1962-3
LT1962-3.3
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LT1962EMS8-3#TRPBF

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Buy LT1962EMS8-3#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LDO Voltage Regulators 300mA, L N, uP LDO Regs
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