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LT3020EDD-1.8#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
10
LT3020/LT3020-1.2/
LT3020-1.5/LT3020-1.8
3020fc
APPLICATIO S I FOR ATIO
WUUU
an output current change of 1mA to 100mA is typically
0.4mV at V
ADJ
= 200mV. At V
OUT
= 1.5V, load regulation is:
(1.5V/200mV) • (0.4mV) = 3mV
Output Capacitance and Transient Response
The LT3020’s design is stable with a wide range of output
capacitors, but is optimized for low ESR ceramic capaci-
tors. The output capacitor’s ESR affects stability, most
notably with small value capacitors. Use a minimum
output capacitor of 2.2µF with an ESR of 0.3Ω or less to
prevent oscillations. The LT3020 is a low voltage device,
and output load transient response is a function of output
capacitance. Larger values of output capacitance decrease
the peak deviations and provide improved transient re-
sponse for larger load current changes. For output capaci-
tor values greater than 20µF a small feedforward capacitor
with a value of 300pF across the upper divider resistor (R2
in Figure 1) is required.
Give extra consideration to the use of ceramic capacitors.
Manufacturers make ceramic capacitors with a variety of
dielectrics, each with a different behavior across tempera-
ture and applied voltage. The most common dielectrics are
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics
provide high C-V products in a small package at low cost,
but exhibit strong voltage and temperature coefficients.
The X5R and X7R dielectrics yield highly stable
characterisitics and are more suitable for use as the output
capacitor at fractionally increased cost. The X5R and X7R
dielectrics both exhibit excellent voltage coefficient char-
acteristics. The X7R type works over a larger temperature
range and exhibits better temperature stability whereas
X5R is less expensive and is available in higher values.
Figures 2 and 3 show voltage coefficient and temperature
coefficient comparisons between Y5V and X5R material.
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, simi-
lar 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 re-
sulting voltages produced can cause appreciable amounts
of noise. A ceramic capacitor produced Figure 4’s trace in
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3020 F02
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
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
3020 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 2. Ceramic Capacitor DC Bias Characteristics
Figure 3. Ceramic Capacitor Temperature Characteristics
1ms/DIV 3020 F04
1mV/DIV
V
OUT
= 1.3V
C
OUT
= 10µF
I
LOAD
= 0
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor
11
LT3020/LT3020-1.2/
LT3020-1.5/LT3020-1.8
3020fc
response to light tapping from a pencil. Similar vibration
induced behavior can masquerade as increased output
voltage noise.
No-Load/Light-Load Recovery
A possible transient load step that occurs is where the
output current changes from its maximum level to zero
current or a very small load current. The output voltage
responds by overshooting until the regulator lowers the
amount of current it delivers to the new level. The regulator
loop response time and the amount of output capacitance
control the amount of overshoot. Once the regulator has
decreased its output current, the current provided by the
resistor divider (which sets V
OUT
) is the only current
remaining to discharge the output capacitor from the level
to which it overshot. The amount of time it takes for the
output voltage to recover easily extends to milliseconds
with microamperes of divider current and a few microfar-
ads of output capacitance.
To eliminate this problem, the LT3020 incorporates a
no-load or light-load recovery circuit. This circuit is a
voltage-controlled current sink that significantly improves
the light load transient response time by discharging the
output capacitor quickly and then turning off. The current
sink turns on when the output voltage exceeds 6% of the
nominal output voltage. The current sink level is then
proportional to the overdrive above the threshold up to a
maximum of approximately 15mA. Consult the curve in
the Typical Performance Characteristics for the No-Load
Recovery Threshold.
If external circuitry forces the output above the no load
recovery circuit’s threshold, the current sink turns on in an
attempt to restore the output voltage to nominal. The
current sink remains on until the external circuitry releases
the output. However, if the external circuitry pulls the
output voltage above the input voltage, or the input falls
below the output, the LT3020 turns the current sink off and
shuts down the bias current/reference generator circuitry.
Thermal Considerations
The LT3020’s power handling capability is limited by its
maximum rated junction temperature of 125°C. The power
dissipated by the device is comprised of two components:
1. Output current multiplied by the input-to-output volt-
age differential: (I
OUT
)(V
IN
– V
OUT
) and
2. GND pin current multiplied by the input voltage:
(I
GND
)(V
IN
).
GND pin current is found by examining the GND pin
current curves in the Typical Performance Characteristics.
Power dissipation is equal to the sum of the two compo-
nents listed above.
The LT3020 regulator has internal thermal limiting (with
hysteresis) designed to protect the device during overload
conditions. For normal continuous conditions, do not
exceed the maximum junction temperature rating of 125°C.
Carefully consider all sources of thermal resistance from
junction to ambient including other heat sources mounted
in proximity to the LT3020.
The underside of the LT3020 DD package has exposed metal
(4mm
2
) from the lead frame to where the die is attached.
This allows heat to directly transfer from the die junction
to the printed circuit board metal to control maximum
operating junction temperature. The dual-in-line pin ar-
rangement allows metal to extend beyond the ends of the
package on the topside (component side) of a PCB. Con-
nect this metal to GND on the PCB. The multiple IN and OUT
pins of the LT3020 also assist in spreading heat to the PCB.
The LT3020 MS8 package has pin 4 fused with the lead
frame. This also allows heat to transfer from the die to the
printed circuit board metal, therefore reducing the thermal
resistance. Copper board stiffeners and plated through-
holes can also be used to spread the heat generated by
power devices.
The following tables list thermal resistance for several
different board sizes and copper areas for two different
packages. Measurements were taken in still air on 3/32"
FR-4 board with one ounce copper.
Table 1. Measured Thermal Resistance for DD Package
COPPER AREA THERMAL RESISTANCE
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
35°C/W
900mm
2
2500mm
2
2500mm
2
40°C/W
225mm
2
2500mm
2
2500mm
2
55°C/W
100mm
2
2500mm
2
2500mm
2
60°C/W
50mm
2
2500mm
2
2500mm
2
70°C/W
APPLICATIO S I FOR ATIO
WUUU
12
LT3020/LT3020-1.2/
LT3020-1.5/LT3020-1.8
3020fc
Table 2. Measured Thermal Resistance for MS8 Package
COPPER AREA THERMAL RESISTANCE
TOPSIDE* BACKSIDE BOARD AREA (JUNCTION-TO-AMBIENT)
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 1.8V, an input voltage
range of 2.25V to 2.75V, an output current range of 1mA
to 100mA, and a maximum ambient temperature of 70°C,
what will the maximum junction temperature be for an
application using the DD package?
The power dissipated by the device is equal to:
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
where
I
OUT(MAX)
= 100mA
V
IN(MAX)
= 2.75V
I
GND
at (I
OUT
= 100mA, V
IN
= 2.75V) = 3mA
so
P = 100mA(2.75V – 1.8V) + 3mA(2.75V) = 0.103W
The thermal resistance is in the range of 35°C/W to
70°C/W depending on the copper area. So the junction
temperature rise above ambient is approximately equal to:
0.103W(52.5°C/W) = 5.4°C
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maxi-
mum ambient temperature or:
T
JMAX
= 70°C + 5.4°C = 75.4°C
Protection Features
The LT3020 incorporates several protection features that
make it ideal for use in battery-powered circuits. In addi-
tion to the normal protection features associated with
monolithic regulators, such as current limiting and ther-
mal limiting, the device also protects against reverse-
input voltages, reverse-output voltages and reverse
output-to-input voltages.
APPLICATIO S I FOR ATIO
WUUU
Current limit protection and thermal overload protection
protect the device against current overload conditions at
the output of the device. For normal operation, do not
exceed a junction temperature of 125°C.
The IN pins of the device withstand reverse voltages of
10V. The LT3020 limits current flow to less than 1µA and
no negative voltage appears at OUT. The device protects
both itself and the load against batteries that are plugged
in backwards.
The LT3020 incurs no damage if OUT is pulled below
ground. If IN is left open circuit or grounded, OUT can be
pulled below ground by 10V. No current flows from the
pass transistor connected to OUT. However, current flows
in (but is limited by) the resistor divider that sets the output
voltage. Current flows from the bottom resistor in the
divider and from the ADJ pin’s internal clamp through the
top resistor in the divider to the external circuitry pulling
OUT below ground. If IN is powered by a voltage source,
OUT sources current equal to its current limit capability
and the LT3020 protects itself by thermal limiting. In this
case, grounding SHDN turns off the LT3020 and stops
OUT from sourcing current.
The LT3020 incurs no damage if the ADJ pin is pulled
above or below ground by 10V. If IN is left open circuit or
grounded and ADJ is pulled above ground, ADJ acts like a
25k resistor in series with a 1V clamp (one Schottky diode
in series with one diode). ADJ acts like a 25k resistor in
series with a Schottky diode if pulled below ground. If IN
is powered by a voltage source and ADJ is pulled below its
reference voltage, the LT3020 attempts to source its
current limit capability at OUT. The output voltage in-
creases to V
IN
– V
DROPOUT
with V
DROPOUT
set by whatever
load current the LT3020 supports. This condition can
potentially damage external circuitry powered by the
LT3020 if the output voltage increases to an unregulated
high voltage. If IN is powered by a voltage source and ADJ
is pulled above its reference voltage, two situations can
occur. If ADJ is pulled slightly above its reference voltage,
the LT3020 turns off the pass transistor, no output current
is sourced and the output voltage decreases to either the
voltage at ADJ or less. If ADJ is pulled above its no load
recovery threshold, the no load recovery circuitry turns on
and attempts to sink current. OUT is actively pulled low
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LT3020EDD-1.8#TRPBF

Mfr. #:
Buy LT3020EDD-1.8#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LDO Voltage Regulators 1.8V Fixed Output 100mA VLDO in DFN
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