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LT3021EDH-1.8#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LT3021/LT3021-1.2/
LT3021-1.5/LT3021-1.8
10
3021fc
APPLICATIONS INFORMATION
The LT3021 is a very low dropout linear regulator capable
of 1V input supply operation. Devices supply 500mA of
output current and dropout voltage is typically 155mV.
Quiescent current is typically 120μA and drops to 3μA in
shutdown. The LT3021 incorporates several protection
features, making it ideal for use in battery-powered sys-
tems. The device protects itself against reverse-input and
reverse-output voltages. In battery backup applications
where the output is held up by a backup battery when the
input is pulled to ground, the LT3021 acts as if a diode is
in series with its output which prevents reverse current
fl ow. In dual supply applications where the regulator
load is returned to a negative supply, the output can be
pulled below ground by as much as 10V without affecting
start-up or normal operation.
Adjustable Operation
The LT3021’s output voltage range is 0.2V to 9.5V. Figure
1 shows that the output voltage is set by the ratio of two
external resistors. The device regulates the output to main-
tain the ADJ pin voltage at 200mV referenced to ground.
The current in R1 equals 200mV/R1 and the current in R2
is the current in R1 minus the ADJ pin bias current. The
ADJ pin bias current of 20nA fl ows out of the pin. Use
the formula in Figure 1 to calculate output voltage. An R1
value of 20k sets the resistor divider current to 10μA. Note
that in shutdown the output is turned off and the divider
current is zero. Curves of ADJ Pin Voltage vs Temperature
and ADJ Pin Bias Current vs Temperature appear in the
Typical Performance Characteristics section.
Specifi cations for output voltages greater than 200mV
are proportional to the ratio of desired output voltage to
200mV; (V
OUT
/200mV). For example, load regulation for
an output current change of 1mA to 500mA 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 LT3021’s design is stable with a wide range of output
capacitors, but is optimized for low ESR ceramic capacitors.
The output capacitor’s ESR affects stability, most notably
with small value capacitors. Use a minimum output ca-
pacitor of 3.3μF with an ESR of 0.2Ω or less to prevent
oscillations. The LT3021 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 response for
larger load current changes. For output capacitor values
greater than 22μF a small feedforward capacitor with a
value of 300pF across the upper divider resistor (R2 in
Figure 1) is required. Under extremely low output current
conditions (I
LOAD
< 30μA) a low frequency small signal
oscillation (200Hz/8mV
P-P
at 1.2V output) can occur.
A minimum load of 100μA is recommended to prevent
this instability.
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 coeffi cients.
The X5R and X7R dielectrics yield highly stable character-
isitics and are more suitable for use as the output capacitor
at fractionally increased cost. The X5R and X7R dielectrics
both exhibit excellent voltage coeffi cient characteristics.
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 coeffi cient and temperature coeffi cient
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,
similar to the way a piezoelectric accelerometer or micro-
phone 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
Figure 1. Adjustable Operation
IN
SHDN
R2
R1
3021 F01
OUT
V
IN
ADJ
GND
LT3021
V
OUT
+
R2
R1
V
OUT
= 200mV
V
ADJ
= 200mV
I
ADJ
= 20nA AT 25°C
OUTPUT RANGE = 0.2V TO 9.5V
1 + – I
ADJ
(R2)
()
LT3021/LT3021-1.2/
LT3021-1.5/LT3021-1.8
11
3021fc
APPLICATIONS INFORMATION
amounts of noise. A ceramic capacitor produced Figure
4’s trace in response to light tapping from a pencil. Similar
vibration induced behavior can masquerade as increased
output voltage noise.
No-Load/Light-Load Recovery
A transient load step occurs when 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 microfarads of output capacitance.
To eliminate this problem, the LT3021 incorporates a
no-load or light-load recovery circuit. This circuit is a
voltage-controlled current sink that signifi cantly improves
the light load transient response time by discharging the
output capacitor quickly and then turning off. The cur-
rent 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 LT3021 turns the current sink off and shuts
down the bias current/reference generator circuitry.
Thermal Considerations
The LT3021’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 voltage
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 components
listed above.
Figure 2. Ceramic Capacitor DC Bias Characteristics
Figure 3. Ceramic Capacitor Temperature Characteristics
Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3021 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
3021 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
1ms/DIVV
OUT
= 1.3V
C
OUT
= 10F
I
LOAD
= 0
3021 F04
1mV/DIV
LT3021/LT3021-1.2/
LT3021-1.5/LT3021-1.8
12
3021fc
APPLICATIONS INFORMATION
The LT3021 regulator has internal thermal limiting (with
hysteresis) designed to protect the device during overload
conditions. For normal continuous conditions, do not ex-
ceed 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 LT3021.
The underside of the LT3021 DH package has exposed
metal (14mm
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 arrangement allows metal to extend beyond the ends
of the package on the topside (component side) of a PCB.
Connect this metal to GND on the PCB. The multiple IN
and OUT pins of the LT3021 also assist in spreading heat
to the PCB.
The LT3021 S8 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 DH Package
COPPER AREA
TOPSIDE* BACKSIDE BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
30°C/W
900mm
2
2500mm
2
2500mm
2
35°C/W
225mm
2
2500mm
2
2500mm
2
50°C/W
100mm
2
2500mm
2
2500mm
2
55°C/W
50mm
2
2500mm
2
2500mm
2
65°C/W
Table 2. Measured Thermal Resistance For S8 Package
COPPER AREA
TOPSIDE* BACKSIDE BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
70°C/W
1000mm
2
2500mm
2
2500mm
2
70°C/W
225mm
2
2500mm
2
2500mm
2
78°C/W
100mm
2
2500mm
2
2500mm
2
84°C/W
50mm
2
2500mm
2
2500mm
2
96°C/W
*Device is mounted on topside.
Calculating Junction Temperature
Example: Given an output voltage of 1.2V, an input voltage
range of 1.8V ±10%, an output current range of 1mA to
500mA, and a maximum ambient temperature of 70°C,
what will the maximum junction temperature be for an
application using the DH 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)
= 500mA
V
IN(MAX)
= 1.98V
I
GND
at (I
OUT
= 500mA, V
IN
= 1.98V) = 10mA
so
P = 500mA(1.98V – 1.2V) + 10mA(1.98V) = 0.41W
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:
0.41W(52.5°C/W) = 21.5°C
The maximum junction temperature equals the maximum
junction temperature rise above ambient plus the maximum
ambient temperature or:
T
JMAX
= 21.5°C + 70°C = 91.5°C
Protection Features
The LT3021 incorporates several protection features
that make it 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 device also protects against reverse-
input voltages, reverse-output voltages and reverse out-
put-to-input voltages.
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 LT3021 limits current fl ow 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.
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LT3021EDH-1.8#PBF

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Buy LT3021EDH-1.8#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LDO Voltage Regulators 1.8V Fixed Output 500mA VLDO in DFN (5x5)
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New from this manufacturer.
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