lt1963aes8-trpbf Linear Technology Corporation, lt1963aes8-trpbf Datasheet - Page 15

• Manufacturer Part Number: lt1963aes8-trpbf
• Description: 1.5a, Low Noise, Fast Transient Response Ldo Regulators
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Overload Recovery
Like many IC power regulators, the LT1963A-X has safe
operating area protection. The safe area protection de-
creases the current limit as input-to-output voltage in-
creases and keeps the power transistor inside a safe
operating region for all values of input-to-output voltage.
The protection is designed to provide some output current
at all values of input-to-output voltage up to the device
breakdown.
When power is first turned on, as the input voltage rises,
the output follows the input, allowing the regulator to start
up into very heavy loads. During the start-up, as the input
voltage is rising, the input-to-output voltage differential is
small, allowing the regulator to supply large output
currents. With a high input voltage, a problem can occur
wherein removal of an output short will not allow the
output voltage to recover. Other regulators, such as the
LT1085, also exhibit this phenomenon, so it is not unique
to the LT1963A-X.
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Com-
mon situations are immediately after the removal of a
short-circuit or when the shutdown pin is pulled high after
the input voltage has already been turned on. The load line
for such a load may intersect the output current curve at
two points. If this happens, there are two stable output
operating points for the regulator. With this double inter-
section, the input power supply may need to be cycled down
to zero and brought up again to make the output recover.
Output Voltage Noise
The LT1963A regulators have been designed to provide
low output voltage noise over the 10Hz to 100kHz band-
width while operating at full load. Output voltage noise is
typically 40nV/√Hz over this frequency bandwidth for the
LT1963A (adjustable version). For higher output voltages
(generated by using a resistor divider), the output voltage
noise will be gained up accordingly. This results in RMS
noise over the 10Hz to 100kHz bandwidth of 14µV
the LT1963A increasing to 38µV
Higher values of output voltage noise may be measured
when care is not exercised with regard to circuit layout
and testing. Crosstalk from nearby traces can induce
U
U
RMS
W
for the LT1963A-3.3.
U
RMS
for
unwanted noise onto the output of the LT1963A-X. Power
supply ripple rejection must also be considered; the
LT1963A regulators do not have unlimited power supply
rejection and will pass a small portion of the input noise
through to the output.
Thermal Considerations
The power handling capability of the device is limited by the
maximum rated junction temperature (125°C). The power
dissipated by the device is made up of two components:
1. Output current multiplied by the input/output voltage
2. GND pin current multiplied by the input voltage:
The GND pin current can be found using the GND Pin
Current curves in the Typical Performance Characteris-
tics. Power dissipation will be equal to the sum of the two
components listed above.
The LT1963A 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 tables list 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 3. Q Package, 5-Lead DD
*Device is mounted on topside
TOPSIDE*
2500mm
1000mm
125mm
differential: (I
(I
GND
COPPER AREA
)(V
2
2
2
IN
BACKSIDE
2500mm
2500mm
2500mm
).
OUT
)(V
2
2
2
IN
BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2500mm
2500mm
– V
OUT
LT1963A Series
2
2
2
), and
THERMAL RESISTANCE
23°C/W
25°C/W
33°C/W
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