LT1963A Series
W U U
U
APPLICATIO S I FOR ATIO
40
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.
20
X5R
0
–20
–40
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.
Y5V
–60
–80
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
–100
50
TEMPERATURE (°C)
100 125
–50 –25
0
25
75
1963 F04
Figure 4. Ceramic Capacitor Temperature Characteristics
an effective value as low as 1mF to 2mF 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.
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 14mVRMS for
the LT1963A increasing to 38mVRMS for the LT1963A-3.3.
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.
Higher values of output voltage noise may be measured
when care is not exercised with regards to circuit layout
and testing. Crosstalk from nearby traces can induce
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.
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.
Theprotectionisdesignedtoprovidesomeoutputcurrent
at all values of input-to-output voltage up to the device
breakdown.
Thermal Considerations
Thepowerhandlingcapabilityofthedeviceislimitedbythe
maximum rated junction temperature (125∞C). The power
dissipated by the device is made up of two components:
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
1. Output current multiplied by the input/output voltage
differential: (IOUT)(VIN – VOUT), and
1963af
12
Linear [ Linear ]
