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LT1585A Datasheet(PDF) 5 Page - ON Semiconductor

Part No. LT1585A
Description  5A LOW DROPOUT FAST RESPONSE POSITIVE ADJUSTABLE AND FIXED VOLTAGE REGULATOR
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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LT1585A
5
MOTOROLA ANALOG IC DEVICE DATA
The use of capacitors with low ESR, low ESL and good
high frequency characteristics is critical in meeting the output
voltage tolerances of these high speed microprocessors.
These requirements dictate a combination of high quality,
surface mount tantalum capacitors and ceramic capacitors.
The location of the decoupling network is critical to
transient response performance. Place the decoupling
network as close as possible to the processor pins because
trace runs from the decoupling capacitors to the processor
pins are inductive. The ideal location for the decoupling
network is actually inside the microprocessor socket cavity.
In addition, use large power and ground plane areas to
minimize distribution drops.
A possible stability problem that occurs in monolithic linear
regulators is current limit oscillations. The LT1585A
essentially has a flat current limit over the range of input
supply voltage. The lower current limit rating and 12V
maximum supply voltage rating for these devices permit this
characteristic.
Current limit oscillations are typically nonexistent, unless
the input and output decoupling capacitors for the regulators
are mounted several inches from the terminals.
Protection Diodes
In normal operation, the LT1585A does not require any
protection diodes. Older 3–terminal regulators require
protection diodes between the output pin and the input pin or
between the adjust pin and the output pin to prevent die
overstress.
Built–in internal resistors limit internal current paths on the
adjust pin. Therefore, even with bypass capacitors on the
adjust pin, no protection diode is needed to ensure device
safety under short–circuit conditions.
A protection diode between the input and output pins is
usually not needed. An internal diode between the input and
output pins on the LT1585A can handle microsecond surge
currents of 50A to 100A. Even with large value output
capacitors it is difficult to obtain those values of surge
currents in normal operation. Only with large values of output
capacitance, such as 1000
µF to 500µF, and with the input pin
instantaneously shorted to ground can damage occur. A
crowbar circuit at the input of the LT1585A can generate
those levels of current, and a diode from output to input is
then recommended. This is shown in Figure 10. Usually,
normal power supply cycling or system “hot plugging and
unplugging” will not generate current large enough to do any
damage.
The adjust pin can be driven on a transient basis
± 7V with
respect to the output, without any device degradation. As
with any IC regulator, exceeding the maximum
input–to–output voltage differential causes the internal
transistors to break down and none of the protection circuitry
is then functional.
Vout
Vin
LT1585A
ADJ
IN
OUT
D1
1N4002
(Optional)
+
+
+
R2
R1
C2
10
mF
C1
10
mF
C
Figure 10.
Ripple Rejection
A bypass capacitor from the adjust pin to ground reduces
the output ripple by the ratio of VOUT /1.25 V. The impedance
of the adjust pin capacitor at the ripple frequency should be
less than the value of R1 (typically in the rage of 100
Ω to
120
Ω) in the feedback divider network in Figure 10.
Therefore, the value of the required adjust pin capacitor is a
function of the input ripple frequency. For example, if R1
equals 100
Ω and the ripple frequency equals 120Hz, the
adjust pin capacitor should be 22
µF. At 10kHz, only 0.22µF is
needed.
Output Voltage
The LT1585A adjustable regulator develops a 1.25V
reference voltage between the output pin and the adjust pin
(see Figure 11). Placing a resistor R1 between these two
terminals causes a constant current to flow through R1 and
down through R2 to set the overall output voltage. Normally,
this current is the specified minimum load current of 10mA.
The current out of the adjust pin adds to the current from R1
and is typically 55
µA. Its output voltage contribution is small
and only needs consideration when very precise output
voltage setting is required.




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