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LT1585A Datasheet(PDF) 4 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
Logo ONSEMI - ON Semiconductor

LT1585A Datasheet(HTML) 4 Page - ON Semiconductor

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LT1585A
4
MOTOROLA ANALOG IC DEVICE DATA
Figure 7. LT1585A Adjust Pin Current vs
Temperature
Figure 8. Ripple Rejection vs Frequency
10
100,000
0
TEMPERATURE (
°C)
10,000
10
20
30
40
50
60
80
70
90
100
1000
–40
–60
20
160
0
TEMPERATURE (
°C)
40
100
10
20
40
30
50
60
70
90
80
100
0
–20
60
80
140
120
OPERATING DESCRIPTION
APPLICATIONS INFORMATION
General
The LT1585A 3–terminal adjustable positive voltage
regulator is easy to use and has all the protection features
expected in high performance linear regulators. The device is
short–circuit protected, safe–area protected and provides
thermal shutdown to turn off the regulator should the junction
temperature exceed about 150
°C.
The LT1585A voltage regulator requires an output
capacitor for stability. However, the improved frequency
compensation permits the use of capacitors with much lower
ESR while still maintaining stability. This is critical in
addressing the needs of modern, low voltage, high speed
microprocessors.
Current generation microprocessors cycle load current
from almost zero to amps in tens of nanoseconds. Output
voltage tolerances are tighter and include transient response
as part of the specification.
The LT1585A is specifically designed to meet the fast
current load–step requirements of these microprocessors
and save total cost by needing less output capacitance in
order to maintain regulation.
Stability
The circuit design in the LT1585A requires the use of an
output capacitor as part of the frequency compensation. For
all operating conditions, the addition of a 22
µF solid tantalum
or a 100
µF aluminium electrolytic on the output ensures
stability. Normally, the LT1585A can use smaller value
capacitors. Many different types of capacitors are available
and have widely varying characteristics.
These capacitors differ in capacitor tolerance (sometimes
ranging up to
±100%), equivalent series resistance,
equivalent series inductance and capacitance temperature
coefficient. The LT1585A frequency compensation optimizes
frequency response with low ESR capacitors. In general, use
capacitors with an ESR of less than 1
Ω.
On the LT1585A, bypassing the adjust pin improves ripple
rejection and transient response. Bypassing the adjust pin
increases the required output capacitor value. The value of
22
µF tantalum or 100µF aluminium covers all cases of
bypassing the adjust terminal. With no adjust pin bypassing,
smaller values of capacitors provide equally good results.
Normally, capacitor values on the order of several hundred
microfarads are used on the output of the regulators to
ensure good transient response with heavy load current
changes.
Output capacitance can increase without limit and larger
values of output capacitance further improve the stability and
transient response of the LT1585A.
Large load current changes are exactly the situation
presented by modern microprocessors. The load current step
contains higher order frequency components that the output
decoupling network must handle until the regulator throttles
to the load current level. Capacitors are not ideal elements
and contain parasitic resistance and inductance. These
parasitic elements dominate the change in output voltage at
the beginning of a transient load step change.
The ESR of the output capacitors produces an
instantaneous step in output voltage (
∆V = ∆I • ESR). The
ESL of the output capacitors produces a droop proportional
to the rate of change of output current (V = L
• ∆I/∆t). The
output capacitance produces a change in output voltage
proportional to the time until the regulator can respond (
∆V =
∆t • ∆l/C). These transient effects are illustrated in Figure 9.
Figure 9.
ESR
Effects
ESL
Effects
Capacitance
Effects
Slope, V
t +
DI
C
Point at which
Regulator Takes Control


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