LT1585A ON Semiconductor, LT1585A Datasheet - Page 4

• Manufacturer Part Number: LT1585A
• Description: 5A LOW DROPOUT FAST RESPONSE POSITIVE ADJUSTABLE AND FIXED VOLTAGE REGULATOR
• Manufacturer: ON Semiconductor

APPLICATIONS INFORMATION
General
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.
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.
from almost zero to amps in tens of nanoseconds. Output
voltage tolerances are tighter and include transient response
as part of the specification.
current load–step requirements of these microprocessors
and save total cost by needing less output capacitance in
order to maintain regulation.
Stability
output capacitor as part of the frequency compensation. For
all operating conditions, the addition of a 22 F solid tantalum
or a 100 F 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.
ranging up to
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 .
rejection and transient response. Bypassing the adjust pin
increases the required output capacitor value. The value of
22 F tantalum or 100 F aluminium covers all cases of
4
The LT1585A 3–terminal adjustable positive voltage
The LT1585A voltage regulator requires an output
Current generation microprocessors cycle load current
The LT1585A is specifically designed to meet the fast
The circuit design in the LT1585A requires the use of an
These capacitors differ in capacitor tolerance (sometimes
On the LT1585A, bypassing the adjust pin improves ripple
100
90
80
70
60
50
40
30
20
10
0
–60
Figure 7. LT1585A Adjust Pin Current vs
–40
–20
100%), equivalent series resistance,
0
Temperature
20
TEMPERATURE ( C)
40
60
80
100
OPERATING DESCRIPTION
120
140
LT1585A
160
bypassing the adjust terminal. With no adjust pin bypassing,
smaller values of capacitors provide equally good results.
microfarads are used on the output of the regulators to
ensure good transient response with heavy load current
changes.
values of output capacitance further improve the stability and
transient response of the LT1585A.
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.
instantaneous step in output voltage ( V = I
ESL of the output capacitors produces a droop proportional
to the rate of change of output current (V = L
output capacitance produces a change in output voltage
proportional to the time until the regulator can respond ( V =
t
Normally, capacitor values on the order of several hundred
Output capacitance can increase without limit and larger
Large load current changes are exactly the situation
The ESR of the output capacitors produces an
90
80
70
60
50
40
30
20
10
0
l/C). These transient effects are illustrated in Figure 9.
10
ESR
Effects
ESL
Effects
Figure 8. Ripple Rejection vs Frequency
Slope, V
t
MOTOROLA ANALOG IC DEVICE DATA
+
100
D
C
I
TEMPERATURE ( C)
Figure 9.
Regulator Takes Control
1000
Point at which
10,000
Capacitance
Effects
ESR). The
I/ t). The
100,000