1. See thermal regulation specifications for changes in output voltage due to heating effects. Load and line regulation are measured at a constant
junction temperature by low duty cycle pulse testing.
2. Line and load regulations are guaranteed up to the maximum power dissipation 28.8W for the LT1585A in T–package. Power dissipation is
determined by input/output differential and the output current. Guaranteed maximum output power will not be available over the full input/output voltage range.
3. The LT1585A has constant current limit with changes in input–to–output voltage.
2
MOTOROLA ANALOG IC DEVICE DATA
LT1585A
Figure 1. Dropout Voltage vs Output Current
1.4
1.3
DROPOUT VOLTAGE (V)
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0
1.0
2.0
3.0
4.0
5.0
6.0
OUTPUT CURRENT (A)
–5°C
25°C
125°C
Isc (A)
7.0
6.8
6.6
6.4
6.2
6.0
5.8
5.6
5.4
5.2
5.0
–60 –40
Figure 2. Short–Circuit Current vs Temperature
–20
0
20
40
60
80
100
120
140
TEMPERATURE (°C)
Figure 3. Load Regulation vs Temperature
0.1
0.05
LOAD REGULATION (%)
0
–0.05
–0.1
–0.15
–0.2
–60 –40
V ref (V)
1.275
1.27
1.265
1.26
1.255
1.25
1.245
1.24
1.235
Figure 4. LT1585A Reference Voltage vs
Temperature
–20
0
20
40
60
80
100
120
140
1.23
1.225
–60 –40 –20
0
20
40
60
80
100 120
140 160
TEMPERATURE (°C)
TEMPERATURE (°C)
Figure 5. LT1585A–1.5 Output Voltage vs
Temperature
1.515
1.51
1.505
Vout (V)
Iq (mA)
1.5
1.495
1.49
1.485
–60 –40
10
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
Figure 6. LT1585A–1.5 Quiescent Current vs
Temperature
–20
0
20
40
60
80
100
120
140
5.5
5.0
–60 –40
–20
0
20
40
60
80
100
120
140
TEMPERATURE (°C)
TEMPERATURE (°C)
MOTOROLA ANALOG IC DEVICE DATA
3
LT1585A
Figure 7. LT1585A Adjust Pin Current vs
Temperature
Figure 8. Ripple Rejection vs Frequency
90
80
70
60
V ref (V)
50
40
30
20
10
0
0
20
40
60
80
100 120
140 160
10
100
1000
TEMPERATURE (°C)
10,000
100,000
TEMPERATURE (°C)
100
90
80
70
Iadj (
m
A)
60
50
40
30
20
10
0
–60 –40 –20
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 22µF solid tantalum
or a 100µF 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
22µF 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
Slope, V
t
Capacitance
Effects
+
D
I
C
Point at which
Regulator Takes Control
4
MOTOROLA ANALOG IC DEVICE DATA
LT1585A
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
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