AS2887
3A Low Dropout Voltage Regulator
Adjustable & Fixed Output,
Fast Response
FEATURES
•
Adjustable Output Down To 1.2V
•
Fixed Output Voltages 1.5, 2.5, 3.3, 5.0V
•
Output Current Of 3A
•
Low Dropout Voltage 1.1V Typ.
•
Extremely Tight Load And Line Regulation
•
Current & Thermal Limiting
•
Standard 3-Terminal Low Cost TO-220, TO-263 & TO-252
•
Similar To Industry Standard LT1085/LT1585
APPLICATIONS
•
Powering VGA & Sound Card
•
Power PCSupplies
•
SMPS Post-Regulator
•
High Efficiency “Green” Computer Systems
•
High Efficiency Linear Power Supplies
•
Portable Instrumentation
•
Constant Current Regulators
•
Adjustable Power Supplies
•
Battery charger
PRODUCT DESCRIPTION
The AS2887 are low power 3A adjustable and fixed voltage regulators that are very easy to use. It requires only 2 external resistors to
set the output voltage for adjustable version. The AS2887 is designed for low voltage applications that offer lower dropout voltage
and faster transient response. This device is an excellent choice for use in powering low voltage microprocessor that require a lower
dropout, faster transient response to regulate from +2.5V to 3.8V supplies and as a post regulator for switching supplies applications.
The AS2887 features low dropout of a maximum of 1.5 volts.
The AS2887 offers full protection against over-current faults, reversed input polarity, reversed load insertion, and positive and
negative transient voltage. On-Chip trimming adjusts the reference voltage to 1%. The I
Q
of this device flows into load which
increases efficiency.
The AS2887 are offered in a 3-pin TO-220, TO-263 & TO-252 packages compatible with other 3 terminal regulators. For a 8A low
dropout regulator refer to the AS2880 data sheet.
ORDERING INFORMATION
TO-220
3-PIN
AS2887YU
AS2887YU-X
DD PLASTIC
3-PIN
AS2887YT
AS2887YT-X
TO-252
DPAK
AS2887YR
AS2887YR-X
TO-263-3 (T)
PIN CONNECTIONS
TO-220-3 (U)
TO-252 (R)
X = Output Voltage (i.e. 1.5 for 1.5V, 2.5 for 2.5V etc.)
Y = Output Tolerance, A for 1%
Blank for 2%
Consult with factory for other fixed output voltages
AS2887
1
2
3
AS2887
AS2887
1
2
3
1
2
3
ADJ/GND V
OUT
V
IN
ADJ/GND V
OUT
V
IN
Front View
ADJ/GND V
OUT
V
IN
Top View
Front View
Rev. 10/11/00
AS2887
ABSOLUTE MAXIMUM RATINGS
Lead Temp. (Soldering, 10 Seconds) .............................. 300°C
Storage Temperature Range ............................ -65° to +150°C
Operating Junction Temperature Range ......................
AS2887 Control Section ............................ -45°C +125°C
AS2887 Power Transistor...........................-45°C +150°C
Input Voltage........................................................ 30V
Input to Output Voltage Differential Max ............ 30V
ELECTRICAL CHARACTERISTICS
(NOTE 1) at I
OUT
= 10mA, T
A
=25°C, unless otherwise specified.
AS2887A
PARAMETER
CONDITIONS
Typ
Min
Max
1.515
1.530
2.525
2.550
3.330
3.360
5.05
5.10
AS2887
Min
Max
1.470
1.455
1.530
1.545
Units
V
1.5V Version
Output Voltage (Note 2)
2.5V Version
Output Voltage (Note 2)
3.3V Version
Output Voltage (Note 2)
5.0V Version
Output Voltage (Note 2)
Adjustable Version
Reference Voltage (V
REF
)
All Voltage Options
Min. Load Current
(Note 3)
Line Regulation (∆V
REF
(V
IN
))
AS2887-1.5V, 0
≤I
OUT
≤3A,
2.75V≤V
IN
≤29V
1.5
1.5
2.5
2.5
3.3
3.3
5.0
5.0
1.485
1.470
2.475
2.450
3.270
3.240
4.95
4.90
AS2887-2.5V,0
≤I
OUT
≤3A,
3.5V≤V
IN
≤29V
2.450
2.425
3.230
3.201
4.900
4.850
2.550
2.575
3.370
3.399
5.100
5.150
V
AS2887-3.3V, 0
≤I
OUT
≤3A,
4.75V≤V
IN
≤29V
V
AS2887-5.0V, 0
≤I
OUT
≤3A,
5.5V≤V
IN
≤29V
V
V
IN
≤
7V, P≤ P
MAX
1.5V≤ (V
IN -
V
OUT
)≤(V
IN
-V
OUT
)
MAX
,
10mA≤I
OUT
≤3A
1.5V≤ (V
IN -
V
OUT
)≤ (V
IN
-V
OUT
)
MAX
2.75V≤V
IN
≤29V,
I
OUT
=10mA, T
J
=25°C (Note 3)
V
IN
≤29V,
I
OUT
=0mA, T
J
=25°C (Note 2)
1.250
1.238
1.263
1.225
1.270
V
5
0.005
0.005
0.05
10
0.2
0.2
0.3
10
0.2
0.2
0.3
mA
%
%
%
Load Regulation(∆V
REF
(I
OUT
))
10mA≤I
OUT
≤3A,
(V
IN
-V
OUT
)=3V, T
J
=25°C (Note
3)
0≤I
OUT
≤3A,
V
IN
=7V, T
J
=25°C (Note 2)
0.05
1.1
0.3
1.5
0.3
1.5
%
V
A
Dropout Voltage
Current Limit
I
OUT
(
MAX
)
Long Term Stability
Thermal Regulation
(∆V
OUT
(Pwr))
Temperature Stability
(∆V
OUT
(T))
Output Noise, RMS
Thermal Resistance
∆V
REF
=1%
I
OUT
=3A (Note 3)
I
OUT
≤
3A (Note 2)
V
IN
=7V
1.4V≤ (V
IN
- V
OUT
) (Note3)
T
A
=125°C, 1000 Hrs.
T
A
=25°C, 20 ms pulse
4.0
0.3
(Note 2)
0.01
0.25
3.2
1
0.020
3.2
1
0.020
%
%/W
%
% V
O
3.0
60
3.0
60
3.0
60
3.0
60
°C/W
°C/W
10Hz to 10kHz
TO-220
DD Package
T
A
=25°C
Junction to Tab
Junction to Ambient
Junction to Tab
Junction to Ambient
0.003
3.0
60
3.0
60
The Bold specifications apply to the full operating temperature range.
Note 1: Changes in output voltage due to heating effects are covered under the specification for thermal regulation.
Note 2: Fixed Version Only
Note 3: Adjustable Version Only
Rev. 10/11/00
AS2887
APPLICATION HINTS
The AS2887 incorporates protection against over-current
faults, reversed load insertion, over temperature operation, and
positive and negative transient voltage. However, the use of
an output capacitor is required in order to insure the stability
and the performances.
Stability
The output capacitor is part of the regulator’s frequency
compensation system. Either a 220µF aluminum electrolytic
capacitor or a 47µF solid tantalum capacitor between the
output terminal and ground guarantees stable operation for all
operating conditions.
However, in order to minimize overshoot and undershoot, and
therefore optimize the design, please refer to the section
‘Ripple Rejection’.
Ripple Rejection
Ripple rejection can be improved by adding a capacitor
between the ADJ pin and ground. When ADJ pin bypassing is
used, the value of the output capacitor required increases to its
maximum (220µF for an aluminum electrolytic capacitor, or
47µF for a solid tantalum capacitor). If the ADJ pin is not
bypass, the value of the output capacitor can be lowered to
100µF for an electrolytic aluminum capacitor or 15µF for a
solid tantalum capacitor.
However the value of the ADJ-bypass capacitor should be
chosen with respect to the following equation:
C = 1 / ( 6.28 * F
R
* R
1
)
Where C
= value of the capacitor in Farads (select an
equal or larger standard value),
F
R
= ripple frequency in Hz,
R
1
= value of resistor R
1
in Ohms.
If an ADJ-bypass capacitor is use, the amplitude of the output
ripple will be independent of the output voltage. If an ADJ-
bypass capacitor is not used, the output ripple will be
proportional to the ratio of the output voltage to the reference
voltage:
M = V
OUT
/ V
REF
Where M = multiplier for the ripple seen when the ADJ pin
is optimally bypassed.
V
REF
= Reference Voltage
Reducing parasitic resistance and inductance
One solution to minimize parasitic resistance and inductance is
to connect in parallel capacitors. This arrangement will
improve the transient response of the power supply if your
system requires rapidly changing current load condition.
Thermal Consideration
Although the AS2887 offers some limiting circuitry for
overload conditions, it is necessary not to exceed the
maximum junction temperature, and therefore to be careful
about thermal resistance. The heat flow will follow the lowest
resistance path, which is the Junction-to-case thermal
resistance. In order to insure the best thermal flow of the
component, a proper mounting is required. Note that the case
of the device is electrically connected to the output. In case
the case has to be electrically isolated, a thermally conductive
spacer can be used. However do not forget to consider its
contribution to thermal resistance.
Assuming:
V
IN
= 10V, V
OUT
= 5V, I
OUT
= 1.5A, T
A
= 50°C
θ
Heatsink
= 6°C/W,
θ
Case-Heatsink
= 0.5°C/W,
θ
JC
= 3°C/W
Power dissipation under this condition
P
D
= (V
IN
– V
OUT
) * I
OUT
= 7.5W
Junction Temperature
T
J
= T
A
+ P
D
* (θ
Case – HS
+
θ
HS
+
θ
JC
)
For the Control Sections
T
J
= 50 + 7.5*(0.5 +6=3) = 121.25°C
121.25°C < T
J(max)
for the control section.
In both case reliable operation is insured by adequate junction
temperature.
Rev. 10/11/00
AS2887
Basic Adjustable Regulator
V
IN
AS2887
V
REF
I
ADJ
50µA
R
2
V
OUT
= V
REF
* ( 1 + R /R
1
) + I
ADJ
* R
2
2
R
1
V
OUT
Fig.2 Basic Adjustable Regulator
Output Voltage
Consider Figure 2. The resistance R
1
generates a constant current flow, normally the specified load current of 10mA. This current will
go through the resistance R
2
to set the overall output voltage. The current I
ADJ
is very small and constant. Therefore its contribution to
the overall output voltage is very small and can generally be ignored
Load Regulation
Parasitic line resistance can degrade load regulation. In order not to affect the behavior of the regulator, it is best to connect directly
the R
1
resistance from the resistor divider to the case, and not to the load. For the same reason, it is best to connect the resistor R
2
to
the Negative side of the load.
V
IN
R
P
Parasitic Line
Resistance
AS2887
Connect R
1
to
Case of Regulator
R
1
R
L
R
2
Connect R
2
to Load
Fig.3 Basic Adjustable Regulator
Rev. 10/11/00
AS2887
TYPICAL APPLICATIONS
V
IN
C
1
IN
AS2887
ADJ
OUT
R
1
V
IN
C
1
IN
AS2887
ADJ
OUT
C
2
R
1
V
OUT
LOAD
V
OUT
= V
REF
(1 + R
2
) + I
ADJ
R
2
R
1
R
2
Fig. 4 3A Current output Regulator
Fig. 5 Typical Adjustable Regulator
(Note A)
V
IN
+
10µF
IN
AS2887
ADJ
5V
OUT
V
OUT
R
1
121Ω
1%
150µF
V
IN
(Note A)
IN
AS2887
ADJ
OUT
5V
121Ω
1%
+
100µF
+
10µF
1k
2N3904
1k
365Ω
1%
*C
1
improves ripple rejection. Xc
should be ~ R
1
at ripple frequency.
R
2
365Ω
1%
TTL
Input
+
C
1
10µF*
Note A: V
IN(MIN)
= (Intended V
OUT
) + (V
DROPOUT (MAX)
)
Note A: V
IN(MIN)
= (Intended V ) + (V
DROPOUT (MAX)
)
OUT
Fig. 6
Improving Ripple Rejection
Fig.7 5V Regulator with Shutdown
Rev. 10/11/00
单片机
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