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Advanced
Monolithic
Systems
AMS2908
800mA LOW DROPOUT VOLTAGE REGULATOR
FEATURES
Three Terminal Adjustable or Fixed Voltages*
1.5V, 2.5V, 2.85V, 3.3V and 5.0V
Output Current of 800mA
Operates Down to 1V Dropout
Line Regulation: 0.2% Max.
Load Regulation: 0.4% Max.
SOT-223 and TO-252 package available
APPLICATIONS
High Efficiency Linear Regulators
Post Regulators for Switching Supplies
5V to 3.3V Linear Regulator
Battery Chargers
Active SCSI Terminators
Power Management for Notebook
Battery Powered Instrumentation
GENERAL DESCRIPTION
TThhee AMS2908 series of adjustable and fixed voltage regulators are designed to provide 800mA output current and to operate
down to 1V input-to-output differential. The dropout voltage of the device is guaranteed maximum 1.3V at maximum output
current, decreasing at lower load currents.
On-chip trimming adjusts the reference voltage to 1%. Current limit is also trimmed, minimizing the stress under overload
conditions on both the regulator and power source circuitry.
The AMS2908 devices are pin compatible with other three-terminal regulators and are offered in the low profile surface
mount SOT-223 package and in the TO-252 (DPAK) plastic package.
ORDERING INFORMATION:
PACKAGE TYPE
OPERATING JUNCTION
TO-252
SOT-223
TEMPERATURE RANGE
AMS2908CD
AMS2908
AMS2908CD-1.5
AMS2908-1.5
AMS2908CD-2.5
AMS2908-2.5
AMS2908CD-2.85 AMS2908-2.85
AMS2908CD-3.3
AMS2908-3.3
AMS2908CD-5.0
AMS2908-5.0
*For additional available fixed voltages contact factory.
0 to 125° C
0 to 125° C
0 to 125° C
0 to 125° C
0 to 125° C
0 to 125° C
PIN CONNECTIONS
FIXED VERSION
ADJUSTABLE VERSION
1- VIN
2- Ground
3- VOUT
1- VIN
2- Adjust
3- VOUT
SOT-223 Top View
123
TO-252 FRONT VIEW
3
2
1
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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ABSOLUTE MAXIMUM RATINGS (Note 1)
Power Dissipation
Internally limited
Input Voltage
15V
Operating Junction Temperature Range
Control Section
0°C to 125°C
Power Transistor
0°C to 150°C
Storage temperature
- 65°C to +150°C
AMS2908
Soldering information
Lead Temperature (10 sec)
300°C
Thermal Resistance
TO-252 package
ϕ JA= 80°C/W
SOT-223 package
ϕ JA= 90°C/W*
* With package soldering to copper area over backside
ground plane or internal power plane ϕ JA can vary from
46°C/W to >90°C/W depending on mounting technique and
the size of the copper area.
ELECTRICAL CHARACTERISTICS
Electrical Characteristics at IOUT = 0 mA, and TJ = +25°C unless otherwise specified.
Parameter
Device
Conditions
Min Typ Max Units
Reference Voltage
(Note 2)
Output Voltage
(Note 2)
Line Regulation
Load Regulation
(Notes 2, 3)
AMS2908
AMS2908-1.5
AMS2908-2.5
AMS2908-2.85
AMS2908-3.3
AMS2908-5.0
AMS2908
AMS2908-1.5
AMS2908-2.5
AMS2908-2.85
AMS2908-3.3
AMS2908-5.0
AMS2908
AMS2908-1.5
AMS2908-2.5
AMS2908-2.85
AMS2908-3.3
AMS2908-5.0
IOUT = 10 mA
10mA IOUT 800mA, 1.5V(VIN - VOUT) 12V
0 IOUT 800mA , 4.35VVIN 12V
0 IOUT 800mA , 4.35VVIN 12V
0 IOUT 800mA , 4.35VVIN 12V
0 IOUT 800mA , 4.75V VIN 12V
0 IOUT 800mA , 6.5V VIN 12V
ILOAD = 10 mA , 1.5V(VIN - VOUT) 12V
3.0VVIN 12V
4.0VVIN 12V
4.35VVIN 12V
4.75VVIN 12V
6.5VVIN 12V
(VIN - VOUT) =3V, 10mA IOUT 800mA
VIN = 5V, 0 IOUT 800mA
VIN = 5V, 0 IOUT 800mA
VIN = 5V, 0 IOUT 800mA
VIN = 5V, 0 IOUT 800mA
VIN = 8V, 0 IOUT 800mA
1.238
1.225
1.485
1.476
2.475
2.460
2.82
2.79
3.267
3.235
4.950
4.900
1.250
1.250
1.500
1.500
2.500
2.500
2.850
2.850
3.300
3.300
5.000
5.000
0.015
0.035
0.3
0.6
0.3
0.6
0.3
0.6
0.5
1.0
0.5
1.0
0.1
0.2
3
6
3
6
3
6
3
7
5
10
1.262
1.270
1.515
1.524
2.525
2.560
2.88
2.91
3.333
3.365
5.050
5.100
0.2
0.2
5
6
6
6
6
6
10
10
10
10
0.3
0.4
10
20
12
20
12
20
15
25
20
35
V
V
V
V
V
V
V
V
V
V
V
V
%
%
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
%
%
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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AMS2908
ELECTRICAL CHARACTERISTICS
Electrical Characteristics at IOUT = 0 mA, and TJ = +25°C unless otherwise specified.
Parameter
Device
Conditions
Min Typ Max Units
Dropout Voltage
(VIN - VOUT)
Current Limit
Minimum Load
Current
Quiescent Current
Ripple Rejection
Thermal Regulation
Adjust Pin Current
AMS2908-1.5/-2.5/-2.85/
-3.3/-5.0
AMS2908-1.5/-2.5/-2.85/
-3.3/-5.0
AMS2908
AMS2908-1.5/-2.5/-2.85/
-3.3/-5.0
AMS2908
AMS2908-1.5
AMS2908-2.5
AMS2908-2.85
AMS2908-3.3
AMS2908-5.0
AMS2908
AMS2908
VOUT , VREF = 1%, IOUT = 800mA (Note 4)
(VIN - VOUT) = 5V
(VIN - VOUT) = 12V (Note 5)
VIN 12V
f =120Hz , COUT = 25µF Tantalum, IOUT = 800mA,
(VIN-VOUT ) = 3V, CADJ =25µF
f =120Hz , COUT = 25µF Tantalum, IOUT = 800mA,
VIN = 6V
f =120Hz , COUT = 25µF Tantalum, IOUT = 800mA,
VIN = 6V
f =120Hz , COUT = 25µF Tantalum, IOUT = 800mA,
VIN = 6V
f =120Hz , COUT = 25µF Tantalum, IOUT = 800mA
VIN = 6.3V
f =120Hz , COUT = 25µF Tantalum, IOUT = 800mA
VIN = 8V
TA = 25°C, 30ms pulse
10mA IOUT 800mA , 1.5V(VIN - VOUT) 12V
Adjust Pin Current
Change
Temperature Stability
Long Term Stability
RMS Output Noise
(% of VOUT )
Thermal Resistance
Junction-to-Case
AMS2908
10mA IOUT 800mA , 1.5V(VIN - VOUT) 12V
TA =125°C, 1000Hrs
TA = 25°C , 10Hz f 10kHz
1.1 1.3
900 1,100 1,500
5 10
5 10
60 75
60 72
60 72
60 72
60 72
60 68
0.008
55
0.2
0.04
120
5
0.5
0.3
0.003
1
15
V
mA
mA
mA
dB
dB
dB
dB
dB
dB
%W
µA
µA
µA
%
%
%
°C/W
Parameters identified with boldface type apply over the full operating temperature range.
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. For guaranteed specifications and test conditions, see the
Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed.
Note 2: Line and Load regulation are guaranteed up to the maximum power dissipation of 1.2 W. Power dissipation is determined by the input/output differential
and the output current. Guaranteed maximum power dissipation will not be available over the full input/output range.
Note 3: See thermal regulation specifications for changes in output voltage due to heating effects. Line and load regulation are measured at a constant junction
temperature by low duty cycle pulse testing. Load regulation is measured at the output lead ~1/8” from the package.
Note 4: Dropout voltage is specified over the full output current range of the device.
Note 5: Minimum load current is defined as the minimum output current required to maintain regulation. When 1.5V (VIN - VOUT) 12V the device is
guaranteed to regulate if the output current is greater than 10mA.
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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APPLICATION HINTS
The AMS2908 series of adjustable and fixed regulators are easy
to use and are protected against short circuit and thermal
overloads. Thermal protection circuitry will shut-down the
regulator should the junction temperature exceed 165°C at the
sense point.
Pin compatible with older three terminal adjustable regulators,
these devices offer the advantage of a lower dropout voltage,
more precise reference tolerance and improved reference stability
with temperature.
Stability
The circuit design used in the AMS2908 series requires the use of
an output capacitor as part of the device frequency compensation.
The addition of 150µF aluminum electrolytic or a 22µF solid
tantalum on the output will ensure stability for all operating
conditions.
When the adjustment terminal is bypassed with a capacitor to
improve the ripple rejection, the requirement for an output
capacitor increases. The value of 22µF tantalum or 150µF
aluminum covers all cases of bypassing the adjustment terminal.
Without bypassing the adjustment terminal smaller capacitors can
be used with equally good results.
To ensure good transient response with heavy load current
changes capacitor values on the order of 100µF are used in the
output of many regulators. To further improve stability and
transient response of these devices larger values of output
capacitor can be used.
Protection Diodes
Unlike older regulators, the AMS2908 family does not need any
protection diodes between the adjustment pin and the output and
from the output to the input to prevent over-stressing the die.
Internal resistors are limiting the internal current paths on the
AMS2908 adjustment pin, therefore even with capacitors on the
adjustment pin no protection diode is needed to ensure device
safety under short-circuit conditions.
Diodes between the input and output are not usually needed.
Microsecond surge currents of 50A to 100A can be handled by the
internal diode between the input and output pins of the device. In
normal operations it is difficult to get those values of surge
currents even with the use of large output capacitances. If high
value output capacitors are used, such as 1000µF to 5000µF and
the input pin is instantaneously shorted to ground, damage can
occur. A diode from output to input is recommended, when a
crowbar circuit at the input of the AMS2908 is used (Figure 1).
AMS2908
D1
AMS2908
VIN IN OUT
VOUT
ADJ
R1
+ COUT
150µF
CADJ
10µF
R2
Figure 1.
Output Voltage
The AMS2908 series develops a 1.25V reference voltage
between the output and the adjust terminal. Placing a resistor
between these two terminals causes a constant current to flow
through R1 and down through R2 to set the overall output
voltage. This current is normally the specified minimum load
current of 10mA. Because IADJ is very small and constant it
represents a small error and it can usually be ignored.
AMS2908
VIN IN OUT
VOUT
ADJ
VREF R1
IADJ
50µA
R2
VOUT = VREF (1+ R2/R1)+IADJR2
Figure 2. Basic Adjustable Regulator
Load Regulation
True remote load sensing it is not possible to provide, because
the AMS2908 is a three terminal device. The resistance of the
wire connecting the regulator to the load will limit the load
regulation. The data sheet specification for load regulation is
measured at the bottom of the package. Negative side sensing is a
true Kelvin connection, with the bottom of the output divider
returned to the negative side of the load.
The best load regulation is obtained when the top of the resistor
divider R1 is connected directly to the case not to the load. If R1
were connected to the load, the effective resistance between the
regulator and the load would be:
RP x ( R2+R1 ) , RP = Parasitic Line Resistance
R1
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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APPLICATION HINTS
Connected as shown , RP is not multiplied by the divider ratio
RP
PARASITIC
AMS2908
LINE RESISTANCE
VIN IN OUT
ADJ
R1* RL
R2*
*CONNECT R1 TO CASE
CONNECT R2 TO LOAD
Figure 3. Connections for Best Load Regulation
In the case of fixed voltage devices the top of R1 is connected
Kelvin internally, and the ground pin can be used for negative
side sensing.
Thermal Considerations
The AMS2908 series have internal power and thermal limiting
circuitry designed to protect the device under overload conditions.
However maximum junction temperature ratings of 125°C should
not be exceeded under continuous normal load conditions.
Careful consideration must be given to all sources of thermal
resistance from junction to ambient. For the surface mount
package SOT-223 additional heat sources mounted near the
device must be considered. The heat dissipation capability of the
PC board and its copper traces is used as a heat sink for the
device. The thermal resistance from the junction to the tab for the
AMS2908 is 15°C/W. Thermal resistance from tab to ambient
can be as low as 30°C/W.
AMS2908
The total thermal resistance from junction to ambient can be as
low as 45°C/W. This requires a reasonable sized PC board with
at least on layer of copper to spread the heat across the board and
couple it into the surrounding air.
Experiments have shown that the heat spreading copper layer
does not need to be electrically connected to the tab of the device.
The PC material can be very effective at transmitting heat
between the pad area, attached to the pad of the device, and a
ground plane layer either inside or on the opposite side of the
board. Although the actual thermal resistance of the PC material
is high, the Length/Area ratio of the thermal resistance between
layers is small. The data in Table 1, was taken using 1/16” FR-4
board with 1 oz. copper foil, and it can be used as a rough
guideline for estimating thermal resistance.
For each application the thermal resistance will be affected by
thermal interactions with other components on the board. To
determine the actual value some experimentation will be
necessary.
The power dissipation of the AMS2908 is equal to:
PD = ( VIN - VOUT )( IOUT )
Maximum junction temperature will be equal to:
TJ = TA(MAX) + PD(Thermal Resistance (junction-to-ambient))
Maximum junction temperature must not exceed 125°C.
Ripple Rejection
The ripple rejection values are measured with the adjustment pin
bypassed. The impedance of the adjust pin capacitor at the ripple
frequency should be less than the value of R1 (normally 100to
200) for a proper bypassing and ripple rejection approaching
the values shown. The size of the required adjust pin capacitor is
a function of the input ripple frequency. If R1=100at 120Hz
the adjust pin capacitor should be >13µF. At 10kHz only 0.16µF
is needed.
The ripple rejection will be a function of output voltage, in
circuits without an adjust pin bypass capacitor. The output ripple
will increase directly as a ratio of the output voltage to the
reference voltage (VOUT / VREF ).
Table 1.
COPPER AREA
TOP SIDE* BACK SIDE
BOARD AREA
2500 Sq. mm 2500 Sq. mm
2500 Sq. mm
1000 Sq. mm 2500 Sq. mm
2500 Sq. mm
225 Sq. mm 2500 Sq. mm
2500 Sq. mm
100 Sq. mm 2500 Sq. mm
2500 Sq. mm
1000 Sq. mm 1000 Sq. mm
1000 Sq. mm
1000 Sq. mm
0
1000 Sq. mm
* Tab of device attached to topside copper.
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
45°C/W
45°C/W
53°C/W
59°C/W
52°C/W
55°C/W
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140