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Advanced
Monolithic
Systems
AMS2501/AMS2502
1A VERY LOW DROPOUT VOLTAGE REGULATORS
FEATURES
Adjustable or Fixed Output
1.5V, 1.8V, 2.5V, 2.85V, 3.0V, 3.3V, 3.5V and 5.0V
Output Current of 1A
Low Dropout, typ. 200mV at 500mA Output Current
Fast Transient Response
Remote Sense
APPLICATIONS
High Efficiency Current Regulators
Post Regulators for Switching Supplies
Audio/Video/Modem Card Supply
Adjustable Power Supply
Notebook/Personal Computer Supplies
GENERAL DESCRIPTION
TThhee AMS2501/AMS2502 series of adjustable and fixed low dropout voltage regulators are designed to provide 1A output
current. The dropout voltage of the device is 100mV at light loads and rising to 200mV at 500mA output current. A second
input voltage of 1.2V or greater than the output is required to achieve this dropout. The AMS2501/AMS2502 can also be used
as a single supply device by connecting pin1 and pin 8 together. In this case the dropout voltage will be typically 1.2V.
New features have been added to the AMS2501: a remote Sense pin is brought out virtually eliminating output voltage
variations due to load changes. AMS2502 has an additional feature of an On/Off pin to keep the device in stand-by mode.
The typical load regulation, measured at the Sense pin, for a load current step of 100mA to 1A is less than 1mV.
The AMS2501/AMS2502 series has fast transient response. On the AMS2502 the reference voltage is brought out to allow
the user to add a bypass capacitor for lower noise and transient response improvement.
The AMS2501/AMS2502 series are ideal for generating supplies of 1.25V to 3V where both 5V and 3.3V supplies are
available.
The AMS2501/AMS2502 devices are offered in 8 lead SOIC package.
ORDERING INFORMATION:
PACKAGE TYPE
8 LEAD SO-8
AMS2501CS
AMS2501CS-1.5
AMS2501CS-1.8
AMS2501CS-2.5
AMS2501CS-2.85
AMS2501CS-3.0
AMS2501CS-3.3
AMS2501CS-3.5
AMS2501CS-5.0
AMS2502CS
AMS2502CS-1.5
AMS2502CS-1.8
AMS2502CS-2.5
AMS2502CS-2.85
AMS2502CS-3.0
AMS2502CS-3.3
AMS2502CS-3.5
AMS2502CS-5.0
OPERATING JUNCTION
TEMPERATURE RANGE
0 to 125° C
0 to 125° C
0 to 125° C
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
AMS2501 8L SOIC
POWER IN 1
OUTPUT 2
OUTPUT 3
SENSE 4
8 CONTROL IN
7 N/C
6 ADJ/GND
5 N/C
Top View
AMS2502 8L SOIC
POWER IN 1
OUTPUT 2
OUTPUT 3
SENSE 4
8 CONTROL IN
7 BYPASS
6 ADJ/GND
5 ON/OFF
Top View
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)
VPOWER Input Voltage
7V
VCONTROL Input Voltage
13V
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
AMS2501/AMS2502
Soldering information
Lead Temperature (10 sec)
Thermal Resistance
SO-8 package
260°C
ϕ JA= 160°C/W
ELECTRICAL CHARACTERISTICS
Electrical Characteristics at ILOAD = 0 mA, and TJ = +25°C unless otherwise specified.
Parameter
Device
Conditions
Min Typ Max Units
Reference Voltage
Output Voltage
Line Regulation
Load Regulation
Minimum Load
Current
Control Pin Current
(Note 4)
Ground Pin Current
(Note 4)
Adjust Pin Current
Current Limit
Ripple Rejection
AMS2501/AMS2502
AMS2501/AMS2502-1.5
AMS2501/AMS2502-1.8
AMS2501/AMS2502-2.5
AMS2501/AMS2502-2.85
AMS2501/AMS2502-3.0
AMS2501/AMS2502-3.3
AMS2501/AMS2502-3.5
AMS2501/AMS2502-5.0
AMS2501/AMS2502-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
AMS2501/AMS2502-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
AMS2501/AMS2502
VCONTROL = 2.75V, VPOWER =2V, ILOAD = 10mA
VCONTROL = 2.7V to 12V, VPOWER =3.3V to 5.5V,
ILOAD = 10mA to 1A
VCONTROL = 4V, VPOWER =2.V, ILOAD = 0mA
VCONTROL = 3V, VPOWER =2.3V, ILOAD = 0mA to 1A
VCONTROL = 4V, VPOWER =2.V, ILOAD = 0mA
VCONTROL = 3V, VPOWER =2.3V, ILOAD = 0mA to 1A
VCONTROL = 5V, VPOWER =3.3V, ILOAD = 0mA
VCONTROL = 4V, VPOWER =3.3V, ILOAD = 0mA to 1A
VCONTROL = 5.35V, VPOWER =3.35V, ILOAD = 0mA
VCONTROL = 4.4V, VPOWER =3.7V, ILOAD = 0mA to 1A
VCONTROL = 5.5V, VPOWER =3.5V, ILOAD = 0mA
VCONTROL = 4.5V, VPOWER =3.8V, ILOAD = 0mA to 1A
VCONTROL = 5.8V, VPOWER =3.8V, ILOAD = 0mA
VCONTROL = 4.8V, VPOWER =4.1V, ILOAD = 0mA to 1A
VCONTROL = 6V, VPOWER =4V, ILOAD = 0mA
VCONTROL = 5V, VPOWER =4.3V, ILOAD = 0mA to 1A
VCONTROL = 7.5V, VPOWER =5.5V, ILOAD = 0mA
VCONTROL = 6.5V, VPOWER =5.8V, ILOAD = 0mA to 1A
ILOAD = 10 mA , 1.5V(VCONTROL - VOUT) 12V
0.8V(VPOWER - VOUT) 5.5V
VCONTROL = VOUT + 2.5V, VPOWER =VOUT + 0.8V,
ILOAD = 10mA to 1A
VCONTROL = 5V, VPOWER =3.3V, VADJ = 0V (Note 3)
AMS2501/AMS2502-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
AMS2501/AMS2502-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
AMS2501/AMS2502
AMS2501/AMS2502-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
AMS2501/AMS2502-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
VCONTROL = VOUT + 2.5V, VPOWER =VOUT + 0.8V,
ILOAD = 10mA
VCONTROL = VOUT + 2.5V, VPOWER =VOUT + 0.8V,
ILOAD = 10mA
VCONTROL = 2.75V, VPOWER = 2.05V, ILOAD = 10mA
VPOWER =VOUT + 0.8V
VCONTROL = VPOWER = VOUT + 2.5V, VRIPPLE = 1VP-P
ILOAD = 1A
1.238
1.232
1.491
1.485
1.782
1.773
2.485
2.475
2.821
2.833
2.982
2.970
3.280
3.235
3.479
3.430
4.930
4.950
1.250
1.250
1.500
1.500
1.800
1.800
2.500
2.500
2.850
2.850
3.000
3.000
3.300
3.300
3.500
3.500
5.000
5.000
1
1
5
10
6
40
1.0 1.2
60 80
1.262
1.268
1.509
1.515
1.818
1.827
2.515
2.525
2.879
2.867
3.018
3.030
3.320
3.333
3.521
3.535
5.030
5.050
3
5
10
16
10
120
1.5
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
mV
mV
mA
mA
mA
µA
A
dB
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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AMS2501/AMS2502
ELECTRICAL CHARACTERISTICS
Electrical Characteristics at IOUT = 0 mA, and TJ = +25°C unless otherwise specified.
Parameter
Device
Conditions
Min Typ Max Units
Thermal Regulation
Dropout Voltage
AMS2501/AMS2502
TA = 25°C, 30ms pulse, ILOAD = 1A
Note 2
0.002
0.020
%W
Control Dropout
(VCONTROL - VOUT)
Power Dropout
(VPOWER - VOUT)
AMS2501/AMS2502/-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
AMS2501/AMS2502/-1.5/-
1.8/-2.5/-2.85/-3.0/-3.3/-
3.5/-5.0
VPOWER =VOUT + 0.8V, ILOAD = 10mA
VPOWER =VOUT + 0.8V, ILOAD = 1A
VCONTROL =VOUT + 2.5V, ILOAD = 1A
1.00 1.15
1.15 1.30
.05 0.15
.30 0.50
V
V
V
V
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: Unless otherwise specified VOUT = VSENSE. For the adjustable device VADJ = 0V.
Note 3: The dropout voltage for the AMS2501/AMS2502 is caused by either minimum control voltage or minimum power voltage. The specifications represent
the minimum input/output voltage required to maintain 1% regulation.
Note 4: For the adjustable device the minimum load current is the minimum current required to maintain regulation. Normally the current in the resistor divider
used to set the output voltage is selected to meet the minimum load current requirement.
Note 5: The control pin current is the drive current required for the output transistor. This current will track output current with a ratio of about 1:100. The
minimum value is equal to the quiescent current of the device.
PIN FUNCTIONS
VPOWER (Pin 1): This pin is the collector to the power
device of the AMS2501/AMS2502. The output load
current is supplied through this pin. The voltage at this
pin must be between 0.1V and 0.8V greater than the
output voltage for the device to regulate.
Output (Pin 2 and 3): These are the power output of the
device. Pin 2 and 3 are fused together and with the
package paddle serving also as heat sink.
Sense (Pin 4): This pin is the positive side of the
reference voltage for the device. With this pin it is
possible to Kelvin sense the output voltage at the load.
On/Off (Pin 5 AMS2502 only): This pin puts the device
in a stand-by mode.
Reference (Pin 7 AMS2502 only): This pin allows the
user to add a bypass capacitor on the reference voltage.
Adjust (Pin 6): This pin is the negative side of the
reference voltage for the device. Adding a small bypass
capacitor from the Adjust pin to ground improves the
transient response. For fixed voltage devices the Adjust
pin is also brought out to allow the user to add a bypass
capacitor.
GND (Pin 6): For fixed voltage devices this is the
bottom of the resistor divider that sets the output
voltage.
VCONTROL (Pin 8): This pin is the supply pin for the
control circuitry of the device. The current flow into
this pin will be about 1% of the output current. The
voltage at this pin must be between 1.0V and 1.3V
greater than the output voltage for the device to
regulate.
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 AMS2501/AMS2502 series of adjustable and fixed
regulators is designed to make use of multiple power supplies,
existing in most systems, to reduce the dropout voltage. One of
the advantages of the two supply approach is maximizing the
efficiency.
The second supply is at least 1.2V greater than output voltage and
is providing the power for the control circuitry and supplies the
drive current to the NPN output transistor. This allows the NPN
to be driven into saturation; thereby reducing the dropout voltage
by a VBE compared to conventional designs. For the control
voltage the current requirement is small, equal to about 1% of the
output current or approximately 10mA for a 1A load. Most of this
current is drive current for the NPN output transistor. This drive
current becomes part of the output current. The maximum voltage
on the Control pin is 13V. The maximum voltage at the Power
pin is 7V. Ground pin current for fixed voltage devices is typical
6mA and is constant as a function of load. Adjust pin current for
adjustable devices is 40µA at 25°C and varies proportional to
absolute temperature.
The improved frequency compensation of AMS2501/AMS2502
permits the use of capacitors with very low ESR. Output voltage
tolerances are tighter and include transient response as part of the
specification. Designed to meet the fast current load step, the
AMS2501/AMS2502 also saves total cost by needing less output
capacitance to maintain regulation.
Careful design of the AMS2501/AMS2502 has eliminated any
supply sequencing issues associated with a dual supply system.
The output voltage will not turn on until both supplies are
operating. If the control voltage comes up first, the output current
will be limited to a few milliamperes until the power input
voltage comes up. If power input comes up first the output will
not turn on at all until the control voltage comes up. The output
can never come up unregulated. By tying the control and power
inputs together the AMS2501/AMS2502 can also be operated as a
single supply device. In single supply operation the dropout will
be determined by the minimum control voltage.
Both the fixed and adjustable versions have remote sense pins,
permitting very accurate regulation of output voltage at the load,
rather than at the regulator. As a result, over an output current
range of 100mA to 1A with a 2.5V output, the typical load
regulation is less than 1mV. For AMS2502 the reference voltage
is brought out to pin 7, allowing the user to improve transient
response by bypassing the internal resistor divider. Optimum
transient response is provided using a capacitor in the range of
0.1µF to 1µF for bypassing the Reference pin. The value chosen
will depend on the amount of output capacitance in the system.
This devices can hold 1% accuracy over the full temperature
range and load current range, guaranteed, when combined with
ratiometrically accurate internal divider resistors and operating
with an input/output differential of well under 1V.
AMS2501/AMS2502
Typical applications for the AMS2501/AMS2502 include 3.3V to
2.5V conversion with a 5V control supply, 5V to 4.2V conversion
with a 12V control supply or 5V to 3.6V conversion with a 12V
control supply. Capable of 1A of output current with a maximum
dropout of 0.8V the AMS2501/AMS2502 also has a fast transient
response that allows it to handle large current changes. The
device is fully protected against overcurrent and overtemperature
conditions.
Grounding and Output Sensing
The AMS2501/AMS2502 allows true Kelvin sensing for both the
high and low side of the load. As a result the voltage regulation at
he load can be easily optimized. Voltage drops due to parasitic
resistances between the regulator and the load can be placed
inside the regulation loop of the AMS2501/AMS2502. The
advantages of remote sensing are illustrated in figures 1 through
3.
Figure 1 shows the device connected as a conventional 3 terminal
regulator with the Sense lead connected directly to the output of
the device. RP is the parasitic resistance of the connections
between the device and the load. RP is made up of the PCB traces
and /or connector resistances (in the case of a modular regulator)
between the regulator and the load. Trace A of figure 3 illustrates
the effect of RP. Very small resistances cause significant load
regulation steps.
Figure 2 shows the device connected to take advantage of the
remote sense feature. The Sense pin and the top of the resistor
divider are connected to the top of the load; the bottom of the
resistor divider is connected to the bottom of the load. RP is now
connected inside the regulation loop of the AMS2501/AMS2502
and for reasonable values of RP the load regulation at the load
will be negligible. The effect on output regulation can be seen in
trace B of figure 3.
3.3V
5V
CONTROL
POWER SENSE
AMS2501
OUTPUT
ADJ
R1
R2
RP
LOAD
RP
+
VOUT
-
Figure 1. Conventional Load Sensing
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

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APPLICATION HINTS
5V
3.3V
CONTROL
POWER SENSE
AMS2501
OUTPUT
ADJ
R1
R2
RP
LOAD
RP
+
VOUT
-
Figure 2. Remote Load Sensing
VOUT
FIGURE 1
VOUT
FIGURE 2
(IOUT)(RP)
IOUT
TIME
Figure 3. Remote Sensing Improves Load Regulation
Voltage drops due to RP are not eliminated; they will add to the
dropout voltage of the regulator regardless of whether they are
inside or outside the regulation loop. The AMS2501/AMS2502
can control the voltage at the load as long as the input-output
voltage is greater than the total of the dropout voltage of the
device plus the voltage drop across RP.
Stability
The circuit design used in the AMS2501/AMS2502 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. For best frequency response
use capacitors with an ESR of less than 1.
In order to increase the transient response larger value capacitors
are needed. To limit the high frequency noise generated by the
load high quality bypass capacitors must be used. In order to limit
parasitic inductance (ESL) and resistance (ESR) in the capacitors
to acceptable limits, multiple small ceramic capacitors in addition
to high quality solid tantalum capacitors are required.
When the adjustment terminal is bypassed to improve the ripple
rejection, the requirement for an output capacitor increases. The
reference voltage is brought out specifically to allow this
capability.
AMS2501/AMS2502
Some of the loads generate large high frequency current
transients. The load current step contains higher order frequency
components than the output coupling network must handle until
the regulator throttles to the load current level. Because they
contain parasitic resistance and inductance, capacitors are not
ideal elements. 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
the 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) (I/C). Figure 4 illustrates
these transient effects.
ESR
EFFECTS
ESL
EFFECTS
CAPACITANCE
EFFECTS
SLOPE, V/t = I/C
POINT AT WHICH REGULATOR
TAKES CONTROL
Figure 4.
Output Voltage
The AMS2501/AMS2502 series develops a 1.25V reference
voltage between the Sense pin and the Adjust pin (Figure5).
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. In general R1 is chosen so that this current
is the specified minimum load current of 10mA.The current out
of the Adjust pin is small, typically 40µA and it adds to the
current from R1. Because IADJ is very small it needs to be
considered only when very precise output voltage setting is
required. For best regulation the top of the resistor divider should
be connected directly to the Sense pin.
VCONTROL
+
VPOWER
+
CONTROL
POWER OUTPUT
AMS2501
SENSE
ADJ
IADJ
50µA
VREF
VOUT = VREF (1+ R2/R1)+IADJR2
+
R1
R2
Figure 5. Setting Output Voltage
VOUT
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140