DS2501 Advanced Monolithic Systems, DS2501 Datasheet - Page 5

• Manufacturer Part Number: DS2501
• Description: 1A VERY LOW DROPOUT VOLTAGE REGULATORS
• Manufacturer: Advanced Monolithic Systems

APPLICATION HINTS
Voltage drops due to R
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 R
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
electrolytic or a 22 F 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.
3.3V
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Figure 3. Remote Sensing Improves Load Regulation
FIGURE 1
FIGURE 2
POWER
Figure 2. Remote Load Sensing
AMS2501
CONTROL
V
V
I
5V
OUT
OUT
OUT
ADJ
OUTPUT
R2
SENSE
P
are not eliminated; they will add to the
R1
( I
TIME
OUT
)(R
P
R
.
R
P
P
P
)
LOAD
150
F aluminum
V
+
-
OUT
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.
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 40 A and it adds to the
current from R1. Because I
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.
V
V
OUT
POWER
= V
SLOPE, V/t = I/C
ESR
EFFECTS
ESL
EFFECTS
REF
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+
+
1+ R2/R1)+I
AMS2501/AMS2502
Figure 5. Setting Output Voltage
POWER
POINT AT WHICH REGULATOR
V
I
50 A
ADJ
CONTROL
CONTROL
ADJ
AMS2501
Figure 4.
ADJ
R2
TAKES CONTROL
ADJ
OUTPUT
SENSE
is very small it needs to be
V
REF
CAPACITANCE
EFFECTS
R1
R2
+
V
OUT