AMS1505 Advanced Monolithic Systems, AMS1505 Datasheet - Page 5

• Manufacturer Part Number: AMS1505
• Description: 5A 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 AMS1505 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 AMS1505 series requires the use of
an output capacitor as part of the device frequency compensation.
The addition of
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 meet the transient requirements of the processor larger
value capacitors are needed. Tight voltage tolerances are required
in the power supply. To limit the high frequency noise generated
by the processor 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
Adjust pin is brought out on the fixed voltage device specifically
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3.3V
Figure 3. Remote Sensing Improves Load Regulation
FIGURE 1
FIGURE 2
POWER
Figure 2. Remote Load Sensing
AMS1505
CONTROL
150
V
V
I
5V
OUT
OUT
OUT
ADJ
P
.
F aluminum electrolytic or a 22 F solid
OUTPUT
R2
SENSE
P
are not eliminated; they will add to the
R1
( I
TIME
OUT
)(R
R
R
P
P
P
)
LOAD
V
+
-
OUT
to allow this capability. To ensure good transient response with
heavy load current changes capacitor values on the order of
100 F 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.
The modern processors 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 AMS1505 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 50 A and it adds to the current
from R1. Because I
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
+
+
1+ R2/R1)+I
Figure 5. Setting Output Voltage
ADJ
POWER
POINT AT WHICH REGULATOR
V
I
50 A
is very small it needs to be considered
ADJ
CONTROL
CONTROL
ADJ
AMS1505
Figure 4.
ADJ
R2
TAKES CONTROL
OUTPUT
SENSE
V
AMS1505
REF
CAPACITANCE
EFFECTS
R1
R2
+
V
OUT