AMS2501/AMS2502
APPLICATION HINTS
5V
CONTROL
POWER
SENSE
3.3V
AMS2501
OUTPUT
ADJ
R
P
LOAD
R1
R2
R
P
+
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.
Figure 2. Remote Load Sensing
(∆I
OUT
)(R
P
)
V
OUT
FIGURE 1
V
OUT
FIGURE 2
ESR
EFFECTS
ESL
EFFECTS
SLOPE, V/t =
∆I/C
CAPACITANCE
EFFECTS
POINT AT WHICH REGULATOR
TAKES CONTROL
Figure 4.
I
OUT
Output Voltage
TIME
Figure 3. Remote Sensing Improves Load Regulation
Voltage drops due to R
P
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 R
P
.
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
150
µF
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 1Ω.
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.
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 I
ADJ
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.
V
CONTROL
+
CONTROL
POWER
OUTPUT
V
POWER
+
AMS2501
SENSE
ADJ
+
V
OUT
V
REF
R1
R2
I
ADJ
50µA
V
OUT
= V
REF
(1+
R2/R1)+I
ADJ
R2
Figure 5. Setting Output Voltage
Advanced Monolithic Systems, Inc.
www.advanced-monolithic.com
Phone (925) 443-0722
Fax (925) 443-0723
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
