AMS2501/AMS2502
APPLICATION HINTS
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 (DV=DI)(ESR). The ESL of the output
capacitors produces a droop proportional to the rate of change of
the output current (V= L)(DI/Dt). The output capacitance
produces a change in output voltage proportional to the time until
the regulator can respond (DV=Dt) (DI/C). Figure 4 illustrates
these transient effects.
5V
CONTROL
POWER
SENSE
3.3V
AMS2501
OUTPUT
+
VOUT
-
RP
ADJ
LOAD
R1
R2
RP
Figure 2. Remote Load Sensing
ESR
EFFECTS
(DIOUT)(RP)
VOUT
FIGURE 1
ESL
EFFECTS
CAPACITANCE
EFFECTS
VOUT
SLOPE, V/t = DI/C
POINT AT WHICH REGULATOR
FIGURE 2
TAKES CONTROL
Figure 4.
IOUT
Output Voltage
TIME
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 40mA 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.
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 150mF aluminum
electrolytic or a 22mF solid tantalum on the output will ensure
stability for all operating conditions. For best frequency response
use capacitors with an ESR of less than 1W.
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.
VCONTROL
+
CONTROL
POWER
OUTPUT
VOUT
VPOWER
+
+
AMS2501
SENSE
ADJ
VREF
R1
R2
IADJ
50mA
VOUT = VREF (1+ R2/R1)+IADJR2
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.
Figure 5. Setting Output Voltage
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
