Applications continued
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 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
RP.
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
AMS1505 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.
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
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 1Ω.
In order to meet the transient requirements of the
load larger value capacitors are needed. Tight
voltage tolerances are required in the power supply.
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.
Figure 1. Conventional Load Sensing
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
to allow this capability. To ensure good transient
response with heavy load current changes capacitor
values on the order of 100µF 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.
Figure 2. Remote Load Sensing
(∆IOUT)(RP)
VOUT
FIGURE 1
The modern systems 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).
VOUT
FIGURE 2
IOUT
TIME
Figure 3. Remote Sensing Improves Load
Regulation
Advanced Monolithic Systems http://www.ams-semitech.com
AMS [ Advanced Monolithic Systems Ltd ]
