AS1129
Note 1: Output or reference voltage temperature coefficients defined as the worst-case voltage change divided by the total temperature range.
Note 2: Unless otherwise specified all limits are guaranteed for TJ = 25°C, VIN = 6V, IL = 100µA and CL = 1µF. Additional conditions for the 8-pin versions are
feedback tied to 5V tap and output tied to output sense (VOUT = 5V) and VSHUTDOWN ≤ 0.8V.
Note 3: Regulation is measured at constant junction temperature, using pulse testing with a low duty cycle. Changes in output voltage due to heating effects are
covered under the specification for thermal regulation.
Note 4: Line regulation for the AS1129 is tested at 150°C for IL = 1 mA. For IL = 100µA and TJ = 125°C, line regulation is guaranteed by design to 0.2%. See typical
performance characteristics for line regulation versus temperature and load current.
Note 5: Dropout voltage is defined as the input to output differential at which the output voltage drops 100 mV below its nominal value measured at 1V differential at
very low values of programmed output voltage, the minimum input supply voltage of 2V (2.3V over temperature) must be taken into account.
Note 6: Comparator thresholds are expressed in terms of a voltage differential at the feedback terminal below the nominal reference voltage measured at 6V input. To
express these thresholds in terms of output voltage change, multiply by the error amplifier gain = VOUT/VREF = (R1 + R2)/R2. For example, at a programmed output
voltage of 5V, the Error output is guaranteed to go low when the output drops by 95 mV x 5V/1.235 = 384 mV. Thresholds remain constant as a percent of VOUT as
VOUT is varied, with the dropout warning occurring at typically 5% below nominal, 7.5% guaranteed.
APPLICATION HINTS
This problem can be fixed by adding a 100pF capacitor between
output and feedback and increasing the output capacitor to at least
3.3µF.
EXTERNAL CAPACITORS
The stability of the AS1129 requires a 2.2µF or greater capacitor
between output and ground. Oscillation could occur without this
capacitor. Most types of tantalum or aluminum electrolytic
works fine here. For operations of below -25°C solid tantalum is
recommended since the many aluminum types have electrolytes
the freeze at about -30°C. The ESR of about 5Ω or less and
resonant frequency above 500kHz are the most important
parameters in the value of the capacitor. The capacitor value can
be increased without limit.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the AS1129
output falls out of regulation by more than around 5%. This is around
60mV offset divided by the 1.235 reference voltage. This trip level
remains 5% below normal regardless of the programmed output
voltage of the regulator. Figure 1 shows the timing diagram depicting
the ERROR signal and the regulator output voltage as the AS1129
input is ramped up and down. The ERROR signal becomes low at
around 1.3V input, and goes high around 5V input (input voltage at
which Vout = 4.75). Since the AS1129’s dropout voltage is load
dependent, the input voltage trip point (around 5V) will vary with the
load current. The output voltage trip point (approx. 4.75V) does not
vary with load.
At lower values of output current, less output capacitance is
required for stability. For the currents below 10mA the value of
the capacitor can be reduced to 0.5µF and 0.15µF for 1mA. More
output capacitance needed for the 8-pin version at voltages below
5V since it runs the error amplifier at lower gain. At worst case
5µF or greater must be used for the condition of 250mA load at
1.23V output.
The error comparator has an open-collector output, which requires an
external pull-up resistor. Depending on the system requirements the
resistor may be returned to 5V output or other supply voltage. In
determining the value of this resistor, note that the output is rated to
sink 400µA, this value adds to battery drain in a low battery
condition. Suggested values range from 100K to 1MΩ. If the output
is unused this resistor is not required.
The AS1129, unlike other low dropout regulators will remain
stable and in regulation with no load in addition to the internal
voltage divider.
This feature is especially important in
application like CMOS RAM keep-alive. When setting the output
voltage of the AS1129,
recommended.
a minimum load of 10mA is
If there is more than 10 inches of wire between the input and the
AC filter capacitor or if a battery is used as the input then a 0.1µF
tantalum or aluminum electrolytic capacitor should be placed
from the input to the ground.
PROGRAMMING THE OUTPUT VOLTAGE OF AS1129
The AS1129 may be pin-strapped for 5V using its internal voltage
divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback)
to Pin 6 (5V Tap).
Instability can occur if there is stray capacitance to the AS1129
feedback terminal (pin 7). This could cause more problems when
using a higher value of external resistors to set the output voltage.
SHUTDOWN
4.75V
VOUT
OUTPUT
VOLTAGE
VIN
8
5
1
2
AS1129
ERROR*
GND
3, 6, 7
ADJ
+
5.0V
+
INPUT
VOLTAGE
+
1.3V
+
* See Application Information.
Figure 1. ERROR Output Timing
Figure .2. Adjustable Voltage Regulator
Rev. 10/11/00
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