AS2955/56/57
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 AS2955/56/57 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
AS2955/56/57 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 AS2955/56/57 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
AS2955/56/57’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 AS2955/56/57, 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 AS2955/56/57, a minimum load of 10mA is
recommended.
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
AS2955/56/57
The AS2956/57 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
AS2955/56/57 feedback terminal (pin 7). This could cause more
problems when using a higher value of external resistors to set
the output voltage.
+VIN
4.75V
8
100K
O U T PU T
+V
______
IN
ERROR
5
V O L TA G E
ERROR
1.2 to 29V
1
OUTPUT
VOUT
_______
ER R O R *
AS2956/57
+
3
R1
SHUTDOWN
INPUT
10uF
SD
.01uF
FB
GND
4
7
+
5.0V
+
1.23V
VREF
IN PU T
V O L TA G E
+
1.3V
+
R2
* See A pplication Info.
_______
F igure 1. E R R O R O utput T im ing
Figure 2. Adjustable Regulator
Rev. 10/6/00
5秒后页面跳转
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