AS2700/01/02
APPLICATION HINTS
EXTERNAL CAPACITORS
high around 5V input (input voltage at which Vout = 4.75 ). Since
the AS2700/01/02’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.
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 250µA, 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 stability of the AS2700/01/02 requires a 10µF 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 below -25°C solid
tantalum is recommended since the many aluminum types have
electrolytes that freeze at about -30°C. The ESR of about 5Ω or less
and resonant frequency above 500 kHz are the most important
parameters in the value of the capacitor. The capacitor value may be
increased without limit.
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 22µF and 0.15µF for 1mA. More output
capacitance is needed for the 8-pin version at voltages below 5V
since it runs the error amplifier at lower gain. At worst case 22µF or
greater must be used for the condition of 750mA load at 1.23V
output.
PROGRAMMING THE OUTPUT VOLTAGE OF AS2700/01/02
The AS2700/01/02 may be programmed for any output voltage
between its 1.235V reference and its 30V maximum rating . As seen
in Figure 2, an external pair of resistors is required. Refer to the
below equation for the programming of the output voltage:
The AS2700/01/02, 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 applications
like CMOS RAM keep-alive. When setting the output voltage of the
AS2700/01/02 version with external resistors, a minimum load of
5mA 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 1µA tantalum or aluminum electrolytic capacitor
should be placed from the input to the ground.
V
OUT = VREF × (1 + R1\ R2)+ IFBR1
The VREF is 1.235 and IFB is the feedback bias current, nominally -20
nA. The minimum recommended load current of 1µA forces an
upper limit of 1.2 MΩ on value of R2. If no load is presented the IFB
produces an error of typically 2% in VOUT, which may be eliminated
at room temperature by trimming R1. To improve the accuracy
choose the value of R2 = 100k this reduces the error by 0.17% and
increases the resistor program current by 12µA. Since the
AS2700/01/02 typically draws 100µA at no load with SHUTDOWN
open-circuited this is a small price to pay
ERROR DETECTION COMPARATOR OUTPUT
4.75V
REDUCING OUTPUT NOISE
OUTPUT
It may be an advantage to reduce the AC noise present at the output.
One way is to reduce the regulator bandwidth by increasing the size
of the output capacitor. This is the only way that noise can be
reduced on the 3 lead AS2700/01/02 but is relatively inefficient, as
increasing the capacitor from 1µF to 220µF only decreases the noise
from 430µV to 160µVRMS for a 100 kHz bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor across R1,
since it reduces the high frequency gain from 4 to unity. Pick
VOLTAGE
_______
ERROR*
C
BYPASS ≅ 1 / 2πR1 × 200 Hz
+
+
5.0V
INPUT
VOLTAGE
or choose 0.01µF. When doing this, the output capacitor must be
increased to 3.3µF to maintain stability. These changes reduce the
output noise from 430µV to 100µVRMS for a 100 kHz bandwidth at
5V output. With the bypass capacitor added, noise no longer scales
with output voltage so that improvements are more dramatic at higher
+
1.3V
+
* See Application Info.
_______
Figure 1. ERROR Output Timing
output voltages.
SHUTDOWN
The Comparator produces
a
logic low output whenever the
VOUT
V
IN
AS2700/01/02 output falls out of regulation by more than around
5%. This is around 75mV 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 AS2700/01/02 input is ramped up and down. The
ERROR signal becomes low at around 1.3V input, and goes
AS2702
GND
ADJ
Fig.2 Adjustable Voltage Regulator
Rev. 10/11/00
SIPEX [ SIPEX CORPORATION ]
