SPX2975 View Datasheet(PDF) - Signal Processing Technologies
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SPX2975 Datasheet PDF : 10 Pages
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APPLICATION HINTS
EXTERNAL CAPACITORS
The stability of the SPX2975 requires a 1.0µF 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 500kHz are the most
important parameters in the value of the capacitor. The
capacitors 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 0.33µF and 0.1µF 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 3.3µF or greater must be used for the condition of 10mA
load at 1.23V output.
The SPX2975, 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 SPX2975, a minimum load of 1µA 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.
Instability can occur if there is stray capacitance to the SPX2975
feedback terminal (pin 7). This could cause more problems when
using a higher value of external resistors to set the output voltage.
This problem can be fixed by adding a 100 pF capacitor between
output and feedback and increasing the output capacitor to at
least 3.3µF.
ERROR DETECTION COMPARATOR OUTPUT
The Comparator produces a logic low output whenever the
SPX2975 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 SPX2975 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 SPX2975 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.
SPX2975
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.
PROGRAMMING THE OUTPUT VOLTAGE OF
SPX2975
The SPX2975 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). Also, it 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:
VOUT = VREF × (1 + R1\ R2)+ IFBR1
The Vref is 1.235 and IFB is the feedback bias current, nominally
-20nA. 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 SPX2975 typically
draws 60µA at no load with Pin 2 open-circuited this is a small price
to pay
REDUCING OUTPUT NOISE
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 SPX2975 but is relatively inefficient, as increasing the
capacitor from 1µF to 220µF only decreases the noise from 430µV to
160µV rms for a 100kHz 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
CBYPASS ≅ 1 / 2πR1 × 200 Hz
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 100kHz bandwidth at 5V
output. With the bypass capacitor added, noise no longer scales with
output voltage so that improvements are more dramatic at higher
output voltages.
Rev. 10/30/00
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