AS2955/56/57
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::
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 AS2955/56/57 but is relatively inefficient, as increasing
the capacitor from 1µF to 220µF only decreases the noise from 430µV
to 160µV Vrms for a 100kHz bandwidth at 5V output.
V
OUT = VREF × ( 1 + R1\ R2 )+ IFBR1
Noise could also be reduced fourfold by a bypass capacitor across R1,
since it reduces the high frequency gain from 4 to unity. Pick
The VREF is 1.235 and IFB is the feedback bias current, nominally
-20nA. The minimum recommended load current of 1 µA 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 AS2955/56/57 typically draws 60 µA at no load with Pin 2
open-circuited this is a small price to pay
C
BYPASS ≅ 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 100µV Vrms 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.
HEAT SINK REQUIREMENTS
Depending on the maximum ambient temperature and maximum
power dissipation a heat sink may be required with the
AS2955/56/57. The junction temperature range has to be within
the range specified under Absolute Maximum Ratings under all
possible operating conditions. To find out if a heat sink is
required, the maximum power dissipation of the device needs to
be calculated. This is the maximum specific AC voltage that
must be taken into consideration at input. Figure 3 shows the
condition and power dissipation which should be calculated with
the following formula:
Iin
5V
Vin
IN
OUT
IL
P
TOTAL = (VIN - 5) IL + (VIN)IG
AS2955
LOAD
+
+
Next step is to calculate the temperature rise TR (max). TJ (max)
maximum allowable junction temperature, TA (max) maximum
ambient temperature :
2.2 uF
GND
IG
IIN = IL + IG
TR (max) = TJ (max) - TA (max)
Junction to ambient thermal resistance θ(j-A) can be calculated
after determining of PTOTAL & TR (max):
FIGURE 3.
5V Regulator Circuit
θ(J-A) = TR (max)/P(max)
If the θ(J-A) is 60°C/W or higher, the device could be operated
without a heat sink. If the value is below 60°C/W then the heat
sink is required and the thermal resistance of the heat sink can be
calculated by the following formula, θ(J-C) junction to case, θ(C-H)
case to heat sink, θ(H-A) heat sink to ambient:
θ(J-A) = θ(J-C) + θ(C-H) + θ(H-A)
Rev. 10/6/00
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