B1121-Z-3.0 View Datasheet(PDF) - Bay Linear
Part Name
Description
MFG CO.
'B1121-Z-3.0' PDF : 9 Pages View PDF
B1121
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 B1121 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.
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 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 B1121. 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:
PTOTAL = (VIN - 5) IL + (VIN)IG
Next step is to calculate the temperature rise TR (max). TJ
(max) maximum allowable junction temperature, TA (max)
maximum ambient temperature :
TR (max) = TJ (max) - TA (max)
Junction to ambient thermal resistance θ(j-A) can be
calculated after determining of PTOTAL & TR (max):
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)
I in
Vin
+
IIN = IL + IG
IN
OUT
B1121
GND
IG
5V
+
2.2 uF
FIGURE 3.
5V Regulator Circuit
IL
LOAD
Bay Linear, Inc 2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 989-7144, Fax: (925) 940-9556
www.baylinear.com
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