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US1050 Fiches technique(PDF) 5 Page - UNISEM

No de pièce US1050
Description  5A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR
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US1050 Fiches technique(HTML) 5 Page - UNISEM

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US1050
2-37
Rev. 1.3
10/27/00
V
V
V
V
I
A
T
C
IN
O
OUT
A
MAX
=
=
=
= °
5
3 5
4 6
35
.
.
(
)
(
)
P
I
V
V
P
.
.
. W
D
O UT
IN
O UT
D
=
×
=
× −
=
4 6 5 3 5
6 9
θJC
C W
= °
2 7
.
/
(
)
(
)
T
T P
.
.
.
S
J
D
= −
×
+
=
×
+
=
°
θ
θ
JC
CS
S
T
C
135 6 9 2 7 0 05
116
T
S
A
T
T
C
=
=
− =
°
116 35 81
Thermal Design
The US1050 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the maximum allowable junction temperature.
Although this device can operate with junction tempera-
tures in the range of 150
°C ,it is recommended that the
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. The example below
shows the steps in selecting the proper Regulator heat
sink for the worst case current consumption using Intel
200MHz microprocessor as the load .
Assuming the following specifications :
The steps for selecting a proper heat sink to keep the
junction temperature below 135
°C is given as :
1) Calculate the maximum power dissipation using :
2) Select a package from the Regulator data sheet
and record its junction to case (or Tab) thermal
resistance.
Selecting TO220 package gives us :
3) Assuming that the heat sink is Black Anodized, cal-
culate the maximum Heat sink temperature allowed :
Assume ,
θcs=0.05°C/W (Heat sink to Case thermal
resistance for Black Anodized)
4) With the maximum heat sink temperature calcu-
lated in the previous step, the Heat Sink to Air thermal
resistance (
θsa) is calculated by first calculating the
temperature rise above the ambient as follows :
T=Temperature Rise Above Ambient
5) Next , a heat sink with lower
θsa than the one calcu-
lated in step 4 must be selected. One way to do this is
to simply look at the graphs of the “Heat Sink Temp
Rise Above the Ambient” vs. the “Power Dissipation” and
select a heat sink that results in lower temperature rise
than the one calculated in previous step. The following
heat sinks from AAVID and Thermaloy meet this crite-
ria.
Air Flow (LFM)
0
100
200
300
400
Thermalloy
6021PB
6021PB
6073PB
6109PB
7141D
AAVID
534202B 534202B 507302
575002
576802B
Note : For further information regarding the above com-
panies and their latest product offerings and application
support contact your local representative or the num-
bers listed below:
AAVID
PH# (603) 528 3400
Thermalloy
PH# (214) 243-4321
Designing for Microprocessor Applications
As it was mentioned before the US1050 is designed spe-
cifically to provide power for the new generation of the
low voltage processors requiring voltages in the range
of 2.5V to 3.6V generated by stepping down the
5V supply. These processors demand a fast regulator
that supports their large load current changes. The worst
case current step seen by the regulator is anywhere in
the range of 1 to 7A with the slew rate of 300 to 500 nS
which could happen when the processor transitions from
“Stop Clock” mode to the “Full Active” mode. The load
current step at the processor is actually much faster ,in
the order of 15 to 20 nS,however the decoupling capaci-
tors placed in the cavity of the processor socket handle
this transition until the regulator responds to the load
current levels. Because of this requirement the selec-
tion of high frequency low ESR and low ESL output ca-
pacitor is imperative in the design of these regulator cir-
cuits.
Figure 4 shows the effects of a fast transient on the
θ
θ
SA
T
D
SA
P
C W
=
=
=
°
81
6 9
117
.
.
/


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