US1260
Stability
The US1260 requires the use of an output capacitor as
part of the frequency compensation in order to make the
regulator stable. Typical designs for the microproces-
sor applications use standard electrolytic capacitors with
typical ESR in the range of 50 to 100 mΩ and the output
capacitance of 500 to 1000uF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The US1260 takes advantage of
this phenomena in making the overall regulator loop
stable. For most applications a minimum of 100uF alu-
minum electrolytic capacitor with the maximum ESR of
0.3Ω such as Sanyo, MVGX series ,Panasonic FA se-
ries as well as the Nichicon PL series insures both sta-
bility and good transient response. The US1260 also
requires a 1 uF ceramic capacitor connected from Vin
to Vctrl and a 10Ω, 0.1W resistor in series with Vctrl pin
in order to further insure stability.
Thermal Design
The US1260 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. Two examples are given
which shows the steps in selecting the proper regulator
heat sink for driving the Pentium II processor GTL+ ter-
mination resistors and the Clock IC using 1260 in TO220
or TO-263 packages.
Example # 1
Assuming the following specifications :
V
IN
=
3.3V
V
OUT 2
=
1.5 V
V
OUT 1
= 2.5 V
I
OUT 2
MAX
=
5.4A
I
OUT 1
MAX
= 0.4 A
T
A
=
35
°
C
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 :
P
D
=
I
OUT1
×
(
V
IN
−
V
OUT1
)
+
I
OUT2
×
(
V
IN
−
V
OUT2
)
P
D
=
0.4
×
(
3.3
−
2.5
)
+ 5.4
×
(
3.3
−
1.5
)
=
10 W
2) Select a package from the datasheet and record its
junction to case (or Tab) thermal resistance.
Selecting TO220 package gives us :
θ
JC
=
2.7
°
C / W
3) Assuming that the heat sink is Black Anodized, cal-
culate the maximum Heat sink temperature allowed :
Assume ,
θ
SA
= 0.05
°C/W
(Heat sink to Case thermal
resistance for Black Anodized)
T
S
=
T
J
−
P
D
×
(
θ
JC
+ θ
CS
)
T
S
=
135
−
10
×
(
2.7
+
0.05
)
=
107.4
°
C
4) With the maximum heat sink temperature calculated
in the previous step, the Heat Sink to Air thermal resis-
tance
θ
SA
is calculated as follows :
∆
T
=
T
S
−
T
A
=
107.4
−
35
=
72.4
°
C
∆
T
P
D
72.4
=
7.24
°
C / W
θ
SA
=
10
θ
SA
=
5) Next , a heat sink with lower
θ
SA
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
7021B 7020B 6021PB 7173D
7141D
593101B 551002B 534202B 577102B 576802B
Thermalloy
AAVID
Note : For further information regarding the above com-
panies and their latest product offering and application
support contact your local representative or the num-
bers listed below:
Thermalloy
AAVID
PH# (214) 243-4321
PH# (603) 528-3400
Rev. 1.9
3/22/99
3-5
UNISEM [ UNISEM ]
