US1010 Datasheet, PDF (5/5 Pages) UNISEM – 1A LOW DROPOUT POSITIVE ADJUSTABLE REGULATOR

US1010
Thermal Design
The US1010 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 for
a SCSI terminator application shows the steps in se-
lecting the proper regulator in a surface mount pack-
age .(See US1015 for non surface mount packages)
Assuming the following specifications :
VIN = 5V
VF = 0.5V
VO = 2.85V
IOUTMAX = 0.8A
TA = 35° C
Where ; VF is the forward voltage drop of the D1diode
as shown in figure 4.
The steps for selecting the right package with proper
board area for heatsinking to keep the junction tem-
perature below 135°C is given as :
To set the output DC voltage, we need to select R1 and
R2 :
3) Assuming R1=121 Ω , 1%
R2
=


VOUT
VREF
− 1^{}
× 121=


2.85
1.25
− 1^{}
× 121 =
154.8
Ω
Select R2=154 Ω ,1%
D1
5V
C1
10uF
Vin
Vout
US1010
Adj
2.85V
C2
22uF
R1
121
1%
R2
154
1%
1010app4-1.3
Figure 4 - Final Schematic for half of the
GTL+ termination regulator.
1) Calculate the maximum power dissipation using :
Layout Consideration
( ) PD = IOUT × VIN − VF − VOUT
PD = 0.8 × (5 − 0.5 − 2.85) = 1.32 W
2) Calculate the maximum θJA allowed for our ex-
ample:
The output capacitors must be located as close to
the Vout terminal of the device as possible. It is recom-
mended to use a section of a layer of the PC board as a
plane to connect the Vout pin to the output capacitors to
prevent any high frequency oscillation that may result
due to excessive trace inductance.
θJAMAX = ^{TJ − TA}
PD
θJAMAX = ^{135 − 35} = 75.6° C / W
1.32
2) Select a package from the datasheet with lower θJA
than the one calculated in the previous step.
Selecting TO252 (D Pak) with at least 0.5" square of
0.062" FR4 board using 1OZ copper has 70°C/W
which is lower than the calculated number.
Rev. 1.3
10/30/00
2-5