US1175
The US1175 keeps a constant 1.25V between the Vsense
pin and the Vadj pin. By placing a resistor R1 across
these two pins and connecting the Vsense and Vout pin
together , a constant current flows through R1, adding
to the Iadj current and into the R2 resistor producing a
voltage equal to the (1.25/R1)*R2 + Iadj*R2 .This voltage
is then added to the 1.25V to set the output voltage.
This is summarized in the above equation. Since the
minimum load current requirement of the US1175 is 10
mA , R1 is typically selected to be a 121W resistor so
that it automatically satisfies this condition. Notice that
since the Iadj is typically in the range of 50uA it only
adds a small error to the output voltage and should be
considered when very precise output voltage setting is
required.
For most applications a minimum of 100uF aluminum
electrolytic capacitor such as Sanyo, MVGX series
,Panasonic FA series as well as the Nichicon PL series
insures both stability and good transient response.
Thermal Design
The US1175 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the allowable maximum 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 continuos load operation the junction temperature
is kept below this number. The example below shows
the steps in selecting the proper surface mount pack-
age.
Load Regulation
Assuming, the following conditions:
Vout=2.7V
Vin=3.3V
Vctrl=5V
Iout=2A DC Avg
Calculate the maximum power dissipation using the fol-
lowing equation:
Pd=Iout*(Vin-Vout) + (Iout/60)*(Vctrl - Vout)
Pd=2*(3.3-2.7) + (2/60)*(5-2.7)=1.28 W
Using table below select the proper package and the
amount of copper board needed.
Since the US1175 has separate pins for the output (Vout)
and the sense (Vsense), it is ideal for providing true re-
mote sensing of the output voltage at the load.This
means that the voltage drops due to parasitic resistance
such as PCB traces between the regulator and the load
are compensated for using remote sensing. Figure 3
shows a typical application of the US1175 with remote
sensing.
Vin
Vout
Vin
US1175
Pkg
Copper qJA(°C/W)
Max Pd Max Pd
Area
(Ta=25°C)
4.4W
3.7W
3.1W
2.4W
2.0W
(Ta=45°C)
3.6W
Vctrl
Vsense
Vctrl
R
L
Adj
R1
R2
TO263 1.4"X1.4"
TO263 1.0"X1.0"
TO263 0.7"X0.7"
TO263 Pad Size
25
30
35
45
55
3.0W
2.6W
2.0W
1.63W
SO8
1.0"X1.0"
1175app3-1.0
Note: Above table is based on the maximum junction
temperature of 135°C.
As shown in the above table, any of the two packages
will do the job. For low cost applications the SO8 pack-
age is recommended.
Figure 3 - Schematic showing connection for best
load regulation
Stability
The US1175 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 mW and an output
capacitance of 500 to 1000uF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The US1175 takes advantage of
this phenomena in making the overall regulator loop
stable.
Rev. 1.2
11/29/99
2-5
UNISEM [ UNISEM ]
