US1075
Output Voltage Setting
The US1075 can be programmed to any voltages in the
range of 1.25V to 5.5V with the addition of R1 and R2
external resistors according to the following formula:
R
2
V
OUT
=
V
REF
1
+ +
I
ADJ
×
R
2
R
1
Where : V
REF
= 125 V Typically
.
I
ADJ
= 50 uA Typically
R
1
& R
2
as shown in figure 2
regulation is achieved when the bottom side of R2 is
connected to the load and the top side of R1 resistor is
connected directly to the case or the Vout pin of the
regulator and not to the load. In fact , if R1 is connected
to the load side, the effective resistance between the
regulator and the load is gained up by the factor of (1+R2/
R1) ,or the effective resistance will be ,Rp(eff)=Rp*(1+R2/
R1).It is important to note that for high current applica-
tions, this can represent a significant percentage of the
overall load regulation and one must keep the path from
the regulator to the load as short as possible to mini-
mize this effect.
Vin
Vin
Vout
Vout
Vin
Vref
R1
Adj
Vin
Vout
PARASITIC LINE
RESISTANCE
US1075
Adj
Rp
US1075
R1
RL
IAdj = 50uA
R2
1075app2-1.0
R2
Figure 2 - Typical application of the US1075
for programming the output voltage.
The US1075 keeps a constant 1.25V between the out-
put pin and the adjust pin. By placing a resistor R1 across
these two pins a constant current flows through R1, add-
ing to the Iadj current and into the R2 resistor producing
a voltage equal to the (1.25/R1)*R2 +
I
adj*R2 which will
be 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 US1075 is 10 mA , R1 is
typically selected to be 121
Ω
resistor so that it auto-
matically satisfies the minimum current requirement.
Notice that since Iadj is typically in the range of 50uA it
only adds a small error to the output voltage and should
only be considered when a very precise output voltage
setting is required. For example, in a typical 3.3V appli-
cation where R1=121
Ω
and R2=200
Ω
the error due to
Iadj is only 0.3% of the nominal set point.
1075app3-1.0
Figure 3 - Schematic showing connection for best load
regulation
Stability
The US1075 requires the use of an output capacitor as
part of the frequency compensation in order to make the
regulator stable. Typical designs for microprocessor ap-
plications use standard electrolytic capacitors with a
typical ESR in the range of 50 to 100 m
Ω
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 US1075 takes advantage of
this phenomena in making the overall regulator loop
stable.For most applications a minimum of 100uF alu-
minum electrolytic capacitor such as Sanyo MVGX se-
ries ,Panasonic FA series as well as the Nichicon PL
series insures both stability and good transient response.
Load Regulation
Since the US1075 is only a 3 terminal device , it is not
possible to provide true remote sensing of the output
voltage at the load.Figure 3 shows that the best load
2-36
Rev. 1.0
9/14/99
UNISEM [ UNISEM ]
