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SLVS859 – JUNE 2008
Setting the Output Voltage
The output voltage is set by an external resistor divider. Typically, a minimum current of 50
µA
flowing through
the feedback divider gives good accuracy and noise covering. A standard low side resistor of 18 kΩ is typically
selected. The resistors are then calculated as:
R
2
=
Vref
»
18
k
W
70
m
A
æ
V
S
ö
-
1
÷
R
1
=
R
2
´ ç
è
Vref
ø
(2)
Compensation (COMP)
The regulator loop can be compensated by adjusting the external components connected to the COMP pin. The
COMP pin is the output of the internal transconductance error amplifier. Standard values of R
COMP
= 13 kΩ and
C
COMP
= 3.3 nF will work for the majority of the applications.
Please refer to
for dedicated compensation networks giving an improved load transient response. The
following equations can be used to calculate R
COMP
and C
COMP
:
R
COMP
=
125 × V
IN
× V
S
× Cout
L × Iout_max
C
COMP
=
V
S
× Cout
5 × Iout_max × R
COMP
(3)
Table 3. Recommended Compensation Network Values at High/Low Frequency
FREQUENCY
L
V
S
15 V
High (1.2 MHz)
3.3 µH
12 V
9V
15 V
Low (650 kHz)
6.8 µH
12 V
9V
V
IN
± 20%
5V
3.3 V
5V
3.3 V
5V
3.3 V
5V
3.3 V
5V
3.3 V
5V
3.3 V
R
COMP
82 kΩ
75 kΩ
51 kΩ
47 kΩ
30 kΩ
27 kΩ
43 kΩ
39 kΩ
27 kΩ
24 kΩ
15 kΩ
13 kΩ
C
COMP
1.1 nF
1.6 nF
1.1 nF
1.6 nF
1.1 nF
1.6 nF
2.2 nF
3.3 nF
2.2 nF
3.3 nF
2.2 nF
3.3 nF
gives conservatives Rcomp and Comp values for certain inductors, input and output voltages providing a
very stable system. For a faster response time, a higher Rcomp value can be used to enlarge the bandwidth, as
well as a slightly lower value of Ccomp to keep enough phase margin. These adjustments should be performed
in parallel with the load transient response monitoring of TPS61085.
Input Capacitor Selection
For good input voltage filtering low ESR ceramic capacitors are recommended. TPS61085 has an analog input
IN. Therefore, a 1
µF
bypass is highly recommended as close as possible to the IC from IN to GND.
One 10
µF
ceramic input capacitors are sufficient for most of the applications. For better input voltage filtering
this value can be increased. Refer to
and typical applications for input capacitor recommendations.
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