Datasheet

Setting the Output Voltage
2 18
70
1.238
m
= » W
=
FB
FB
V
R k
A
V V
1 2 1
S
FB
V
R R
V
æ ö
= × -
ç ÷
è ø
V
FB
R1
R2
V
S
(8)
Compensation (COMP)
110
IN S
out
COMP
out
V V C
R
L I
× ××
=
×
7.5
s out
COMP
out COMP
V C
C
I R
×
=
× ×
(9)
Input Capacitor Selection
TPS61086
SLVSA05 – AUGUST 2009 ................................................................................................................................................................................................ www.ti.com
The output voltage is set by an external resistor divider. Typically, a minimum current of 50 µ A flowing through
the feedback divider gives good accuracy and noise covering. A standard low side resistor of 18 k Ω is typically
selected. The resistors are then calculated as:
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
= 16 k Ω and C
COMP
= 2.7 nF will work for the majority of the applications.
Please refer to Table 3 for dedicated compensation networks giving an improved load transient response. The
following equations can be used to calculate R
COMP
and C
COMP
:
with
V
IN
Minimum input voltage
V
S
Output voltage
C
out
Output capacitance
L Inductor value, e.g. 3.3 µ H or 4.7 µ H
I
out
Maximum output current in the application
Make sure that R
COMP
< 120 k Ω and C
COMP
> 820 pF, independent of the results of the above formulas.
Table 3. Recommended Compensation Network Values at High/Low Frequency
L V
S
V
IN
± 20% R
COMP
C
COMP
5 V 100 k Ω 820 pF
15 V
3.3 V 91 k Ω 1.2 nF
5 V 68 k Ω 820 pF
3.3 µ H 12 V
3.3 V 68 k Ω 1.2 nF
5 V 39 k Ω 820 pF
9 V
3.3 V 39 k Ω 1.2 nF
Table 3 gives conservative R
COMP
and C
COMP
values for certain inductors, input and output voltages providing a
very stable system. For a faster response time, a higher R
COMP
value can be used to enlarge the bandwidth, as
well as a slightly lower value of C
COMP
to keep enough phase margin. These adjustments should be performed in
parallel with the load transient response monitoring of TPS61086.
For good input voltage filtering low ESR ceramic capacitors are recommended. TPS61086 has an analog input
IN. Therefore, a 1 µ F bypass is highly recommended as close as possible to the IC from IN to GND.
One 10 µ F ceramic input capacitors are sufficient for most of the applications. For better input voltage filtering
this value can be increased. Refer to Table 4 and typical applications for input capacitor recommendation
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