Datasheet

S IN OUT
C
S S OUT
V V I
V
V f C
-
D = ×
×
_ ( ) _
D = ×
C ESR L peak C ESR
V I R
TPS61085
SLVS859A –JUNE 2008–REVISED APRIL 2012 www.ti.com
Table 3. Recommended Compensation Network Values at High/Low Frequency
FREQUENCY L V
S
V
IN
± 20% R
COMP
C
COMP
Used I
OUT
5 V 82 kΩ 1.1 nF 0.7A
15 V
3.3 V 75 kΩ 1.6 nF 0.5A
5 V 51 kΩ 1.1 nF 0.9A
High (1.2 MHz) 3.3 µH 12 V
3.3 V 47 kΩ 1.6 nF 0.6A
5 V 30 kΩ 1.1 nF 1.2A
9 V
3.3 V 27 kΩ 1.6 nF 0.8A
5 V 43 kΩ 2.2 nF 0.7A
15 V
3.3 V 39 kΩ 3.3 nF 0.5A
5 V 27 kΩ 2.2 nF 0.9A
Low (650 kHz) 6.8 µH 12 V
3.3 V 24 kΩ 3.3 nF 0.6A
5 V 15 kΩ 2.2 nF 1.2A
9 V
3.3 V 13 kΩ 3.3 nF 0.8A
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 TPS61087.
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 Table 4 and typical applications for input capacitor recommendations.
Output Capacitor Selection
For best output voltage filtering a low ESR output capacitor like ceramic capcaitor is recommended. Two 10 μF
ceramic output capacitors (or one 22 μF) work for most of the applications. Higher capacitor values can be used
to improve the load transient response. Refer to Table 4 for the selection of the output capacitor.
Table 4. Rectifier Input and Output Capacitor Selection
CAPACITOR VOLTAGE RATING SUPPLIER COMPONENT CODE
C
IN
10 μF/1206 16 V Taiyo Yuden EMK212 BJ 106KG
IN bypass 1 μF/0603 16 V Taiyo Yuden EMK107 BJ 105KA
C
OUT
10 μF/1206 25 V Taiyo Yuden TMK316 BJ 106KL
To calculate the output voltage ripple, the following equation can be used:
(10)
with
ΔV
C
Output voltage ripple dependent on output capacitance,output current and switching frequency
V
S
Output voltage
V
IN
Minimum input voltage of boost converter
f
S
Converter switching frequency (typically 1.2 MHz or 650 kHz)
I
out
Output capacitance
ΔV
C_ESR
Output voltage ripple due to output capacitors ESR (equivalent series resistance)
I
SWPEAK
Inductor peak switch current in the application
R
C_ESR
Output capacitors equivalent series resistance (ESR)
ΔV
C_ESR
can be neglected in many cases since ceramic capacitors provide low ESR.
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