Datasheet

D avg forward
P I V= ×
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
110
IN S
OUT
COMP
OUT
V V C
R
L I
× ××
=
×
7.5
s OUT
COMP
OUT COMP
V C
C
I R
×
=
× ×
TPS61085
www.ti.com SLVS859A –JUNE 2008–REVISED APRIL 2012
Usually a Schottky diode with 2 A maximum average rectified forward current rating is sufficient for most
applications. The Schottky rectifier can be selected with lower forward current capability depending on the output
current I
out
but has to be able to dissipate the power. The dissipated power, P
D
, is the average rectified forward
current times the diode forward voltage, V
forward
.
(7)
Typically the diode should be able to dissipate around 500mW depending on the load current and forward
voltage.
Table 2. Rectifier Diode Selection
CURRENT COMPONENT PACKAGE
Vr V
forward / Iavg
SUPPLIER
RATING Iavg CODE TYPE
750 mA 20 V 0.425 V / Fairchild Semiconductor FYV0704S SOT 23
750 mA
1 A 20 V 0.39 V / 1 A NXP PMEG2010AEH SOD 123
1 A 20 V 0.52 V / 1 A Vishay Semiconductor B120 SMA
1 A 20 V 0.5 V / 1 A Vishay Semiconductor SS12 SMA
1 A 20 V 0.44 V / 1 A Vishay Semiconductor MSS1P2L μ-SMP (Low
Profile)
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:
(8)
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.
See 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
:
(9)
with
V
IN
Minimum input voltage
V
S
Output voltage
C
out
Output capacitance
L Inductor value, e.g. 3.3 μH or 6.8 μH
I
OUT
Maximum output current in the application
Make sure that R
COMP
< 120 kΩ and C
COMP
> 820 pF, independent of the results of the above formulas.
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