Datasheet

110
IN S
out
COMP
out
V V C
R
L I
× ××
=
×
7.5
s out
COMP
out COMP
V C
C
I R
×
=
× ×
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
D avg forward
P I V= ×
avg out
I I=
TPS61087
www.ti.com SLVS821C –MAY 2008–REVISED JULY 2013
Rectifier Diode Selection
To achieve high efficiency a Schottky type should be used for the rectifier diode. The reverse voltage rating
should be higher than the maximum output voltage of the converter. The averaged rectified forward current I
avg
,
the Schottky diode needs to be rated for, is equal to the output current I
out
:
(6)
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
V
r
V
forward
/I
avg
SUPPLIER COMPONENT CODE
RATING I
avg
2 A 20 V 0.44 V / 2 A Vishay Semiconductor SL22
2 A 20 V 0.5 V / 2 A Fairchild Semiconductor SS22
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
= 16 kΩ and C
COMP
= 2.7 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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