D
Vout
+
Iout
Cout
ǒ
1
fS(Iout)
–
I
P
L
Vout
)
Vd–Vin
Ǔ
)
I
P
ESR
TPS61040
TPS61041
SLVS413F – OCTOBER 2002 – REVISED DECEMBER 2010
www.ti.com
The larger the feedforward capacitor the worse the line regulation of the device. Therefore, when concern for line
regulation is paramount, the selected feedforward capacitor should be as small as possible. See the following
section for more information about line and load regulation.
LINE AND LOAD REGULATION
The line regulation of the TPS61040/41 depends on the voltage ripple on the feedback pin. Usually a 50 mV
peak-to-peak voltage ripple on the feedback pin FB gives good results.
Some applications require a very tight line regulation and can only allow a small change in output voltage over a
certain input voltage range. If no feedforward capacitor C
FF
is used across the upper resistor of the voltage
feedback divider, the device has the best line regulation. Without the feedforward capacitor the output voltage
ripple is higher because the TPS61040/41 shows output voltage bursts instead of single pulses on the switch pin
(SW), increasing the output voltage ripple. Increasing the output capacitor value reduces the output voltage
ripple.
If a larger output capacitor value is not an option, a feedforward capacitor C
FF
can be used as described in the
previous section. The use of a feedforward capacitor increases the amount of voltage ripple present on the
feedback pin (FB). The greater the voltage ripple on the feedback pin (
≥
50 mV), the worse the line regulation.
There are two ways to improve the line regulation further:
1. Use a smaller inductor value to increase the switching frequency which will lower the output voltage ripple,
as well as the voltage ripple on the feedback pin.
2. Add a small capacitor from the feedback pin (FB) to ground to reduce the voltage ripple on the feedback pin
down to 50 mV again. As a starting point, the same capacitor value as selected for the feedforward capacitor
C
FF
can be used.
OUTPUT CAPACITOR SELECTION
For best output voltage filtering, a low ESR output capacitor is recommended. Ceramic capacitors have a low
ESR value but tantalum capacitors can be used as well, depending on the application.
Assuming the converter does not show double pulses or pulse bursts on the switch node (SW), the output
voltage ripple can be calculated as:
where:
I
P
= Peak current as described in the
Peak Current Control
section
L = Selected inductor value
I
out
= Nominal load current
fS (I
out
) = Switching frequency at the nominal load current as calculated previously
Vd = Rectifier diode forward voltage (typically 0.3 V)
C
out
= Selected output capacitor
ESR = Output capacitor ESR value
(7)
See
Table 5
and the typical applications section for choosing the output capacitor.
Table 5. Recommended Input and Output Capacitors
DEVICE
CAPACITOR
VOLTAGE RATING
COMPONENT SUPPLIER
COMMENTS
4.7
m
F/X5R/0805
6.3 V
Tayo Yuden JMK212BY475MG
C
IN
/C
OUT
10
m
F/X5R/0805
6.3 V
Tayo Yuden JMK212BJ106MG
C
IN
/C
OUT
TPS61040/41
1
m
F/X7R/1206
25 V
Tayo Yuden TMK316BJ105KL
C
OUT
1
m
F/X5R/1206
35 V
Tayo Yuden GMK316BJ105KL
C
OUT
4.7
m
F/X5R/1210
25 V
Tayo Yuden TMK325BJ475MG
C
OUT
12
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