TPS61040
TPS61041
SLVS413 – FEBRUARY 2002
APPLICATION INFORMATION
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 next
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
is used across the upper resistor of the voltage
FF
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 can be used as described in the
FF
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 can be used.
FF
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:
I
L
I
out
1
P
DV
+
ǒfS(Iout) Vout ) Vd–VinǓ) I ESR
–
out
P
C
out
Where:
I = Peak current as described in the previous peak current control section
P
L = Selected inductor value
I
= Nominal load current
out
fS (I ) = Switching frequency at the nominal load current as calculated previously
out
Vd = Rectifier diode forward voltage (typically 0.3V)
C
= Selected output capacitor
out
ESR = Output capacitor ESR value
Refer to Table 2 and typical applications section for choosing the output capacitor.
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