C
FF
+
1
2
p
fS
20
R1
V
OUT
+
1.233 V
ǒ
1
)
R1
R2
Ǔ
13
TPS61040, TPS61041
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SLVS413I – OCTOBER 2002 – REVISED DECEMBER 2016
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Copyright © 2002–2016, Texas Instruments Incorporated
8.2.2.2 Setting the Output Voltage
The output voltage is calculated as:
(5)
For battery-powered applications, a high-impedance voltage divider should be used with a typical value for R2 of
≤
200 k
Ω
and a maximum value for R1 of 2.2 M
Ω
. Smaller values might be used to reduce the noise sensitivity of
the feedback pin.
A feedforward capacitor across the upper feedback resistor R1 is required to provide sufficient overdrive for the
error comparator. Without a feedforward capacitor, or one whose value is too small, the TPS6104x shows
double
pulses
or a pulse burst instead of single pulses at the switch node (SW), causing higher output voltage ripple. If
this higher output voltage ripple is acceptable, the feedforward capacitor can be left out.
The lower the switching frequency of the converter, the larger the feedforward capacitor value required. A good
starting point is to use a 10-pF feedforward capacitor. As a first estimation, the required value for the feedforward
capacitor at the operation point can also be calculated using the following formula:
where
•
R1 = Upper resistor of voltage divider
•
fS = Switching frequency of the converter at the nominal load current (See
Inductor Selection, Maximum Load
Current
for calculating the switching frequency)
•
C
FF
= Choose a value that comes closest to the result of the calculation
(6)
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.
8.2.2.3 Line and Load Regulation
The line regulation of the TPS6104x 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 TPS6104x 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.
8.2.2.4 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:
