V
OUT
+
1.233 V
ǒ
1
)
R1
R2
Ǔ
C
FF
+
1
2
p
fS
20
R1
TPS61040
TPS61041
www.ti.com
SLVS413F – OCTOBER 2002 – REVISED DECEMBER 2010
The maximum load current of the converter is the current at the operation point where the converter starts to
enter the continuous conduction mode. Usually the converter should always operate in discontinuous conduction
mode.
Last, the selected inductor should have a saturation current that meets the maximum peak current of the
converter (as calculated in the
Peak Current Control
section). Use the maximum value for I
LIM
for this calculation.
Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher the efficiency
of the converter. See
Table 4
and the typical applications for the inductor selection.
Table 4. Recommended Inductor for Typical LCD Bias Supply (see
Figure 15
)
DEVICE
INDUCTOR VALUE
COMPONENT SUPPLIER
COMMENTS
10
m
H
Sumida CR32-100
High efficiency
10
m
H
Sumida CDRH3D16-100
High efficiency
TPS61040
10
m
H
Murata LQH4C100K04
High efficiency
4.7
m
H
Sumida CDRH3D16-4R7
Small solution size
4.7
m
H
Murata LQH3C4R7M24
Small solution size
High efficiency
TPS61041
10
m
H
Murata LQH3C100K24
Small solution size
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 TPS61040/41 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 the
Inductor Selection, Maximum
Load Current
section for calculating the switching frequency)
C
FF
= Choose a value that comes closest to the result of the calculation
(6)
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