TPS61040
TPS61041
SLVS413 – FEBRUARY 2002
APPLICATION INFORMATION
The maximum load current of the conveter is the current at the operation point where the coverter 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
for thiscalculation.
LIM
Another important inductor parameter is the dc resistance. The lower the dc resistance, the higher the efficiency
of the converter. Refer to the Table 1 and the typical applications for the inductor selection.
Table 1. Recommended Inductor for Typical LCD Bias Supply (see Figure 15)
DEVICE
INDUCTOR VALUE
10 µH
COMPONENT SUPPLIER
Sumida CR32-100
COMMENTS
High efficiency
10 µH
Sumida CDRH3D16-100
Murata LQH4C100K04
Sumida CDRH3D16-4R7
Murata LQH3C4R7M24
High efficiency
10 µH
High efficiency
TPS61040
4.7 µH
Small solution size
Small solution size
4.7 µH
High efficiency
Small solution size
TPS61041
10 µH
Murata LQH3C100K24
setting the output voltage
The output voltage is calculated as:
R1
R2
ǒ
+ 1.233 V 1 )
Ǔ
V
out
For battery powered applications a high impedance voltage divider should be used with a typical value for R2
of ≤200 kΩ and a maximum value for R1 of 2.2 MΩ. 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:
1
C
+
FF
fS
2 p
R1
20
Where:
R1 = Upper resistor of voltage divider
fS = Switching frequency of the converter at the nominal load current (See previous section for calculating the
switching frequency)
C
= Choose a value that comes closest to the result of the calculation
FF
12
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