Texas Instruments TPS6104 Series - Page 12

2
P S (m a x)
lo a d(m ax)
O U T IN
I L f
I
2 ( V V )
´ ´
= h
´ -
load OUT IN d
S load
2
P
2 I (V V V )
f (I )
I L
´ ´ - +
=
´
IN(min) OUT IN
S(max)
P OUT
V (V V )
f
I L V
´ -
=
´ ´
12
TPS61040 , TPS61041
SLVS413I –OCTOBER 2002–REVISED DECEMBER 2016 www.ti.com
Product Folder Links: TPS61040 TPS61041
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The inductor value determines the maximum switching frequency of the converter. Therefore, select the inductor
value that ensures the maximum switching frequency at the converter maximum load current is not exceeded.
The maximum switching frequency is calculated by the following formula:
where
• I
P
= Peak current as described in Peak Current Control
• L = Selected inductor value
• V
IN(min)
= The highest switching frequency occurs at the minimum input voltage (2)
If the selected inductor value does not exceed the maximum switching frequency of the converter, the next step
is to calculate the switching frequency at the nominal load current using the following formula:
where
• I
P
= Peak current as described in Peak Current Control
• L = Selected inductor value
• I
load
= Nominal load current
• Vd = Rectifier diode forward voltage (typically 0.3 V) (3)
A smaller inductor value gives a higher converter switching frequency, but lowers the efficiency.
The inductor value has less effect on the maximum available load current and is only of secondary order. The
best way to calculate the maximum available load current under certain operating conditions is to estimate the
expected converter efficiency at the maximum load current. This number can be taken out of the efficiency
graphs shown in Figure 1 through Figure 4. The maximum load current can then be estimated as follows:
where
• I
P
= Peak current as described in Peak Current Control
• L = Selected inductor value
• fS
max
= Maximum switching frequency as calculated previously
• η = Expected converter efficiency. Typically 70% to 85% (4)
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 Peak Current Control). 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 3 and the typical applications for the inductor selection.
Table 3. Recommended Inductor for Typical LCD Bias Supply (see Figure 23 )
DEVICE INDUCTOR VALUE COMPONENT SUPPLIER COMMENTS
TPS61040
10 μH Sumida CR32-100 High efficiency
10 μH Sumida CDRH3D16-100 High efficiency
10 μH Murata LQH4C100K04 High efficiency
4.7 μH Sumida CDRH3D16-4R7 Small solution size
4.7 μH Murata LQH3C4R7M24 Small solution size
TPS61041 10 μH Murata LQH3C100K24
High efficiency
Small solution size