Datasheet

I
swpeak
+ I
L
)
Di
L
2
+ 1.11 A )
304 mA
2
+ 1.26 A
Di
L
+
ƪ
V
in
* V
sw
ƫ
D
f
s
L
+
(3.3 V * 0.5 V) 0.73
1.6 MHz 4.2 mH
+ 304 mA
I
L
+
I
out
1 * D
+
300 mA
1 * 0.73
+ 1.11 A
D +
V
out
) V
D
* V
in
V
out
) V
D
* V
sw
+
10 V ) 0.8 V * 3.3 V
10 V ) 0.8 V * 0.5 V
+ 0.73
TPS65140, TPS65141
TPS65145
www.ti.com SLVS497E –SEPTEMBER 2003–REVISED NOVEMBER 2012
APPLICATION INFORMATION
BOOST CONVERTER DESIGN PROCEDURE
The first step in the design procedure is to calculate the maximum possible output current of the main boost
converter under certain input and output voltage conditions. Below is an example for a 3.3-V to 10-V conversion:
V
in
= 3.3 V, V
out
= 10 V, Switch voltage drop V
sw
= 0.5 V, Schottky diode forward voltage V
D
= 0.8 V
1. Duty cycle:
2. Average inductor current:
3. Inductor peak-to-peak ripple current:
4. Peak switch current:
The integrated switch, the inductor, and the external Schottky diode must be able to handle the peak switch
current. The calculated peak switch current has to be equal or lower than the minimum N-MOSFET switch
current limit as specified in the electrical characteristics table (1.6 A for the TPS65140/41 and 0.96 A for the
TPS65145). If the peak switch current is higher, then the converter cannot support the required load current. This
calculation must be done for the minimum input voltage where the peak switch current is highest. The calculation
includes conduction losses like switch r
DS(on)
(0.5 V) and diode forward drop voltage losses (0.8 V). Additional
switching losses, inductor core and winding losses, etc., require a slightly higher peak switch current in the actual
application. The above calculation still allows for a good design and component selection.
Inductor Selection
Several inductors work with the TPS6514x. Especially with the external compensation, the performance can be
adjusted to the specific application requirements. The main parameter for the inductor selection is the saturation
current of the inductor which should be higher than the peak switch current as calculated above with additional
margin to cover for heavy load transients and extreme start-up conditions. Another method is to choose the
inductor with a saturation current at least as high as the minimum switch current limit of 1.6 A for the
TPS65140/41 and 0.96 A for the TPS65145. The different switch current limits allow selection of a physically
smaller inductor when less output current is required. The second important parameter is the inductor DC
resistance. Usually, the lower the DC resistance, the higher the efficiency. However, the inductor DC resistance
is not the only parameter determining the efficiency. Especially for a boost converter where the inductor is the
energy storage element, the type and material of the inductor influences the efficiency as well. Especially at high
switching frequencies of 1.6 MHz, inductor core losses, proximity effects, and skin effects become more
important. Usually, an inductor with a larger form factor yields higher efficiency. The efficiency difference
between different inductors can vary between 2% to 10%. For the TPS6514x, inductor values between 3.3 μH
and 6.8 μH are a good choice but other values can be used as well. Possible inductors are shown in Table 1.
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