Datasheet

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APPLICATION INFORMATION
BOOST CONVERTER DESIGN PROCEDURE
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
I
L

I
out
1  D

300 mA
1  0.73
 1.11 A
i
L


V
in
 V
sw

 D
f
s
 L

(3.3 V  0.5 V)  0.73
1.6 MHz  4.2 H
 304 mA
I
swpeak
 I
L

i
L
2
 1.11 A 
304 mA
2
 1.26 A
Inductor Selection
TPS65100 , TPS65101
TPS65105
SLVS496C – SEPTEMBER 2003 – REVISED APRIL 2006
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. The following example is 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 to or lower than the minimum N-MOSFET switch
current limit as specified in the electrical characteristics table (1.6 A for the TPS65100/01 and 0.96 A for the
TPS65105). 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
DSon
(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 a for good design and component selection.
Several inductors work with the TPS6510x series. Especially with the external compensation, the performance
can be adjusted to the specific application requirements. The main parameter for 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
TPS65100/01 and 0.96 A for the TPS65105. The different switch current limits allow selection of a physically
smaller inductor when less output current is required. The second important parameter is inductor dc resistance.
Usually, the lower the DC resistance, the higher the efficiency. However, inductor DC resistance, is not the only
parameter determining the efficiency. Especially for a boost converter where the inductor is the energy storage
elemen,t the type and material of the inductor influences the efficiency as well. Especially at the high switching
frequency 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 TPS6510x series, inductor values between 3.3 µ H and 6.8 µ H are a good
choice but other values can be used as well. Possible inductors are shown in Table 1 .
13
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