Datasheet

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
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
swpeak
+ I
L
)
Di
L
2
+ 1.11 A )
304 mA
2
+ 1.26 A
TPS65100-Q1
www.ti.com SLVS849A –JULY 2008–REVISED APRIL 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. This 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 must be equal to or lower than the minimum N-MOSFET switch
current limit specified in electrical characteristics. If the peak switch current is higher, 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. This calculation still allows for good design and component
selection.
Inductor Selection
Several inductors work with the TPS65100 and, particularly with the external compensation, performance can be
adjusted to 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 previously calculated, with additional margin to
allow for heavy load transients and extreme start-up conditions. Another method is to choose an inductor with a
saturation current at least as high as the minimum switch current limit of 1.6 A. The different switch-current limits
allow selection of a physically smaller inductor when less output current is required. Another 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 element, the type and material of the inductor influences the efficiency as well. At
the high switching frequency of 1.6 MHz, inductor core losses, proximity effects, and skin effects are 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%. Inductor values between 3.3 μH and 6.8 μH are a
good choice, but other values can be used. Possible inductors are shown in Table 1.
Table 1. Inductor Selection
INDUCTOR VALUE COMPONENT SUPPLIER DIMENSIONS (mm) ISAT/DCR
4.7 μH Coilcraft DO1813P-472HC 8,89 × 6,1 × 5 2.6 A/54 mΩ
4.2 μH Sumida CDRH5D28 4R2 5,7 × 5,7 × 3 2.2 A/23 mΩ
4.7 μH Sumida CDC5D23 4R7 6 × 6 × 2,5 1.6 A/48 mΩ
3.3 μH Wuerth Elektronik 744042003 4,8 × 4,8 × 2 1.8 A/65 mΩ
4.2 μH Sumida CDRH6D12 4R2 6,5 × 6,5 × 1,5 1.8 A/60 mΩ
3.3 μH Sumida CDRH6D12 3R3 6,5 × 6,5 × 1,5 1.9 A/50 mΩ
Copyright © 2008–2012, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): TPS65100-Q1