Datasheet
RHPZ
2
(1 D) Vout
=
2 Iout L
- ´
´ ´
f
p
PEAK
Iout Vin D
I = +
η (1 D) 2 L
´
´ - ´ ´f
Vout Vin
D =
Vout
-
Duty Cycle Boost
TPS63020
TPS63021
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SLVS916C –JULY 2010–REVISED MARCH 2013
APPLICATION INFORMATION
DESIGN PROCEDURE
The TPS6302X series of buck-boost converter has internal loop compensation. Therefore, the external L-C filter
has to be selected to work with the internal compensation. As a general rule of thumb, the product L×C should
not move over a wide range when selecting a different output filter. However, when selecting the output filter a
low limit for the inductor value exists to avoid subharmonic oscillation which could be caused by a far too fast
ramp up of the amplified inductor current. For the TPS6302X series the minimum inductor value should be kept
at 1uH.
In particular either 1uH or 1.5uH is recommended working at output current between 1.5A and 2A. If operating
with lower load current is also possible to use 2.2uH.
Selecting a larger output capacitor value is less critical because the corner frequency moves to lower
frequencies.
Inductor Selection
For high efficiencies, the inductor should have a low dc resistance to minimize conduction losses. Especially at
high-switching frequencies the core material has a higher impact on efficiency. When using small chip inductors,
the efficiency is reduced mainly due to higher inductor core losses. This needs to be considered when selecting
the appropriate inductor. The inductor value determines the inductor ripple current. The larger the inductor value,
the smaller the inductor ripple current and the lower the conduction losses of the converter. Conversely, larger
inductor values cause a slower load transient response. To avoid saturation of the inductor, with the chosen
inductance value, the peak current for the inductor in steady state operation can be calculated. Equation (1) and
(5) show how to calculate the peak current I
PEAK
. Only the equation which defines the switch current in boost
mode is reported because this is providing the highest value of current and represents the critical current value
for selecting the right inductor.
(1)
(2)
With,
D =Duty Cycle in Boost mode
f = Converter switching frequency (typical 2.4 MHz)
L = Selected inductor value
η = Estimated converter efficiency (use the number from the efficiency curves or 0.80 as an assumption)
Note: The calculation must be done for the maximum input voltage which is possible to have in boost mode
Consideration must be given to the load transients and error conditions that can cause higher inductor currents.
This must be taken into consideration when selecting an appropriate inductor. Please refer to Table 3 for typical
inductors.
The size of the inductor can also affect the stability of the feedback loop. In particular the boost transfer function
exhibits a right half-plane zero, whose frequency is inverse proportional to the inductor value and the load
current. This means higher is the value of inductance and load current more possibilities has the right plane zero
to be moved at lower frequency which could degrade the phase margin of the feedback loop. It is recommended
to choose the inductor's value in order to have the frequency of the right half plane zero >400KHz. The frequency
of the RHPZ can be calculated using equation (6)
(3)
With,
D =Duty Cycle in Boost mode
Note: The calculation must be done for the maximum input voltage which is possible to have in boost mode
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