Datasheet

I
Vin x D
2 x f x L
PEAK
= I
SW_MAX
+
Vout Vin
D =
Vout
-
Duty Cycle Boost
TPS63036
SLVSB76 –AUGUST 2012 www.ti.com
APPLICATION INFORMATION
DESIGN PROCEDURE
The TPS63036 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 TPS63036 the minimum inductor value should be kept at 1uH. To simplify this
process Table 2 outlines possible inductor and capacitor value combinations.
Table 2. Output Filter Selection (Average Inductance current up to 1A)
OUTPUT CAPACITOR VALUE [µF]
(2)
INDUCTOR VALUE [µH]
(1)
30 44 66
1.0 √ √ √
1.5 √
(3)
√ √
2.2 √
(1) Inductor tolerance and current de-rating is anticipated. The effective inductance can vary by 20% and –30%.
(2) Capacitance tolerance and bias voltage de-rating is anticipated. The effective capacitance can vary by 20% and –50%.
(3) Typical application. Other check mark indicates recommended filter combinations
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. 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.
(5)
(6)
With,
D =Duty Cycle in Boost mode
f = Converter switching frequency (typical 2 MHz)
L = Selected inductor value
η = Estimated converter efficiency (use the number from the efficiency curves or 0.80 as an assumption)
I
SW_MAX
=Maximum average input current (Figure 4)
Note: The calculation must be done for the minimum 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. This 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 (3)
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