Datasheet

PEAK
Iout Vin D
I = +
η (1 D) 2 L
´
´ - ´ ´f
RHPZ
2
(1 D) Vout
=
2 Iout L
- ´
´ ´
f
p
TPS63060
TPS63061
SLVSA92A –DECEMBER 2011– REVISED FEBRUARY 2012 www.ti.com
APPLICATION INFORMATION
DESIGN PROCEDURE
The TPS6306x 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. 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 TPS6306x series, the minimum inductor value should be kept at 1µH.
Selecting a larger output capacitor value is less critical because the corner frequency moves to lower
frequencies. To simplify this process, Table 2 outlines possible inductor and capacitor value combinations.
Table 2. Output Filter Selection (Average Inductance current up to 2A)
OUTPUT CAPACITOR VALUE [µF]
(2)
INDUCTOR VALUE [µH]
(1)
44 66 100
1.0 √ √
(3)
√
1.5 √ √ √
(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. Equation 1 and
Equation 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.
(5)
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 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. See 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 half 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 6.
(6)
With,
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