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SETTING THE BOOST VOLTAGE
CC
0.5 (R1 + R2)
V =
R1
´
æ ö
ç ÷
è ø
(1)
INDUCTOR SELECTION
CC
L CC
DD
V
I = I
V 0.8
æ ö
´
ç ÷
´
è ø
(2)
DD CC DD
L CC
V (V V )
L =
I f V
boost
´ -
D ´ ´
(3)
TPA2013D1
SLOS520 – AUGUST 2007
Use Equation 1 to determine the value of R1 for a given V
CC
. The maximum recommended value for V
CC
is
5.5 V. The typical value of the V
CC
FB pin is 500 mV. The current through the resistor divider should be about 100
times greater than the current into the V
CC
FB pin, typically 0.01 μ A. Based on those two values, the
recommended value of R2 is 500 k Ω . V
CC
must be greater than 3 V and less than or equal to 5.5 V.
SURFACE MOUNT INDUCTORS
Working inductance decreases as inductor current increases. If the drop in working inductance is severe enough,
it may cause the boost converter to become unstable, or cause the TPA2013D1 to reach its current limit at a
lower output power than expected. Inductor vendors specify currents at which inductor values decrease by a
specific percentage. This can vary by 10% to 35%. Inductance is also affected by dc current and temperature.
TPA2013D1 INDUCTOR EQUATIONS
Inductor current rating is determined by the requirements of the load. The inductance is determined by two
factors: the minimum value required for stability and the maximum ripple current permitted in the application.
Use Equation 2 to determine the required current rating. Equation 2 shows the approximate relationship between
the average inductor current, I
L
, to the load current, load voltage, and input voltage (I
CC
, V
CC
, and V
DD
,
respectively). Insert I
CC
, V
CC
, and V
DD
into Equation 2 to solve for I
L
. The inductor must maintain at least 90% of
its initial inductance value at this current.
The minimum working inductance is 2.2 μ H. A lower value may cause instability.
Ripple current, Δ I
L
, is peak-to-peak variation in inductor current. Smaller ripple current reduces core losses in the
inductor as well as the potential for EMI. Use Equation 3 to determine the value of the inductor, L. Equation 3
shows the relationship between inductance L, V
DD
, V
CC
, the switching frequency, f
boost
, and Δ I
L
. Insert the
maximum acceptable ripple current into Equation 3 to solve for L.
Δ I
L
is inversely proportional to L. Minimize Δ I
L
as much as is necessary for a specific application. Increase the
inductance to reduce the ripple current. Note that making the inductance too large will prevent the boost
converter from responding to fast load changes properly. Typical inductor values for the TPA2013D1 are 4.7 μ H
to 6.8 μ H.
Select an inductor with a small dc resistance, DCR. DCR reduces the output power due to the voltage drop
across the inductor.
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Product Folder Link(s): TPA2013D1