TPS55340-EP_15 Datasheet, PDF (15/38 Pages) Texas Instruments – TPS55340-EP Integrated 5-A, 40-V Wide Input Range Boost/SEPIC/Flyback DC-DC Regulator

www.ti.com
TPS55340-EP
SLVSCG7 – JULY 2014
All converters using a diode as the freewheeling or catch component have a load current level at which they
transit from DCM to CCM. This is the point where the inductor current just falls to 0 during the off-time of the
power switch. At higher load currents, the inductor current does not fall to 0, and diode and switch current
assume a trapezoidal wave shape as opposed to a triangular wave shape. The load current boundary between
discontinuous conduction and continuous conduction can be found for a set of converter parameters as follows.
( _{( )} ) _IOUT(crit)
=
V_OUT +
2 ´ V_OUT
V_D -
+ V_D
V_IN ´ V_IN²
2 _{´ ¦SW ´ L}
(10)
For loads higher than the result of Equation 10, the duty cycle is given by Equation 8. For loads less than the
results of Equation 10, the duty cycle is given Equation 9. For Equation 7 through Equation 10, the variable
definitions are as follows.
• V_OUT is the output voltage of the converter in V
• V_D is the forward conduction voltage drop across the rectifier or catch diode in V
• V_IN is the input voltage to the converter in V
• I_OUT is the output current of the converter in A
• L is the inductor value in H
• ƒ_SW is the switching frequency in Hz
NOTE
Unless otherwise stated, the design equations that follow assume that the converter is
running in CCM, which typically results in a higher efficiency for the power levels of this
converter.
10.2.1.2.3 Selecting the Inductor (L1)
The selection of the inductor affects steady-state operation as well as transient behavior and loop stability. These
factors make it the most important component in power regulator design. There are three important inductor
specifications: inductor value, DC resistance, and saturation current. Considering inductor value alone is not
enough. Inductor values can have ±20% tolerance with no current bias. When the inductor current approaches
saturation level, the effective inductance can fall to a fraction of the zero current value.
The minimum value of the inductor should be able to meet inductor current ripple (ΔI_L) requirement at worst
case. In a boost converter, maximum inductor current ripple occurs at 50% duty cycle. For the applications where
duty cycle is always smaller or larger than 50%, Equation 12 should be used with the duty cycle closest to 50%
and corresponding input voltage to calculate the minimum inductance. For applications that need to operate with
50% duty cycle when input voltage is somewhere between the minimum and the maximum input voltage, use
Equation 13. K_IND is a coefficient that represents the amount of inductor ripple current relative to the maximum
input current (I_INDC = I_Lavg). The maximum input current can be estimated with Equation 11, with an estimated
efficiency based on similar applications (η_EST). The inductor ripple current will be filtered by the output capacitor.
Therefore, choosing high inductor ripple currents impacts the selection of the output capacitor because the
output capacitor must have a ripple current rating equal to or greater than the inductor ripple current. In general,
the inductor ripple value (K_IND) is at the discretion of the designer. However, the following guidelines may be
used.
For CCM operation, TI recommends to use K_IND values in the range of 0.2 to 0.4. Choosing K_IND closer to 0.2
results in a larger inductance value, maximizes the converter’s potential output current, and minimizes EMI.
Choosing K_IND closer to 0.4 results in a smaller inductance value, a physically smaller inductor, and improved
transient response, but potentially worse EMI and lower efficiency. Using an inductor with a smaller inductance
value may result in the converter operating in DCM. This reduces the boost converter’s maximum output current
and causes larger input voltage and output voltage ripple and reduced efficiency. For this design, choose K_IND =
0.3 and a conservative efficiency estimate of 85% with the minimum input voltage and maximum output current.
Equation 12 is used with the maximum input voltage because this corresponds to duty cycle closest to 50%. The
maximum input current is estimated at 4.52 A and the minimum inductance is 7.53 µH. A standard value of 10
µH is chosen.
I_INDC
=
V_OUT ´ I_OUT
h_EST ´ V_IN min
(11)
Copyright © 2014, Texas Instruments Incorporated
Submit Documentation Feedback
15