TPS61130 Datasheet, PDF (17/28 Pages) Texas Instruments – SYNCHRONOUS SEPIC / FLYBACK CONVERTER WITH 1.1A SWITCH AND INTEGRATED LDO

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TPS61130
TPS61131
TPS61132
SLVS431A – JUNE 2002 – REVISED AUGUST 2003
APPLICATION INFORMATION (continued)
Parameter f is the switching frequency and∆ I_L is the ripple current in the inductor, i.e., 40% × I_L. In this example,
the desired inductance value is in the range of 20 µH. With this calculated value and the calculated currents, it is
possible to choose a suitable inductor. Care has to be taken that load transients and losses in the circuit can
lead to higher currents as estimated in Equation 6. Also, the losses in the inductor caused by magnetic
hysteresis losses and copper losses are a major parameter for total circuit efficiency.
The following inductor series from different suppliers were tested. All work with the TPS6113x converter within
their specified parameters.
VENDOR
Sumida
Wurth Electronik
Cooper Electronics Technologies
EPCOS
List of Inductors
RECOMMENDED INDUCTOR SERIES
CDRH73
CDRH74
CDRH5D18
7447789___
7447779___
DR73
DR74
B82462G
COUPLED INDUCTOR SERIES
744878220
744877220
DRQ73
DRQ74
Capacitor Selection
Input Capacitor
At least a 10-µF input capacitor is recommended to improve transient behavior of the regulator and EMI behavior
of the total power supply circuit. A ceramic capacitor or a tantalum capacitor with a 100-nF ceramic capacitor in
parallel, placed close to the IC, is recommended.
Flying Capacitor DC/DC Converter
In the normal operating mode, the flying capacitor (C7) must be large enough so that the voltage across the
capacitor is small. This means the resonance frequency formed by the flying capacitor and the inductors must be
at least ten times lower than the switching frequency (see Equation 7).
Cmin
+
100
4p² ƒ²
L
(7)
Where L is the inductance of L1-A or L1-B.
To optimize efficiency, capacitors with very low ESR such as ceramic capacitors are recommended. The voltage
rating of the flying capacitor must be higher than the input voltage V_BAT.
Output Capacitor DC/DC Converter
The major parameter necessary to define the output capacitor is the maximum allowed output voltage ripple of
the converter. This ripple is determined by two parameters of the capacitor, the capacitance and the ESR. It is
possible to calculate the minimum capacitance needed for the defined ripple, supposing that the ESR is zero, by
using Equation 8:
ǒ Ǔ ^{Cmin +} _ƒ
IOUT VOUT
DV VOUT ) VBAT
(8)
Parameter f is the switching frequency and ∆V is the maximum allowed ripple.
With a chosen ripple voltage of 15 mV, a minimum capacitance of 26 µF is needed. The total ripple is larger due
to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 9:
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