Datasheet

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Capacitor Selection
Input Capacitor
Flying Capacitor DC/DC Converter
C
min
+
100
4p
2
ƒ
2
L
(7)
Output Capacitor DC/DC Converter
C
min
+
I
OUT
V
OUT
ƒ DV
ǒ
V
OUT
) V
BAT
Ǔ
(8)
DV
ESR
+ I
OUT
R
ESR
(9)
TPS61130
TPS61131
TPS61132
SLVS431B – JUNE 2002 – REVISED JANUARY 2008
has been optimized to operate with inductance values between 10 µ H and 47 µ H. Nevertheless operation with
higher inductance values may be possible in some applications. Detailed stability analysis is recommended. 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 have been used with the TPS6113X converters:
List of Inductors
VENDOR RECOMMENDED INDUCTOR SERIES COUPLED INDUCTOR SERIES
LPS4012 LPD4012
Coilcraft
LPS3015
Cooper Electronics Technologies DR73 DRQ73
DR74 DRQ74
EPCOS B82462G
Sumida CDRH5D18
7447789___ 744878220
Wurth Electronik
7447779___ 744877220
At least a 10- µ F 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.
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 ).
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
.
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 :
Parameter f is the switching frequency and Δ V is the maximum allowed ripple.
With a chosen ripple voltage of 15 mV, a minimum capacitance of 26 µ F 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 :
An additional ripple of 24 mV is the result of using a tantalum capacitor with a low ESR of 80 m Ω . The total ripple
is the sum of the ripple caused by the capacitance and the ripple caused by the ESR of the capacitor. In this
example, the total ripple is 39 mV. Additional ripple is caused by load transients. This means that the output
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