Datasheet

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Capacitor Selection
C
min

I
OUT


V
OUT
 V
BAT

ƒ  V  V
OUT
(7)
V
ESR
 I
OUT
 R
ESR
(8)
A
REG

d
V
FB

10  (R3  R6)
R6  (1  i    1.6  s)
(9)
TPS61120
TPS61122, TPS61121
SLVS427C – JUNE 2002 – REVISED APRIL 2004
List of Inductors
VENDOR RECOMMENDED INDUCTOR SERIES
CDRH5D18
Sumida
CDRH6D28
Wurth Electronik 7447789___
7447779___
DR73
Coiltronics
DR74
TDK SLF 7032
EPCOS B82462G
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.
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 7 :
Parameter f is the switching frequency and ∆V is the maximum allowed ripple.
With a chosen ripple voltage of 10 mV, a minimum capacitance of 22 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 8 :
An additional ripple of 20 mV is the result of using a tantalum capacitor with a low ESR of 80 mΩ. 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 30 mV. Additional ripple is caused by load transients. This means that the output
capacitance needs to be larger than calculated above to meet the total ripple requirements. The output capacitor
has to completely supply the load during the charging phase of the inductor. A reasonable value of the output
capacitance depends on the speed of the load transients and the load current during the load change. In typical
applications a 100 µF capacitance is recommended. For economical reasons this usually is a tantalum capacitor.
Because of this the control loop has been optimized for using output capacitors with an ESR of above 30 mΩ.
The minimum value for the output capacitor is 22 µF.
Small Signal Stability
When using output capacitors with lower ESR, like ceramics, it is recommended to use the adjustable voltage
version. The missing ESR can be easily compensated there in the feedback divider. Typically a capacitor in the
range of 10 pF in parallel with R3 helps to obtain small signal stability, with the lowest ESR output capacitors.
For more detailed analysis the small signal transfer function of the error amplifier and regulator, which is given in
Equation 9 , can be used.
16