Datasheet

C
min
+
I
OUT
ǒ
V
OUT
* V
BAT
Ǔ
ƒ DV V
OUT
DV
ESR
+ I
OUT
R
ESR
TPS61020, TPS61024, TPS61025
TPS61026, TPS61027, TPS61028
TPS61029
www.ti.com SLVS451F –SEPTEMBER 2003–REVISED APRIL 2012
Output Capacitor
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 6:
(6)
Parameter f is the switching frequency and ΔV is the maximum allowed ripple.
With a chosen ripple voltage of 10 mV, a minimum capacitance of 24 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 7:
(7)
An additional ripple of 16 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 26 mV. Additional ripple is caused by load transients. This means that 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.
With the calculated minimum value of 24 µF and load transient considerations the recommended output
capacitance value is in a 47 to 100 µF range. For economical reasons, this is usually a tantalum capacitor.
Therefore, the control loop has been optimized for using output capacitors with an ESR of above 30 mΩ.
Additionally, a ceramic output capacitor of 2.2 µF must be added in parallel with the tantalum output capacitor.
The device is not designed to operate with ceramic capacitors only, unless a discrete resistor is added in series
with them to replicate the required ESR. Large amounts of low ESR capacitance on the output causes instability.
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