TPS61280_15 Datasheet, PDF (36/55 Pages) Texas Instruments – TPS6128x Low-, Wide- Voltage Battery Front-End DC/DC Converter Single-Cell Li-Ion, Ni-Rich, Si-Anode Applications

TPS61280, TPS61281, TPS61282
SLVSBI1A – OCTOBER 2013 – REVISED SEPTEMBER 2014
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11.2.1.2.2 Output Capacitor
For the output capacitor, it is recommended to use small ceramic capacitors placed as close as possible to the
VOUT and GND pins of the IC. If, for any reason, the application requires the use of large capacitors which can
not be placed close to the IC, using a smaller ceramic capacitor in parallel to the large one is highly
recommended. This small capacitor should be placed as close as possible to the V_OUT and GND pins of the IC.
To get an estimate of the recommended minimum output capacitance, Equation 10 can be used.
C_MIN =
I_OUT x (V_OUT - V_IN )
f x DV x V_OUT
(10)
Where f is the switching frequency which is 2.3MHz (typ.) and ΔV is the maximum allowed output ripple.
With a chosen ripple voltage of 20mV, a minimum effective capacitance of 10μF is needed. The total ripple is
larger due to the ESR and ESL of the output capacitor. This additional component of the ripple can be calculated
using Equation 11
ΔV_OUT(ESR)
=
ESR
x
æ
çè
I_OUT
1-D
+
ΔI_L
2
ö
÷ø
(11)
ΔV_OUT(ESL)
=
ESL x
æ
çè
I_OUT
1-D
+
ΔI_L
2
ö
- I_{OUT ÷ø} x
1
t _SW(RISE)
(12)
ΔV_OUT(ESL)
= ESL x
æ
çè
I_OUT
1-D
-
ΔI_L
2
ö
- I_{OUT ÷ø} x
1
t _SW(FALL)
(13)
with:
• I_OUT = output current of the application
• D = duty cycle
• ΔI_L = inductor ripple current
• t_SW(RISE) = switch node rise time
• t_SW(FALL) = switch node fall time
• ESR = equivalent series resistance of the used output capacitor
• ESL = equivalent series inductance of the used output capacitor
An MLCC capacitor with twice the value of the calculated minimum should be used due to DC bias effects. This
is required to maintain control loop stability. The output capacitor requires either an X7R or X5R dielectric. Y5V
and Z5U dielectric capacitors, aside from their wide variation in capacitance over temperature, become resistive
at high frequencies. There are no additional requirements regarding minimum ESR. Larger capacitors cause
lower output voltage ripple as well as lower output voltage drop during load transients.
In applications featuring high (pulsed) load currents (e.g. ≥2Amps), it is recommended to run the converter with a
reasonable amount of effective output capacitance and low-ESL device, for instance x2 22µF X5R 6.3V (0603)
MLCC capacitors connected in parallel with a 1µF X5R 6.3V (0306-2T) MLCC LL capacitor.
DC bias effect: high cap. ceramic capacitors exhibit DC bias effects, which have a strong influence on the
device's effective capacitance. Therefore the right capacitor value has to be chosen carefully. Package size and
voltage rating in combination with material are responsible for differences between the rated capacitor value and
it's effective capacitance. For instance, a 10µF X5R 6.3V (0603) MLCC capacitor would typically show an
effective capacitance of less than 5µF (under 3.5V bias condition, high temperature).
For RF Power Amplifier applications, the output capacitor loading is combined between the dc/dc converter and
the RF Power Amplifier (x2 10µF X5R 6.3V (0603) + PA input cap 4.7µF X5R 6.3V (0402)) are recommended.
36
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