Datasheet

OUT
OUT
IN
RMSC OUT
V
1
V
1
I V
L f
2 3
-
= ´ ´
´
OUT
IN
OUT OUT
OUT
V
1
V
1
V V ESR
L f 8 C f
-
æ ö
D = ´ ´ +
ç ÷
´ ´ ´
è ø
TPS62560, TPS62561
TPS62562
www.ti.com SLVS815C –JANUARY 2008–REVISED DECEMBER 2009
A more conservative approach is to select the inductor current rating just for the switch current limit I
LIMF
of the
converter.
Accepting larger values of ripple current allows the use of lower inductance values, but results in higher output
voltage ripple, greater core losses, and lower output current capability.
The total losses of the coil have a strong impact on the efficiency of the dc/dc conversion and consist of both the
losses in the dc resistance (R
(DC)
) and the following frequency-dependent components:
• The losses in the core material (magnetic hysteresis loss, especially at high switching frequencies)
• Additional losses in the conductor from the skin effect (current displacement at high frequencies)
• Magnetic field losses of the neighboring windings (proximity effect)
• Radiation losses
Output Capacitor Selection
The advanced fast-response voltage-mode control scheme of the TPS62560 allows the use of tiny ceramic
capacitors. Ceramic capacitors with low ESR values have the lowest output voltage ripple and are
recommended. 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.
At nominal load current, the device operates in PWM mode, and the RMS ripple current is calculated as:
(3)
At nominal load current, the device operates in PWM mode, and the overall output voltage ripple is the sum of
the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and discharging
the output capacitor:
(4)
At light load currents, the converter operates in power-save mode, and the output voltage ripple is dependent on
the output capacitor and inductor values. Larger output capacitor and inductor values minimize the voltage ripple
in PFM mode and tighten dc output accuracy in PFM mode.
Input Capacitor Selection
An input capacitor is required for best input voltage filtering and minimizing the interference with other circuits
caused by high input voltage spikes. For most applications, a 4.7-μF to 10-μF ceramic capacitor is
recommended. Because a ceramic capacitor loses up to 80% of its initial capacitance at 5 V, it is recommended
that 10-μF input capacitors be used for input voltages > 4.5 V. The input capacitor can be increased without any
limit for better input voltage filtering. Take care when using only small ceramic input capacitors. When a ceramic
capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a
load step at the output or V
IN
step on the input can induce ringing at the V
IN
terminal. This ringing can couple to
the output and be mistaken as loop instability or could even damage the part by exceeding the maximum ratings.
Table 3. List of Capacitors
CAPACITANCE TYPE SIZE SUPPLIER
4.7 μF GRM188R60J475K 0603—1,6 × 0,8 × 0,8 mm Murata
10 μF GRM188R60J106M69D 0603—1,6 × 0,8 × 0,8 mm Murata
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