Datasheet

TPS62125
www.ti.com SLVSAQ5C –MARCH 2012–REVISED DECEMBER 2013
In DC/DC converter applications, the efficiency is essentially affected by the inductor AC resistance (i.e. quality
factor) and by the inductor DCR value. To achieve high efficiency operation, care should be taken in selecting
inductors featuring a quality factor above 25 at the switching frequency. Increasing the inductor value produces
lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance
usually results in an inductor with lower saturation current.
The total losses of the coil 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
The following inductor series from different suppliers have been used with the TPS62125.
Table 4. List of Inductors
INDUCTANCE DIMENSIONS INDUCTOR
DCR [Ω] SUPPLIER
[µH] [mm
3
] TYPE
10 / 15 0.33 max / 0.44 max 3.3 x 3.3 x 1.4 LPS3314 Coilcraft
22 0.36 max 3.9 x 3.9 x 1.8 LPS4018 Coilcraft
15 0.33 max 3.0 x 2.5 x 1.5 VLF302515 TDK
10/15 0.44 max / 0.7 max 3.0 x 3.0 x 1.5 LPS3015 Coilcraft
10 0.38 typ. 3.2 × 2.5 × 1.7 LQH32PN Murata
OUTPUT CAPACITOR SELECTION
Ceramic capacitors with low ESR values provide 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 light load currents the converter operates in Power Save Mode and the output voltage ripple is dependent on
the output capacitor value and the PFM peak inductor current. Higher output capacitor values minimize the
voltage ripple in PFM Mode and tighten DC output accuracy in PFM Mode. In order to achieve specified
regulation performance and low output voltage ripple, the DC-bias characteristic of ceramic capacitors must be
considered. The effective capacitance of ceramic capacitors drops with increasing DC - bias Voltage. Due to this
effect, it is recommended for output voltages above 3.3V to use at least 1 x 22µF or 2 x 10µF ceramic capacitors
on the output.
INPUT CAPACITOR SELECTION
Because of the nature of the buck converter having a pulsating input current, a low ESR 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 10µF ceramic capacitor is recommended. The voltage rating and DC
bias characteristic of ceramic capacitors need to be considered. The input capacitor can be increased without
any limit for better input voltage filtering.
For applications powered from high impedance sources, a tantalum polymer capacitor should be used to buffer
the input voltage for the TPS62125. Tantalum polymer capacitors provide a constant capacitance vs. DC bias
characteristic compared to ceramic capacitors. In this case, a 10µF ceramic capacitor should be used in parallel
to the tantalum polymer capacitor to provide low ESR.
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 large ringing at the VIN pin. This ringing can couple to the output and be mistaken as loop
instability or could even damage the part by exceeding the maximum ratings. In case the power is supplied via a
connector e.g. from a wall adapter, a hot-plug event can cause voltage overshoots on the VIN pin exceeding the
absolute maximum ratings and can damage the device, too. In this case a tantalum polymer capacitor or
overvoltage protection circuit reduces the voltage overshoot, see Figure 41.
Table 5 shows a list of input/output capacitors.
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