Datasheet

TPS5402
SLVSBK4 –SEPTEMBER 2012 www.ti.com
The TPS5402 provides robust protection during short circuits. There is potential for overcurrent runaway in the
output inductor during a short circuit at the output. The TPS5402 solves this issue by increasing the off time
during short circuit conditions by lowering the switching frequency. The switching frequency is divided by 8, 4, 2,
and 1 as the voltage ramps from 0 V to 0.8 V on the VSENSE pin. The relationship between the switching
frequency and the VSENSE pin voltage is shown in Table 1.
Table 1. Switching Frequency Conditions
SWITCHING FREQUENCY VSENSE PIN VOLTAGE
f
SW
V
SENSE
≥ 0.6 V
f
SW
/2 0.6 V > V
SENSE
≥ 0.4 V
f
SW
/4 0.4 V > V
SENSE
≥ 0.2 V
f
SW
/8 0.2 V > V
SENSE
Spread Spectrum
In order to reduce EMI, TPS5402 introduces frequency spread spectrum. The jittering span is ±6% of the
switching frequency with 1/512 swing frequency.
Overvoltage Transient Protection
The TPS5402 incorporates an overvoltage transient protection (OVTP) circuit to minimize output voltage
overshoot when recovering from output fault conditions or strong unload transients. The OVTP circuit includes an
overvoltage comparator to compare the VSENSE pin voltage and internal thresholds. When the VSENSE pin
voltage goes above 109% × V
ref
, the high-side MOSFET will be forced off. When the VSENSE pin voltage falls
below 107% × V
ref
, the high-side MOSFET will be enabled again.
Inductor Selection
The higher operating frequency allows the use of smaller inductor and capacitor values. A higher frequency
generally results in lower efficiency because of switching loss and MOSFET gate charge losses. In addition to
this basic trade-off, the effect of the inductor value on ripple current and low current operation must also be
considered. The ripple current depends on the inductor value. The inductor ripple current (i
L
) decreases with
higher inductance or higher frequency and increases with higher input voltage (V
IN
). Accepting larger values of i
L
allows the use of low inductances, but results in higher output voltage ripple and greater core losses.
To calculate the value of the output inductor, use Equation 3. LIR is a coefficient that represents inductor peak-
to-peak ripple to DC load current. It is recommended to set LIR to 0.1 ~ 0.3 for most applications.
Actual core loss of the inductor is independent of core size for a fixed inductor value, but it is very dependent on
the inductance value selected. As inductance increases, core losses go down. Unfortunately, increased
inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low
core loss and are preferred for high switching frequencies, so design goals can concentrate on copper loss and
preventing saturation. Ferrite core material saturates hard, which means that inductance collapses abruptly when
the peak design current is exceeded. It results in an abrupt increase in inductor ripple current and consequent
output voltage ripple. Do not allow the core to saturate. It is important that the RMS current and saturation
current ratings are not exceeding the inductor specification. The RMS and peak inductor current can be
calculated from Equation 5 and Equation 6.
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