TPS5403

SLVSBK5 – SEPTEMBER 2012

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The TPS5403 provides robust protection during short circuits. There is potential for overcurrent runaway in the

output inductor during a short circuit at the output. The TPS5403 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 3.3 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

Spread Spectrum

In order to reduce EMI, TPS5403 introduces frequency spread spectrum. The jittering span is ±6% of the

switching frequency with 1/512 swing frequency.

Switching Node Anti-Ringing

When the non-synchronous buck converter operates in DCM mode, the filter inductor and the parasitic

capacitance in the switching node (LX) form an LC resonant circuit; due to its high Q factor, lengthy high

frequency oscillation can be observed in the switching node. This ringing could cause radiated EMI issues in

some systems. TPS5403 adds an anti-ringing circuit to prevent the ringing from happening, when the inductor

current crosses zero and LX starts to climb up, an internal MOSFET between LX and VSENSE is turned on,

providing a damping path for the resonant circuit so as to eliminate the ringing.

Overvoltage Transient Protection

The TPS5403 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

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

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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