LM5145 datasheet(33/73 Pages) TI2

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OUT

˜ ¨

' ˜

(11)

L(peak)

OUT

(12)

Check the inductor data sheet to make sure that the saturation current rating is well above the peak inductor

current of a particular design. Ferrite-cored inductors have very low core loss and are preferred at high switching

frequencies, so design goals can then concentrate on copper loss and preventing saturation. Low inductor

core loss is evidenced by reduced no-load input current and higher light-load efficiency. However, ferrite core

materials exhibit a hard saturation characteristic where the inductance collapses abruptly when the saturation

current is exceeded. This action results in an outsized increase in inductor ripple current and higher output

voltage ripple, not to mention reduced efficiency and compromised reliability. Note that the saturation current

rating of an inductor generally decreases as the core temperature increases. Of course, accurate overcurrent

protection is critical to avoiding inductor saturation.

8.1.1.3 Output Capacitors

Ordinarily, the energy stored by the output capacitors combined with the control loop response prescribe to

maintain the integrity of the output voltage within the dynamic (transient) tolerance specifications. The usual

boundaries restricting the output capacitors in power management applications are driven by finite available

PCB area, component footprint and profile, and cost. As the load step amplitude and slew rate increase, the

capacitor parasitics – equivalent series resistance (ESR) and equivalent series inductance (ESL) – take greater

precedence in shaping the load transient response of the regulator.

The output capacitor, COUT, filters the inductor ripple current and provides a reservoir of charge for step-load

transient events. Typically, ceramic capacitors provide extremely low ESR to reduce the output voltage ripple

and noise spikes, while tantalum and polymer electrolytic capacitors provide a large bulk capacitance in a

relatively compact footprint for transient loading events.

Based on the static specification of peak-to-peak output voltage ripple denoted by ΔVOUT, choose an output

capacitance that is higher than that given by Equation 13.

OUT

ESR

8 F

˜ '

(13)

Figure 8-1 conceptually illustrates the relevant current waveforms during both load-on and load-off transitions.

As shown, the slew rate of the inductor current represent a large-signal constraint as the inductor current ramps

to match the new load-current level following a load transient. This slew-rate limiting exacerbates the deficit of

charge in the output capacitor, which must be replenished as rapidly as possible during and after a load-on

transient. Similarly, during and after a load-off transient, the slew rate limiting of the inductor current adds to the

surplus of charge in the output capacitor that must be depleted as quickly as possible.

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LM5145 Datasheet(PDF) 33 Page - Texas Instruments

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