TLE6389_07 datasheet(33/38 Pages) INFINEON

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TLE 6389

Datasheet Rev. 2.1

2007-08-13

7.10.3

PFM and PWM thresholds

The crossover thresholds PFM to PWM and vice versa strongly depend on the input

voltage VIN, the Buck converter inductance L1, the sense resistor value RSENSE and the

turn on and turn off delays of the external PMOS.

For more details on the PFM to PWM and PWM to PFM thresholds please refer to the

application note “TLE6389 - Determining PFM/PWM current thresholds”.

7.10.4

Buck output capacitor (COUT) selection:

The choice of the output capacitor effects straight to the minimum achievable ripple

which is seen at the output of the buck converter. In continuous conduction mode the

ripple of the output voltage can be estimated by the following equation:

From the formula it is recognized that the ESR has a big influence in the total ripple at

the output, so low ESR tantalum capacitors are recommended for the application.

One other important thing to note are the requirements for the resonant frequency of the

output LC-combination. The choice of the components L and C have to meet also the

specified range given in section 3 otherwise instabilities of the regulation loop might

occur.

7.10.5

Input capacitor (CIN1) selection:

At high load currents, where the current through the inductance flows continuously, the

input capacitor is exposed to a square wave current with its duty cycle VOUT/VI. To

prevent a high ripple to the battery line a capacitor with low ESR should be used. The

maximum RMS current which the capacitor has to withstand is calculated to:

For low ESR an e.g. Al-electrolytic capacitance in parallel to an ceramic capacitance

could be used.

VRipple

∆IRESRCOUT

8fSW COUT

⋅⋅

------------------------------------





⋅

IRMS

ILOAD

VOUT

VIN

--------------

---

∆I

2ILOAD

⋅

-----------------------



 2

⋅

⋅⋅

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