TPS54341_15 datasheet(28/48 Pages) TI

28 / 48 page

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) =

RIPPLE

OUT

L peak

0.905 A

3.5 A

3.95 A

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OUT

IN max

OUT

L rms

IN max

3.3 V

42 V – 3.3 V

3.5 A

42 V

5.6 µH

600 kHz

(

)

(

)

OUT

IN max

RIPPLE

IN max

)

3.3 V

(42 V

3.3 V)

0.905 A

42 V

5.6 µH

600 kHz

–

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(

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(

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OUT

IN max

OUT

O min

OUT

IND

IN max

42 V

3.3 V

4.8 µH

3.5 A

0.3

42 V

600 kHz

–

TPS54341

SLVSC61 – NOVEMBER 2013

www.ti.com

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Output Capacitor

There are three primary considerations for selecting the value of the output capacitor. The output capacitor

determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in

load current. The output capacitance needs to be selected based on the most stringent of these three criteria.

The desired response to a large change in the load current is the first criteria. The output capacitor needs to

supply the increased load current until the regulator responds to the load step. The regulator does not respond

immediately to a large, fast increase in the load current such as transitioning from no load to a full load. The

regulator usually needs two or more clock cycles for the control loop to sense the change in output voltage and

adjust the peak switch current in response to the higher load. The output capacitance must be large enough to

supply the difference in current for two clock cycles to maintain the output voltage within the specified range.

Equation 35 shows the minimum output capacitance necessary, where

ΔIOUT is the change in output current, ƒsw

is the regulators switching frequency and

ΔVOUT is the allowable change in the output voltage. For this example,

the transient load response is specified as a 4% change in VOUT for a load step from 0.875 A to 2.625 A.

Therefore,

ΔIOUT is 2.625 A – 0.875 A = 1.75 A and ΔVOUT = 0.04 × 3.3 = 0.13 V. Using these numbers gives a

minimum capacitance of 44.9

μF. This value does not take the ESR of the output capacitor into account in the

output voltage change. For ceramic capacitors, the ESR is usually small enough to be ignored. Aluminum

electrolytic and tantalum capacitors have higher ESR that must be included in load step calculations.

The output capacitor must also be sized to absorb energy stored in the inductor when transitioning from a high to

low load current. The catch diode of the regulator can not sink current so energy stored in the inductor can

produce an output voltage overshoot when the load current rapidly decreases. A typical load step response is

shown in Figure 47. The excess energy absorbed in the output capacitor will increase the voltage on the

capacitor. The capacitor must be sized to maintain the desired output voltage during these transient periods.

Equation 36 calculates the minimum capacitance required to keep the output voltage overshoot to a desired

value, where LO is the value of the inductor, IOH is the output current under heavy load, IOL is the output under

light load, Vf is the peak output voltage, and VI is the initial voltage. For this example, the worst case load step

will be from 2.625 A to 0.875 A. The output voltage increases during this load transition and the stated maximum

in our specification is 4 % of the output voltage. This makes Vf = 1.04 × 3.3 = 3.432. VI is the initial capacitor

voltage which is the nominal output voltage of 3.3 V. Using these numbers in Equation 36 yields a minimum

capacitance of 38.6

μF.

Equation 37 calculates the minimum output capacitance needed to meet the output voltage ripple specification,

where ƒsw is the switching frequency, VORIPPLE is the maximum allowable output voltage ripple, and IRIPPLE is the

inductor ripple current. Equation 37 yields 11.4

μF.

Equation 38 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple

specification. Equation 38 indicates the ESR should be less than 18 m

Ω.

The most stringent criteria for the output capacitor is 44.9

μF required to maintain the output voltage within

regulation tolerance during a load transient.

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