TPS51116MPWPREP datasheet(18/38 Pages) TI1

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ESR

wz2 +

ESR

+ wp3 +

C + 2.8

OUT

O [mF]

S [mW]

C v 2

OUT

0.75

IND(peak) +

TRIP

DS(on)

)

IN(max) * VOUT

OUT

IN(max)

L +

IND(ripple)

IN(max) * VOUT

OUT

IN(max)

OUT(max)

IN(max) * VOUT

OUT

IN(max)

0 +

R1 ) R2

0.75

OUT

wZ2 +

ESR

wZ1 +

wP3 +

TPS51116-EP

SLUSB52A – OCTOBER 2012 – REVISED NOVEMBER 2012

www.ti.com

(12)

(13)

(14)

Usually, each frequency of those poles and zeros is lower than the 0 dB frequency, f0. However, the f0 should be

kept under 1/3 of the switching frequency to avoid effect of switching circuit delay. The f0 is given by Equation 15.

(15)

Based on small signal analysis above, the external components can be selected by following manner.

1. Choose the inductor. The inductance value should be determined to give the ripple current of

approximately 1/4 to 1/2 of maximum output current.

(16)

The inductor also needs to have low DCR to achieve good efficiency, as well as enough room above peak

inductor current before saturation. The peak inductor current can be estimated as shown in Equation 17.

(17)

2. Choose rectifying (bottom) MOSFET. When RDS(on) sensing scheme is selected, the rectifying MOSFET’s

on-resistance is used as this RS so that lower RDS(on) does not always promise better performance. In order

to clearly detect inductor current, minimum RS recommended is to give 15 mV or larger ripple voltage with

the inductor ripple current. This promises smooth transition from CCM to DCM or vice versa. Upper side of

the RDS(on) is of course restricted by the efficiency requirement, and usually this resistance affects efficiency

more at high-load conditions. When using external resistor current sensing, there is no restriction for low

RDS(on). However, the current sensing resistance RS itself affects the efficiency

3. Choose output capacitor(s). When organic semiconductor capacitors (OS-CON) or specialty polymer

capacitors (SP-CAP) are used, ESR to achieve required ripple value at stable state or transient load

conditions determines the amount of capacitor(s) need, and capacitance is then enough to satisfy stable

operation. The peak-to-peak ripple value can be estimated by ESR times the inductor ripple current for stable

state, or ESR times the load current step for a fast transient load response. When ceramic capacitor(s) are

used, the ESR is usually small enough to meet ripple requirement. In contrast, transient undershoot and

overshoot driven by output capacitance becomes the key factor in determining the capacitor(s) required.

4. Determine f0 and calculate RC using Equation 18. Note that higher RC shows faster transient response in

cost of unstableness. If the transient response is not enough even with high RC value, try increasing the out

put capacitance. Recommended f0 is fOSC/4. Then RC can be derived by Equation 19.

(18)

(19)

5. Calculate CC2 . Purpose of this capacitance is to cancel zero caused by ESR of the output capacitor. When

ceramic capacitor(s) are used, no need for CC2.

(20)

(21)

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