TPS53317RGBR datasheet(22/35 Pages) TI1

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è × 4% × 2B59

è × 3.9 G! × 2 × 800 G*V

= 25.5

BL =

è × 4% ×

%% × %2

%% + %2

è × 4% × %2

BV =

B%1

è × 4% × %%

4% =

B%1 × 45 × 2è × %176

G*V × 53 I! × 2è × 160 ä(

= 4.26

B%1 =

2è

%176

= 80 G*V

:IEJ ; = 2.5 # ×

0.55 × (1 . 0.55)

12 mV × 800 G*V

= 64.45 µ(

%+0:IEJ ; = +KQP ×

× (1 . &)

8+0(2.2) × B59

176

:IEJ ;,75 =

:3 A;2 × 0.25 µ* × @

0.6 8

1.2 8

800 G*V

+ 270 JO

2 × 0.6 8 × 0.03 8 ×

1.2 8 . 0.6 8

1.2 8

800 G*V .

270 JO

= 157.6 µ(

TPS53317

SLUSAK4D – JUNE 2011 – REVISED JULY 2015

www.ti.com

Typical Applications (continued)

Again, choose 30 mV to account for normal output voltage ripple.

(13)

The undershoot requirements determine, so there must be a minimum of 157.6 µF. Because this is a DDR

application where size is also a consideration, this design uses only ceramic capacitors. To account for voltage

de-rating of capacitors and provide additional margin, this design includes eleven 22-µF output capacitors.

8.2.1.2.4

Step 4. Input Capacitance

This design requires sufficient input capacitance to filter the input current from the host source. Use Equation 14

to calculate the necessary input capacitance.

where

•

ΔVIN(P-P) is the desired input voltage ripple (typically 1% of the input voltage)

(14)

(15)

As with the output capacitance selection, this design accounts for voltage de-rating of capacitors and provides

additional margin, using four 22-µF input capacitors.

8.2.1.2.5

Step 5. Compensation Network

In order to achieve stable operation, the crossover frequency should be less than 1/5 of the switching frequency.

where

•

RS = 53 mΩ

(16)

Account for capacitor de-rating here and set the value of COUT to 160 µF, so that Equation 17 is true.

(17)

Choose an RC value of 3.9 kΩ. Determine CC by choosing the value of the zero created by RC and CC. Using the

relationship described in Equation 18.

(18)

Equation 18 yields a CC value of 2.55 nF. Choose the closest common capacitor value of 2.2 nF. To determine a

value for CP, first consider the relationship described in Equation 19.

•

CC >> CP

(19)

Because CC >> CP , set the pole to be two times the switching frequency as described in Equation 20.

(20)

To boost the gain margin, set CP to 33 pF.

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