TPS54341_15 datasheet(32/48 Pages) TI

32 / 48 page

´ p ´

p(mod)

5740 pF

R4 x

11.5 k

x 2411 Hz

OUT

REF

26.9 kHz

70 µF

3.3 V

11.6 k

x gm

12 A / V

0.8 V

350 µA / V

´ p ´

p(mod) x

600 kHz

2411 Hz x

26.9 kHz

p(mod) x

z(mod)

2411 Hz x 455 kHz

33.1 kHz

(

)

´ p ´

W ´

Z mod

ESR

OUT

455 kHz

5 m

(

)

(

)

´ p ´

OUT max

P mod

OUT

3.5 A

2411 Hz

3.3 V

OUT

- 0.8 V

3.3 V - 0.8 V

10.2 k

31.9 k

0.8 V

TPS54341

SLVSC61 – NOVEMBER 2013

www.ti.com

Output Voltage and Feedback Resistors Selection

The voltage divider of R5 and R6 sets the output voltage. For the example design, 10.2 k

Ω was selected for R6.

Using Equation 2, R5 is calculated as 31.9 k

Ω. The nearest standard 1% resistor is 31.6 kΩ. Due to the input

current of the FB pin, the current flowing through the feedback network should be greater than 1

μA to maintain

the output voltage accuracy. This requirement is satisfied if the value of R6 is less than 800 k

Ω. Choosing higher

resistor values decreases quiescent current and improves efficiency at low output currents but may also

introduce noise immunity problems.

(47)

Compensation

There are several methods to design compensation for DC-DC regulators. The method presented here is easy to

calculate and ignores the effects of the slope compensation that is internal to the device. Because the slope

compensation is ignored, the actual crossover frequency will be lower than the crossover frequency used in the

calculations. This method assumes the crossover frequency is between the modulator pole and the ESR zero

and the ESR zero is at least ten-times greater the modulator pole.

To get started, the modulator pole, ƒp(mod), and the ESR zero, ƒz1 must be calculated using Equation 48 and

Equation 49. For COUT, use a derated value of 70 μF. Use equations Equation 50 and Equation 51 to estimate a

starting point for the crossover frequency, ƒco. For the example design, ƒp(mod) is 2411 Hz and ƒz(mod) is 455 kHz.

Equation 49 is the geometric mean of the modulator pole and the ESR zero and Equation 51 is the mean of

modulator pole and the switching frequency. Equation 50 yields 33.1 kHz and Equation 51 gives 26.9 kHz. Use

the lower value of Equation 50 or Equation 51 for an initial crossover frequency. For this example, the target ƒco

is 26.9 kHz.

Next, the compensation components are calculated. A resistor in series with a capacitor is used to create a

compensating zero. A capacitor in parallel to these two components forms the compensating pole.

(48)

(49)

(50)

(51)

To determine the compensation resistor, R4, use Equation 52. Assume the power stage transconductance,

gmps, is 12 A/V. The output voltage, VO, reference voltage, VREF, and amplifier transconductance, gmea, are 5

V, 0.8 V and 350

μA/V, respectively. R4 is calculated to be 11.6 kΩ and a standard value of 11.5 kΩ is selected.

Use Equation 53 to set the compensation zero to the modulator pole frequency. Equation 53 yields 5740 pF for

compensating capacitor C5. 5600 pF is used for this design.

(52)

(53)

Submit Documentation Feedback

Product Folder Links: TPS54341

TPS54341 Datasheet(PDF) 32 Page - Texas Instruments

TPS54341 Datasheet(HTML) 32 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Datasheet Download

Link URL