SLMA002 datasheet(21/55 Pages) TI1

21 / 55 page

www.ti.com

ZERO(desired)

ESR(zero)

(4)

ESR(zero)

p ´

(5)

(6)

POLE(desired)

p ´

(7)

(8)

TPS54283,, TPS54286

SLUS749C – JULY 2007 – REVISED OCTOBER 2007

The value of the resistor is calculated using a ratio of impedances to match the ratio of ESR zero frequency to

the desired zero frequency.

where

• f

ESR(zero) is the ESR zero frequency of the output capacitor

• f

ZERO(desired) is the desired frequency of the zero added to the feedback. This frequency should be placed

between 20 kHz and 60 kHz to ensure good loop stability.

The value of the capacitor is calculated in Equation 5.

where:

• R

EQ is an equivalent impedance created by the parallel combination of the voltage setting divider resistors (R1

and R2) in series with R3.

Using All Ceramic Output Capacitors

With low ESR ceramic capacitors, there may not be enough phase margin at the crossover frequency. In this

case, (Ref Figure 29) resistor R3 is set equal to 1/2 R2. This will lower the gain by 6dB, reduce the crossover

frequency, and improve phase margin.

The value of C1 is found by determining the frequency to place the low frequency pole. The minimum frequency

to place the pole is 1 kHz. Any lower, and the time constant will be too slow and interfere with the internal soft

start. (Ref. Soft Start) The upper bound for the pole frequency is determined by the operating frequency of the

converter. It is 3 kHz for the TPS54x83, and 6 kHz for the TPS54x86. C1 is then found from Equation 7. Keep

component tolerances in mind when selecting the desired pole frequency.

where:

• f

POLE(desired)is the desired pole frequency between 1 kHz and 3 kHz (TPS54x83) or 1 kHz and 6 kHz

(TPS54x86).

• R

EQ is an equivalent impedance created by the parallel combination of the voltage setting divider resistors (R1

and R2) in series with R3.

If it is necessary to increase phase margin, place a capacitor in parallel with the upper voltage setting divider

resistor (Ref. C2 in Equation 9).

Submit Documentation Feedback

Product Folder Link(s): TPS54283 TPS54286

SLMA002 Datasheet(PDF) 21 Page - Texas Instruments

SLMA002 Datasheet(HTML) 21 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 49 50 51 52 53 54 55

Datasheet Download

Link URL