TL3577 datasheet(16/23 Pages) TI

16 / 23 page

www.ti.com

4. Input Capacitor Selection (CIN)

TL3577

100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR

SLVS633 – OCTOBER 2006

C. Calculate the minimum value of CC.

CC ≥ 58.5 × VOUT

× C

OUT × RC

× V

IN(min)

The compensation capacitor also is used in the soft-start function of the regulator. When the input voltage is

applied to the part, the switch duty cycle is increased slowly at a rate defined by the compensation capacitor and

the soft-start current, thus eliminating high input currents. Without the soft-start circuitry, the switch duty cycle

would instantly rise to about 90% and draw large currents from the input supply. For proper soft starting, the

value for CC should be equal to or greater than 0.22 µF.

Table 3 lists several types of aluminum electrolytic capacitors that could be used for the output filter. Use the

following parameters to select the capacitor:

Working Voltage (WVDC): Choose a capacitor with a working voltage at least 20% higher than the regulator

output voltage.

Ripple Current: This is the maximum RMS value of current that charges the capacitor during each switching

cycle. For step-up and flyback regulators, the formula for ripple current is:

IRIPPLE(rms) = (ILOAD(max) × Dmax)/(1 – Dmax)

Choose a capacitor that is rated at least 50% higher than this value at 100 kHz.

Equivalent Series Resistance (ESR): This is the primary cause of output ripple voltage, and it also affects the

values of RC and CC needed to stabilize the regulator. As a result, the preceding calculations for CC and RC are

only valid if the ESR does not exceed the maximum value specified by the following equations.

ESR

≤ (0.01 × 15 V)/I

RIPPLE(P-P) and ≤ (8.7 × 10

-3

× V

IN)/ILOAD(max) where

IRIPPLE(P-P) = (1.15 × ILOAD(max))/(1 – Dmax)

Select a capacitor with an ESR, at 100 kHz, that is less than or equal to the lower value calculated. Most

electrolytic capacitors specify ESR at 120 kHz, which is 15% to 30% higher than at 100 kHz. Also, note that

ESR increases by a factor of 2 when operating at –20

°C.

In general, low values of ESR are achieved by using large-value capacitors (C

≥ 470 µF) and capacitors with

high WVDC, or by paralleling smaller-value capacitors.

To reduce noise on the supply voltage caused by the switching action of a step-up regulator (ripple current

noise), VIN should be bypassed to ground. A good quality 0.1-µF capacitor with low ESR should provide

sufficient decoupling. If the TL3577 is located far from the supply-source filter capacitors, an additional

electrolytic (47

µF, for example) is required.

Table 3. Aluminum Electrolytic Capacitors Recommended for Switching Regulators

Nichicon – Types PF, PX, or PZ

United Chemi-CON – Types LX, SXF, or SXJ

927 East State Parkway, Schaumburg, IL 60173

9801West Higgens, Rosemont, IL 60018

(708) 843-7500

(708) 696-2000

Submit Documentation Feedback

TL3577 Datasheet(PDF) 16 Page - Texas Instruments

TL3577 Datasheet(HTML) 16 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

Datasheet Download

Link URL