TL3577 Datasheet, PDF (16/21 Pages) Texas Instruments – 100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR

TL3577
100-kHz CURRENT-MODE SIMPLE STEP-UP/FLYBACK SWITCHING REGULATOR
SLVS633 – OCTOBER 2006
www.ti.com
C. Calculate the minimum value of C_C.
C_C ≥ 58.5 × V_OUT2 × C_OUT × R_C2 × 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 C_C 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:
I_RIPPLE(rms) = (I_LOAD(max) × D_max)/(1 – D_max)
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 R_C and C_C needed to stabilize the regulator. As a result, the preceding calculations for C_C and R_C 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)/I_LOAD(max) where
I_RIPPLE(P-P) = (1.15 × I_LOAD(max))/(1 – D_max)
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.
4. Input Capacitor Selection (C_IN)
To reduce noise on the supply voltage caused by the switching action of a step-up regulator (ripple current
noise), V_IN 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
927 East State Parkway, Schaumburg, IL 60173
[phone]
United Chemi-CON – Types LX, SXF, or SXJ
9801West Higgens, Rosemont, IL 60018
[phone]
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