AN2928 datasheet(9/21 Pages) STMICROELECTRONICS

9 / 21 page

AN2928

Design equations for the modified buck converter

9/21

VZCD = voltage on the ZCD pin of the L6562A (V)

VZCD_CLAMP = clamp voltage on the ZCD pin of the L6562A (V)

VZCD_TRIGGER = trigger voltage on the ZCD pin of the L6562A (V)

Capacitor C4 can be selected, and the resistor is easily calculated using Equation 14, or the

inverse can be calculated with Equation 15.

Equation 14

Equation 15

Resistor R5 (shown in Figure 2) is designed to limit the charge current flowing to capacitor

C4 during the tON time, and must be within the following range (as described in application

note AN1792. See Section 3: Reference and related materials).

Equation 16

VF = diode D2 forward voltage (V) (typically 0.7 V)

VZCD_CLAMP = clamp voltage on the ZCD pin of L6562A (V) (5.7 V)

VGD_MAX = output high - maximum gate driver voltage (V) (15 V)

VGD_MIN = output high - minimum gate driver voltage (V) (9.8 V)

IZCD_MAX = maximum sink capability for the ZCD pin (A) (0.01 A)

If the tON time is very short (light load or low output voltage), then capacitor C4 cannot be

quickly charged via resistor R5. Therefore it is recommended to connect capacitor C3 in

parallel with resistor R5. The maximum size of capacitor C3 can be calculated with the

following equation:

Equation 17

OFF

2.1

⋅

----------------------

OFF

2.1

⋅

----------------------

GD MAX

ZCD C LAMP

–

ZCD M AX

ZCD C LAMP

-----------------------------------

-------------------------------------------------------------------------------

GD MIN

ZC D CLAMP

–

ZCD CL AM P

------------------------------------------------------------------------------

⎝⎠

⎜⎟

⎛⎞

⋅

ZCD CL AM P

GD M AX

ZCD C LAMP

–

()

------------------------------------------------------------------------------------

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