AN557 datasheet(29/52 Pages) STMICROELECTRONICS

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AN557 APPLICATION NOTE

Figure 36. Test and Evaluation Board Circuit.

TYPICAL PERFORMANCES (using evaluation board) :

n = 83% (Vi = 35V ; Vo = VREF ; Io = 3.5A ; fsw = 100KHz); Vo RIPPLE = 30mV (at 1A); Line regulation =

12mV (Vi = 15 to 50V); Load regulation = 7mV (Io = 0.5 to 3.5A); for component values Refer to the fig. 35

(Part list).

PARTS LIST

* 2 capacitors in parallel to increase input RMS current capability

** 3 capacitors in parallel to reduce total output ESR

Table A

Note: In the Test and Application Circuit for L4974D are not mounted C2, C14 and R8.

Table B - SUGGESTED BOOTSTRAP CAPACITORS

R1 = 30K

Ω

C3, C4, C5, C6 = 2.2

µF 50V

R2 = 10K

Ω

C7 = 390pF Film

R3 = 15K

Ω

C8 = 22nF MKT 1837 (ERO)

R4 = 30K

Ω

C9 = 2.7nF KP1830 (ERO)

R5 = 22

Ω

C10 = 0.33

µF Film

R6 = 4K7

C11 = 1nF

R7 = see tab. A

**C12, C13, C14 = 100

µF 40VL EKR (ROE)

R8 = OPTION

C15 = 1

µF Film

* C1, C2 = 1000

µF 63V EYF (ROE)

D1 = SB 560 (or equivalent)

L1 = 150

µH core 58310 MAGNETICS 45 TURNS 0.91mm (AWG 16) COGEMA 949181

R10

12V

4.7K

Ω

6.2K

Ω

15V

4.7K

Ω

9.1K

Ω

18V

4.7K

Ω

12K

Ω

24V

4.7K

Ω

18K

Ω

Operating Frequency

Bootstrap Cap.c10

f = 20KHz

≥680nF

f = 50KHz

≥470nF

f = 100KHz

≥330nF

f = 200KHz

≥220nF

f = 500KHz

≥100nF

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