Electrical Specifications and Performance:

Input Voltage range

8V-30V

Output Voltage

5.1V

±3% (Line, Load and Temperature)

Output Voltage Ripple

47mV (0.92%/Vo)

Output Current range

0 to 7A

Max Output Ripple current

15%

Min Iomax Current limit

8A

Switching frequency

200kHz

Efficiency

87% @ 7A Vin = 24V

Table 2. Output voltage selection

L4973D3.3

Vo (V)

R3 (K

Ω)R4 (KΩ)

3.3

0

4.7

5.1

2.7

4.7

L4973D5.1

5.1

0

4.7

Main Components Description.

It follows a description of the chosen out-

put and input capacitor and of the induc-

tor for each of the two sections.

Input Capacitors

The input capacitors have to be able to support the maximum input operating voltage of the device and

the maximum RMS input current.

At full load, Io = 7A and duty cycle of 50% the RMS current flowing through the input capacitors is

maximum and is given by Io/2 . So the RMS current to be sustained is 3.5A.

The two selected capacitor, FA 680

µF/50V Panasonic, are able to support this current.

Inductor Selection

The minimum duty cycle is:

Dmin = (Vo + Vf )/(Vin max + Vf )= 0.184

where Vf is the freewheeling diode forward voltage.

The inductor ripple current is fixed at 15% of Iomax and it is 0.525A.

The inductor needed for each of the two sections is:

L

=

)

= 43µH

The L

compensate a 40% reduction of inductance at full load due to the DC current level, it is necessary to

wire 34 turns, which correspond to 84

µH of inductance at light load.

01234

5678

70

75

80

85

90

95

Io [A]

η [%]

Vo=5.1V

fsw=200kHz

10V

15V

24V

12V

Vcc=8V

20V

Figure 16. Board efficiency vs. output current.

AN1126 APPLICATION NOTE

10/16