1.5 MHZ, 600MA SYNCHRONOUS STEP-DOWN CONVERTER

2007-1-29

9/11

FSP3100

APPLICATION INFORMATION

Fig.1 Basic Application Circuit with FSP3100 adjustable version

Fig.2 Basic Application Circuit with fixed output versions

SETTING THE OUTPUT VOLTAGE

Figure 1 above shows the basic application circuit with FSP3100 adjustable output version. The external resistor

sets the output voltage according to the following equation:

OUT

R2

V0.6V 1+

R1

⎛⎞

=

⎜⎟

⎝⎠

INDUCTOR SELECTION

For most designs, the FSP3100 operates with inductors of 1uH to 4.7uH. Low inductance values are physically

smaller but require faster switching, which results in some efficiency loss. The inductor value can be derived from

the following equation:

()

OUT

IN

OUT

IN

L

OSC

VV

V

L

VI

f

×−

=

×∆ ×

high ripple currents. Choose inductor ripple current approximately 35% of the maximum load current 600mA, or

For output voltages above 2.0V, when light-load efficiency is important, the minimum recommended inductor is

2.2uH. For optimum voltage-positioning load transients, choose an inductor with DC series resistance in the 50mΩ

to 150mΩ range. For higher efficiency at heavy loads (above 200mA), or minimal load regulation (but some transient

overshoot), the resistance should be kept below 100mΩ. The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple current to prevent core saturation (600mA+105mA). Table 1

lists some typical surface mount inductors that meet target applications for the FSP3100.

Vin

R1

316K

R2

C3

10uF

2

5

3

4

C1

4.7uF

Vout 1.8V

1

GND

VFB

Vin 2.7V-4.2V

SW

Run

2.2uH

L1

22pF

C2

632K

L1

Vin 2.5V-5.5V

C3

10uF

2

3

2.2uH

Vout 1.8V

5

SW

1

4

Vin

Vout

Run

GND

C1

4.7uF