1.5 MHZ, 600MA SYNCHRONOUS STEP-DOWN CONVERTER
APPLICATION INFORMATION
Vin 2.7V-4.2V
C1
4.7uF
1
Run
GND
2
VFB
5
R2 632K
R1
316K
4
Vin
SW
3 L1 2.2uH
C2 22pF
Vout 1.8V
C3
10uF
FSP3100
Fig.1 Basic Application Circuit with FSP3100 adjustable version
3 L1 2.2uH
Vin 2.5V-5.5V
C1
4.7uF
4
Vin
SW
Vout 1.8V
C3
10uF
1
Run
GND
2
Vout
5
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:
⎛
R2
⎞
V
OUT
=
0.6V
⎜
1+
⎟
⎝
R1
⎠
R1=300kΩ for all outputs; R2=300kΩ for V
OUT
=1.2V, R2=200kΩ for V
OUT
=1.5V, R2=150kΩ for V
OUT
=1.8V and
R=95.3kΩ for V
OUT
=2.5V.
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:
L
=
V
OUT
×
(
V
IN
−
V
OUT
)
V
IN
× ∆
I
L
×
f
OSC
Where
∆I
L
is inductor Ripple Current. Large value inductors lower ripple current and small value inductors result in
high ripple currents. Choose inductor ripple current approximately 35% of the maximum load current 600mA, or
∆I
L
=210mA.
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.
9/11
2007-1-29
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
