FAN5234 Fairchild Semiconductor, FAN5234 Datasheet - Page 11

• Manufacturer Part Number: FAN5234
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• Manufacturer: Fairchild Semiconductor

© 2004 Fairchild Semiconductor Corporation
FAN5234 • Rev. 2.0.0
Design and Component Selection
Guidelines
As an initial step, define operating input voltage range,
output voltage, and minimum and maximum load
currents for the controller.
For the examples in the following discussion, select
components for:
V
V
Setting the Output Voltage
The internal reference is 0.9V. The output is divided
down by a voltage divider to the VSEN pin (for example,
R1 and R2 in Figure 1). The output voltage therefore is:
To minimize noise pickup on this node, keep the
resistor to GND (R2) below 2K; for example R2 at
1.82K, then choose R5:
Output Inductor Selection
The minimum practical output inductor value keeps
inductor current just on the boundary of continuous
conduction at some minimum load. The industry
standard practice is to choose the ripple current to be
somewhere from 15% to 35% of the nominal current. At
light-load, the ripple current determines the point where
the converter automatically switches to Hysteretic Mode
to sustain high efficiency. The following equations help
to choose the proper value of the output filter inductor:
where ΔI is the inductor ripple current, which is chosen
for 20% of the full load current and ΔV
maximum output ripple voltage allowed:
For this example, use:
V
∆I = 20% x 3.5A = 0.7A
f
Therefore;
L ≈ 8 µH
R
L
Δ
0
SW
IN
OUT
R
IN
9 .
I
5
=
2
=
V
from 5V to 20V
= 20V, V
= 300KHz.
=
2
V
= 1.8V at I
=
(
. 1
−
IN
f
SW
1
V
82
MIN
−
OUT
K
×
V
=
R
Ω
−
OUT
OUT
Δ
1
Δ
) (
0
I
×
0
ESR
V
9 .
LOAD(MAX)
9 .
= 1.8V
1
OUT
V
×
8 .
V
V
V
OUT
−
IN
0
9 .
= 3.5A
)
=
. 1
82
K
OUT
is the
(11)
(12)
(13)
(14)
(15)
(16)
11
Output Capacitor Selection
The output capacitor serves two major functions in a
switching power supply. Along with the inductor, it filters
the sequence of pulses produced by the switcher and it
supplies the load transient currents. The output
capacitor requirements are usually dictated by ESR,
inductor ripple current (ΔI), and the allowable ripple
voltage (ΔV):
For this example,
In addition, the capacitor's ESR must be low enough to
allow the converter to stay in regulation during a load
step. The ripple voltage due to ESR for the converter in
Figure 1 is 100mV
due to the capacitance value itself:
which is only about 1.5mV for the converter in Figure 1
and can be ignored.
The capacitor must also be rated to withstand the RMS
current, which is approximately 0.3 X (ΔI) or about
210mA for the converter in Figure 1. High-frequency
decoupling capacitors should be placed as close to the
loads as physically possible.
Input Capacitor Selection
The input capacitor should be selected by its ripple
current rating. The input RMS current at maximum load
current (I
where the converter duty cycle;
the circuit in Figure 1, with V
Power MOSFET Selection
Losses in a MOSFET are the sum of its switching (P
and conduction (P
In typical applications, the FAN5234 converter's output
voltage is low with respect to its input voltage.
Therefore, the lower MOSFET (Q2) is conducting the
full-load current for most of the cycle. Q2 should
therefore be selected to minimize conduction losses,
thereby selecting a MOSFET with low R
In contrast, the high-side MOSFET (Q1) has a shorter
duty cycle, and its conduction loss has less impact. Q1,
however, sees most of the switching losses, so Q1's
primary selection criteria should be gate charge.
ESR
I
I
Δ
RMS
RMS
V
=
=
=
<
I
C
1
L
Δ
6 .
OUT
Δ
L
D
V
) is:
A
I
−
.
D
×
Δ
2
8
I
×
ESR
COND
PP
f
SW
. Some additional ripple will appear
) losses.
(
MAX
)
IN
=
=6, calculates to
Δ
Δ
D =
V
I
=
V
0
0
V
OUT
7 .
1 .
IN
V
DS(ON)
A
,
=
which for
142
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.
m
Ω
(17)
(18)
(19)
SW
)