FAN5182_08 Datasheet, PDF (13/19 Pages) Fairchild Semiconductor – Adjustable Output, 1-, 2-, or 3-Phase Synchronous Buck Controller

Designing an Inductor
Once the inductance and DCR are known, the next step
is to either design an inductor or find a suitable
standard inductor if one exists. Inductor design starts
with choosing appropriate core material. Some
candidate materials that have low core loss at high
frequencies are powder cores (e.g. Kool-Mµ® from
Magnetics, Inc. or from Micrometals) and gapped soft
ferrite cores (e.g. 3F3 or 3F4 from Philips). Powdered
iron cores have higher core loss and are used for low-
cost applications.
The best choice for a core geometry is a closed-loop
type, such as a potentiometer core, a PQ/U/E core, or a
toroid core.
Some useful references for magnetics design are:
ƒ Magnetic Designer Software
ƒ Intusoft (www.intusoft.com)
ƒ Designing Magnetic Components for High-
Frequency DC-DC Converters, by William T.
McLyman, Kg Magnetics, Inc., ISBN 1883107008.
Selecting a Standard Inductor
The following power inductor manufacturers can provide
design consultation and deliver power inductors
optimized for high-power applications upon request:
ƒ BI Technologies, 714-447-2345
www.bitechnologies.com
ƒ Taiyo Yuden (USA), 408-573-4150
www.taiyo-yuden.com
Output Current Sense
The output current can be measured by summing the
voltage across each inductor and passing the signal
through a low-pass filter. The CS amplifier is configured
with resistors RPH(X) (for summing the voltage), and RCS
and CCS (for the low-pass filter).
The output current IO is set by the following equations:
I_O
=
R_{PH( x )}
R_CS
×
V_DRP
RL
(7)
Ccs ≥ ^L
R_L × R_CS
(8)
where:
RL is the DCR of the output inductors,
VDRP is the voltage drop from CSCOMP to CSREF.
When load current reaches its limit, VDRP is at its
maximum (VDRPMAX). VDRPMAX can be in the range of
100mV to 200mV. In this example, it is 110mV.
Designers have the flexibility of choosing either RCS or
RPH(X). It is recommended to select RCS equal to 100kΩ,
and then solve for RPH(X) by rearranging Equation 7 as:
R_{PH( x )}
=
R_L
× R_CS
×
I_LIM
V_DRPMAX
(9)
R _{PH( x )}
= 1.4mΩ ×100kΩ × ^110A
110mV
= 140kΩ
(10)
WARNING: The parallel combination of the all the Rph
resistors must be greater than 30kΩ to ensure that the
current sense amplifier does not saturate.
Next, use Equation 8 to solve for CCS:
Ccs ≥ ^320nH ≥ 2.28nF
1.4mΩ × 100kΩ
(11)
Choose the closest standard value that is greater than
the result given by Equation 8. This example uses a CCS
value of 5.6nF.
Output Voltage
FAN5182 has an internal FBRTN referred 800mV
reference voltage VREF. The output voltage can be set
by using a voltage divider consisting of resistors RB1
and RB2:
V_OUT
=
(R_B1 + R_B2
R_B1
)
×
V_REF
(12)
Rearranging Equation 12 to solve RB2 and assuming a
1%, 1kΩ resistor for RB1 yields
R_B2
=
V_OUT − V_FB
V_FB
× R_B1
R_B2
=
1.8V − 0.8V
0.8V
× 1kΩ
=
1.25kΩ
(13)
The closest standard 1% resistor value for RB2 is
1.24kΩ.
Power MOSFETs
For this example, one high-side and one low-side
N-channel power MOSFET per phase have been
selected. The main selection parameters for power
MOSFETs are VGS(TH), QG, CISS, CRSS, and RDS(ON). The
minimum gate-drive voltage (the supply voltage to the
FAN5109) dictates whether standard threshold or logic-
level threshold MOSFETs can be used. With VGATE
~10V, logic-level threshold MOSFETs (VGS(TH) < 2.5V)
are recommended.
The maximum output current (IO) determines the RDS(ON)
requirement for the low-side (synchronous) MOSFETs.
With good current balance among phases, the current
in each low-side MOSFET is the output current divided
by the total number of the low-side MOSFETs (nSF).
Since conduction loss is dominant in low-side MOSFET,
the following expression can represent total power
dissipation in each synchronous MOSFET in terms of
the ripple current per phase (IR) and the total output
current (IO):
P_SF
=
(1
−
D)
×
⎢⎢⎣⎡⎜⎜⎝⎛
I_O
n_SF
⎟⎟⎠⎞²
+¹
12
×
⎜⎜⎝⎛
n ×I_R
n_SF
⎟⎟⎠⎞
2
⎤
⎥
⎥
⎦
×
R_DS(SF)
(14)
© 2005 Fairchild Semiconductor Corporation
FAN5182 • Rev. 1.1.3
13
www.fairchildsemi.com