SC2441A_09 Datasheet, PDF (30/37 Pages) Semtech Corporation – 1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

SC2441A
POWER MANAGEMENT
Applications Information
If D₁>0.5 and D₂ > 0.5, then
I_Cin » (D₁ + D₂ - 1)(I_o1 + I_o2 )² + (1- D₂ )I_o12 + (1- D₁)I_o22 .
Power MOSFET Selection and Gate Drive
Main considerations in selecting the MOSFET’s are power
dissipation, cost and packaging. Switching losses and
conduction losses of the MOSFET’s are directly related
to the total gate charge (C_g) and channel on-resistance
(R_ds(on)). In order to judge the performance of MOSFET’s,
the product of the total gate charge and on-resistance is
used as a figure of merit (FOM). Transistors with the same
FOM follow the same curve in Figure 20.
50
40
Cg(100, Rds)
Cg(200, Rds)
Cg(500, Rds) ²⁰
1 ₀₀
5
10
15
20
1
Rds
20
On-resistance (mOhm)
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
Figure 20. Figure of merit curves.
The losses in power MOSFET’s consist of
a) conduction loss due to the channel resistance R_ds(on),
b) switching loss due to the switch rise time t_r and fall
time t_f and
c) the gate loss due to the gate resistance R_G.
Top Switch:
The RMS value of the top switch current is
I_Q1,rms = I_o
D(1 +
d²
12
).
Its conduction loss is then
P = I 2R .
tc Q1,rms ds(on)
R varies with temperature and gate-source voltage.
ds(on)
Curves showing R_ds(on) variations can be found in
manufacturers’ data sheet. From the Si7882DP
datasheet, R_ds(on) is less than 4.5mOhm when V_gs is greater
than 5V. However R_ds(on) increases by nearly 40% as the
junction temperature increases from 25°C to 125°C.
The switching losses can be estimated using the simple
formula
P_ts
=
1
2
(t
r
+ t _f )(1+
d
2
)I_o
V_in
f
s
.
where t_r is the rise time and t_f is the fall time of the
switching process. To clarify these, we sketch the typical
MOSFET switching characteristics under clamped
inductive mode in Figure 21.
The closer the curve is to the origin, the lower is the FOM.
This means lower switching loss or lower conduction loss
or both. It is difficult to find MOSFET’s with both low C_g
and low R_ds(on). Usually a trade-off between R_ds(on) and C_g
has to be made.
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss
dominates for a particular MOSFET. For synchronous buck
converters with high input to output voltage ratios, the
top MOSFET is hard switched but conducts with very low
duty cycle. The bottom switch conducts at high duty cycle
but switches at near zero voltage. For such applications,
MOSFET’s with low C_g are used for the top switch and
MOSFET’s with low R_ds(on) are used for the bottom switch.
Figure 21. MOSFET switching characteristics
 2006 Semtech Corp.
30
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