sc2542 Semtech Corporation, sc2542 Datasheet - Page 18

• Manufacturer Part Number: sc2542
• Description: Sc2542 High Performance Wide Input Rangedual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

the ESR is:
The peak-to-peak input voltage ripple due to the capaci-
tor is:
From these two expressions, CIN can be found to meet
the input voltage ripple specification. In a multi-phase
converter, channel interleaving can be used to reduce
ripple. The two step-down channels of the SC2542 op-
erate at 180 degrees from each other. If both step-down
channels in the SC2542 are connected to the same in-
put rail, the input RMS currents will be reduced. Ripple
cancellation effect of interleaving allows the use of
smaller input capacitors.
leaved, the total DC input current is simply the sum of
the individual DC input currents. The combined input cur-
rent waveform depends on duty ratio and the output
current waveform. Assuming that the output current
ripple is small, the following formula can be used to esti-
mate the RMS value of the ripple current in the input
capacitor.
Let the duty ratio and output current of Channel 1 and
Channel 2 be D1, D2 and Io1, Io2, respectively.
If D1<0.5 and D2<0.5, then:
If D1>0.5 and (D1-0.5) < D2<0.5, then:
If D1>0.5 and D2 < (D1-0.5) < 0.5, then:
Choosing Power MOSFETs
Main considerations in selecting the MOSFETs are power
dissipation, MOSFETs cost, and packaging. Switching
POWER MANAGEMENT
Applications Information (Cont.)
I
When two channels with a common input are inter-
CIN
2005 Semtech Corp.
I
CIN
( 1
D D
0.5
2 1)(
I
O
1
I
I
CIN
(
V
O
D
1
ESR
1
I
0.5)(
O
V
2
D
C
R
) (1
1
2
I
ESR
O
I
1
O
C
2
DI
1
1 (
IN
O
f
I
D
S
D I
O
2
2
2
2
I
)
O
)
2
I )
2
O
2
O
1
(
2
D D
2
(1
D I
1
1
)
0.5)
O
2
2
I
O
2
2
18
losses and conduction losses of the MOSFET are directly
related to the total gate charge (Cg) and channel on-re-
sistance (Rds(on)). In order to judge the performance of
MOSFET, 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 16
Figure 16. Figure of Merit curves.
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 may be difficult to find MOSFET with both low
Cg and low Rds(on). Usually a trade-off between
Rds(on)and Cg has to be made.
MOSFET selection also depends on applications. In many
applications, either switching loss or conduction loss domi-
nates 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 with low Cg are used for the top switch and
MOSFET with low Rds(on) are used for the bottom switch.
MOSFET power dissipation consists of:
a) conduction loss due to the channel resistance Rds(on);
b) switching loss due to the switch rise time tr and fall
time tf; and
c) the gate loss due to the gate resistance RG.
Cg 100 Rds
Cg 200 Rds
Cg 500 Rds
(
(
(
?
?
?
50
1
)
)
)
40
20
0
1
0
FOM:100*10^{-12}
FOM:200*10^{-12}
FOM:500*10^{-12}
On-resistance (mOhm)
5
Rds
10
www.semtech.com
SC2542
15
20
20