SC2446ITETRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC2446ITETRT' PDF : 38 Pages View PDF
SC2446
POWER MANAGEMENT
Application Information (Cont.)
1) lossless current sensing,
2) higher signal-to-noise ratio, and
3) preventing thermal run-away.
The basic arrangement of the Combi-sense is shown in
Figure 14.
Where, R is the equivalent series resistance of the output
L
inductor. The added R and C form a RC branch for
s
s
inductor current sensing. This branch is driven from a
small totem pole driver (Q3 and Q4) integrated within
SC2446. The base driving signals Vbe3 and Vbe4
Vin
Cin
Rds1
iL(t) L
RL
PN
Rs
Cs
VPN
vC(t)
Vo
Cout Rload
Figure 15 a). Equivalent sub-circuit.
Vin
Cin
V g s1
PN
V g s2
Q1
iL (t)
L
Rs
Q2
RL
Cs
v C (t )
Vo
Cou t Rload
Vbe3
Q3
VP N
Vbe4
Q4
Vin
C in
PN
i L (t )
L
Rs
Rds2
VP N
RL
Cs
vC (t)
Vo
C ou t Rload
Figure 14. The basic structure of Combi-Sense.
Figure 15 b). Equivalent sub-circuit.
are designed to follow the gate drive signals Vgs1 and
Vgs2, respectively, with minimal delay drive. Ideally, the
leading and falling edges of the Virtual Phase Node (VPN)
follow that of the Phase Node (PN) when Q1~Q4 switch
accordingly.
Specifically, when Q1/Q3 are ON and Q2/Q4 are OFF,
the equivalent circuit of Figure 14 reduces to Figure 15
a). Where, Rds1 is the on-resistance of the top MOSFET.
The two branches, consisting of {(Rds1+RL), L} and {Rs,
CS}, are in parallel. The DC voltage drop (Rds1+RL)Io
equals VCs. In this way, the output current is sensed from
VCs when (Rds1+RL) is known.
When Q1/Q3 are OFF and Q2/Q4 are ON, the equivalent
circuit of Figure 14 becomes the sub-circuit as shown in
Figure 15 b). Where, Rds2 is the channel resistance of
the bottom MOSFET. In this case, the branch {Rs,Cs} is in
parallel with {(Rds2+RL), L} and VCs=(Rds2+RL)Io. In
average,
VCs=[D(Rds1+RL)+(1-D)(Rds2+RL)]Io,
or equivalently
VCs=[D Rds1+(1-D)Rds2+RL]Io=ReqIo.
It is noted that the DC value of VCs is independent of the
value of L, Rs and Cs. This means that, if only the average
load current information is needed (such as in average
current mode control), this current sensing method is
effective without time constant matching requirement.
In the current mode control as implemented in SC2446,
the voltage ripple on Cs is critical for PWM operation. In
fact, the AC voltage ripple peak-to-peak value of VCs
(denoted as ∆VCs) directly effects the signal-to-noise ratio
of the PWM operation. In general, smaller ∆VCs leads to
lower signal-to-noise ratio and more noise sensitive
operation. Larger ∆VCs leads to more circuit (power stage)
2004 Semtech Corp.
19
www.semtech.com
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