SC2446 Semtech Corporation, SC2446 Datasheet - Page 19

• Manufacturer Part Number: SC2446
• Description: Dual-Phase Single or Two Output Synchronous Step-Down Controller
• Manufacturer: Semtech Corporation

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
inductor. The added R
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
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 {R
C
equals V
V
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 {R
parallel with {(Rds2+RL), L} and V
average,
POWER MANAGEMENT
Application Information (Cont.)
Cs
S
}, are in parallel. The DC voltage drop (Rds1+RL)I
2004 Semtech Corp.
Vin
when (Rds1+RL) is known.
Figure 14. The basic structure of Combi-Sense.
C i n
Cs
L
. In this way, the output current is sensed from
is the equivalent series resistance of the output
Vbe 3
Vbe 4
Vgs1
Vgs2
VP N
PN
Q 3
Q 4
Q1
Q2
i L (t )
s
and C
L
R s
s
R L
C s
v C (t )
form a RC branch for
Cs
=(Rds2+RL)I
C ou t
R l o ad
s
,C
s
} is in
V o
o
. In
s
o
,
19
or equivalently
It is noted that the DC value of V
value of L, R
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 C
fact, the AC voltage ripple peak-to-peak value of V
(denoted as V
of the PWM operation. In general, smaller V
lower signal-to-noise ratio and more noise sensitive
operation. Larger V
Vin
Vin
Cin
C
V
V
i n
Cs
Cs
Figure 15 b). Equivalent sub-circuit.
=[D(Rds1+RL)+(1-D)(Rds2+RL)]I
=[D Rds1+(1-D)Rds2+RL]I
s
and C
PN
P N
Cs
Figure 15 a). Equivalent sub-circuit.
VPN
) directly effects the signal-to-noise ratio
Rds2
Rds1
s
. This means that, if only the average
Cs
leads to more circuit (power stage)
iL(t)
iL(t)
s
VP N
is critical for PWM operation. In
L
Rs
L
Rs
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Cs
RL
Cs
vC(t)
RL
Cs
vC (t)
is independent of the
o
=R
eq
Cout
C o u t
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SC2446
I
o
o
,
.
Rload
Rload
Cs
leads to
Vo
Vo
Cs