SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC2441ITSTRT
Semtech Corporation
SC2441ITSTRT Datasheet PDF : 35 Pages
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POWER MANAGEMENT
Applications Information (Cont.)
SC2441
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 13. b) Equivalent sub-circuit.
Figure 12. The basic structure of combi-sense.
VCs=[D(Rds1+RL)+(1-D)(Rds2+RL)]Io
follow the gate drive signals Vgs1 and Vgs2 respectively
with minimal delay. The transition edges at the Virtual
Phase Node (VPN) therefore matches closely to those of
the Phase Node (PN).
When Q1/Q3 are on and Q2/Q4 are off, the circuit in Figure
12 is reduced to that of Figure 13 a). Here Rds1 is the on-
resistance of the top MOSFET. The branches {(Rds1+RL),
L} and {Rs, CS}, are in parallel. The DC voltage across L and
the DC current through CS are both zero. There is no DC
voltage across R either. Therefore on average, the voltage
s
drop (Rds1+RL)I equals V . The DC inductor current can
o
Cs
be sensed from VCs if (Rds1+RL) is known.
When Q1/Q3 are off and Q2/Q4 are on, the equivalent
circuit of Figure 12 reduces to the sub-circuit in Figure
13b). Here Rds2 is the channel resistance of the bottom
MOSFET. In this case, the branch {Rs, Cs} is in parallel with
{(Rds2+RL), L} and V =(Rds2+RL)I . Averaging over one
Cs
o
switching cycle,
Vin
Rds1
or
VCs=[D Rds1+(1-D)Rds2+RL]Io:=ReqIo.
The DC voltage across VCs is independent of L, Rs and Cs. If
only the average load current is needed (as in average
current-mode control), this current sensing method will be
sufficient without any additional time-constant matching
constraint.
In peak current-mode control, the voltage ripple on Cs is
used as the modulating ramp. The VCs peak-to-peak
amplitude (denoted as ∆VCs) directly affects the signal-
to-noise ratio of the PWM operation. Small ∆VCs leads to
lower signal-to-noise ratio and more jittery operation.
Large ∆VCs leads to more circuit (power stage) sensitive
operation. A good compromise is to make
∆VCs~ReqδIo.
The above condition holds if the following time-constants
are made equal.
L
R eq
≈ RsCs.
iL(t) L
RL
PN
Cin
Rs
Cs
If Rds1=Rds2, then Req = Rds1+ RL. For example, in the
application circuit, L=1.3µH, RL=1.56mΩ and
VPN
vC(t)
Vo
Cout Rload
Rds1=Rds2=8mΩ , the time constant RsCs should be set
as 136µs. If Cs=33nF, then Rs=4.12kΩ.
Figure 13. a) Equivalent sub-circuit.
2005 Semtech Corp.
20
www.semtech.com
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