sc4510 Semtech Corporation, sc4510 Datasheet - Page 12

• Manufacturer Part Number: sc4510
• Description: Sc4510 High-performance Synchronous Buck Controller With Reference Tracking
• Manufacturer: Semtech Corporation

COMBI-SENSE OPERATION AND CURRENT LIMIT
Effective current sensing is important for the current mode
control of power convertors. Instead of using the traditional
lossy resistive current sense scheme, a novel lossless
Combi-Sense technique is used in SC4510. This SEMTECH
proprietary technology has the advantages of
1) lossless current sensing,
2) bigger signal-to-noise ratio, and
3) thermal run-away prevention.
The basic structure of the Combi-Sense is shown in Fig.1.
Where R
The added R
the output inductor for inductor current sensing. This
branch works with a small signal totem pole (Q3 and Q4)
integrated in SC4510 in order to improve the signal-to-noise
ratio. The base signals Vbe3 and Vbe4 are designed to
closely follow the gate signals Vgs1 and Vgs2, respectively.
Ideally, the leading and falling edges of the Virtual Phase
Node (VPN) follow that of the Phase Node (PH) when Q1~Q4
switch in perfect synchronism.
When Q1/Q3 turn ON and Q2/Q4 turn OFF, the equivalent
circuit of Fig.1 is shown in Fig. 2a). Where, Rds1 is the on-
resistance of the top MOSFET. The two branches, consisting
of {(Rds1+RL), L} and {R
drop (Rds1+RL)I
is sensed from V
POWER MANAGEMENT
Application Information
Vin
2004 Semtech Corp.
Cin
Fig.1 The Basic Structure of Combi-Sense.
L
is the equivalent resistance of the output inductor.
Vbe3
Vbe4
Vgs1
Vgs2
s
VPN
and C
PN
o
Cs
Q3
Q4
equals V
when (Rds1+RL) is known.
Q1
Q2
s
form the RC branch in “parallel” with
iL(t)
s
, C
Cs
S
}, are in parallel. The DC voltage
. In this way, the output current
L
Rs
RL
Cs
vC(t)
Cout
Rload
Vo
12
When Q1/Q3 turn OFF and Q2/Q4 turn ON, the equivalent
circuit of Fig.1 becomes the sub-circuit as shown in Fig.
2b). Where Rds2 is the channel resistance of the bottom
MOSFET. In this case, the branch {R
{(Rds2+RL), L} and V
D is the operating duty ratio. It is noted that the average
DC value in V
If only the average load current information is needed (such
as in average current mode control), this current sensing
method is effective without further requirements.
Vin
When averaged over a complete cycle,
Vin
V
Cs
Cin
Cin
= [D(Rds1+RL)+(1-D)(Rds2+RL)] I
Fig.2 a) Equivalent sub-circuit.
= [D Rds1+(1-D)Rds2+RL]I
= R
eq
I
Cs
o
Fig.2 b) Equivalent sub-circuit.
.
PN
PN
is independent of the value of L, R
VPN
Rds2
Rds1
Cs
=(Rds2+RL)I
iL(t)
iL(t)
VPN
L
Rs
L
Rs
RL
Cs
vC(t)
RL
Cs
vC(t)
o
o
.
s
,C
s
} is in parallel with
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SC4510
o
Cout
Cout
Rload
Rload
s
and C
Vo
Vo
s
.