SC4510ITSTR datasheet(12/16 Pages) SEMTECH

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SC4510

POWER MANAGEMENT

Application Information

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

L is the equivalent resistance of the output inductor.

The added R

s and Cs form the RC branch in “parallel” with

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.

Fig.1 The Basic Structure of Combi-Sense.

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

s, CS}, are in parallel. The DC voltage

drop (Rds1+RL)I

o equals VCs. In this way, the output current

is sensed from V

Cs when (Rds1+RL) is known.

Cin

Cout

Rload

Vin

vC(t)

iL(t)

VPN

Vbe4

Vbe3

Vgs2

Vgs1

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

s,Cs} is in parallel with

{(Rds2+RL), L} and V

Cs=(Rds2+RL)Io.

Fig.2 a) Equivalent sub-circuit.

When averaged over a complete cycle,

Cs = [D(Rds1+RL)+(1-D)(Rds2+RL)] Io

= [D Rds1+(1-D)Rds2+RL]I

= R

eqIo.

D is the operating duty ratio. It is noted that the average

DC value in V

Cs is independent of the value of L, Rs and Cs.

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.

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Rds1

VPN

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Rds2

VPN

Fig.2 b) Equivalent sub-circuit.

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