SC2441ITSTRT datasheet(21/35 Pages) SEMTECH

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 2005 Semtech Corp.

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POWER MANAGEMENT

SC2441

Setting the Current Limit

When the voltage difference between CS1+(CS2+) and

CS1- (CS2-) exceeds 50 mV, the top MOSFET will be

turned OFF and the bottom MOSFET will be turned ON to

limit the output inductor current. In the circuit of Figure

12, the converter output current limit is

LMcp =

In the application circuit, R

eq = 9.56mΩ so ILM = 5.23A. In

other applications, different current limits may be required.

The circuit in Figure 14 allows the user to set current-limit

different from 50mV/R

eq.

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Vgs2

Vgs1

VPN

Vbe4

Vbe3

ISEN

Rs3

Rs 2

Rs 1

Figure 14. Circuit for setting current-limit different

from 50mV/R

a) The required current limit I

LM is higher than ILMcp. Rs3 is

deleted from Figure 14. R

s2 is given by

s is obtained from

s1 is then computed from

−

If the current limit is to be set to I

LM=15A in the example

given in last section with C

s=33nF, then Rs2=4.12kΩ,

s=11.8kΩ and Rs1=6.36kΩ.

b) The required current limit I

LM is less than ILMcp. Rs1 is

deleted from Figure 14. R

s is given by

s3 is obtained from

Lastly R

s2 is computed from

−

If the current limit is to be set to I

LM=2.5A with Vo=1.2V

and C

s=33nF in the same example, then Rs=4.12 kΩ,

s3=190 kΩ and Rs2=4.21 kΩ.

Remark 3: If the current limit I

LM is lower than ILMcp, the

circuit designer will have the option to use MOSFET’s with

higher R

ds(ON) to reduce the cost. As a result, Req is increased

and I

LMcp is reduced. Although the use of low-cost MOSFET’s

is always preferred, the current-limit setting technique

described above allows quick adjustment on a well-tested

prototype without the need to replace the power MOSFET’s.

Over Current Protection and Hiccup Mode

During start-up, the capacitor from the SS/EN pin to ground

functions as a soft-start capacitor. After the converter starts

and enters regulation, the same capacitor operates as

overload shutoff timing capacitor. As the load current

increases, the cycle-by-cycle current-limit comparator will

first limit the inductor current. Further increase in loading

will cause the output voltage (hence the feedback voltage)

to fall. If the feedback voltage falls to less than 70% of the

normal, the controller will shut off both the top and the

bottom MOSFET’s. Meanwhile an internal current source

(1.4µA) discharges the soft start capacitor C

32(C33)

connected to the SS/EN pin.

When the capacitor is discharged to 0.47V, a 2.3µA

current source recharges the SS/EN capacitor and the

controller re-initiates soft-start. If the overload persists,

the controller will shut down the converter as the soft

start capacitor voltage exceeds 3.25V. The converter will

repeatedly start and shut off until it is no longer

overloaded. This hiccup mode of overload protection is a

Applications Information (Cont.)

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