SC2441ITSTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441ITSTRT

Very Low Input Voltage 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441

POWER MANAGEMENT

Applications Information (Cont.)

Setting the Current Limit

Rs=11.8kΩ and Rs1=6.36kΩ.

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

ILMcp

=

50mV

R eq

.

In the application circuit, Req = 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/Req.

Vin

Q1

V gs1

PN

Cin

i L (t )

L

Rs

Q2

V gs2

RL

Rs 1

Cs

vC(t)

V be3

Q3

VP N

V be4

Q4

Vo

C out Rload

b) The required current limit ILM is less than ILMcp. Rs1 is

deleted from Figure 14. Rs is given by

RsCs

=

L

R eq

,

Rs3 is obtained from

ILMR eq

+

Rs

Rs3

VO

=

50mV,

Lastly Rs2 is computed from

Rs2

=

Rs3Rs

Rs3 − Rs

.

If the current limit is to be set to ILM=2.5A with Vo=1.2V

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

Rs3=190 kΩ and Rs2=4.21 kΩ.

Remark 3: If the current limit ILM is lower than ILMcp, the

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

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

and ILMcp 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

ISEN

+1

-2

Rs 2

R s3

Figure 14. Circuit for setting current-limit different

from 50mV/Req

a) The required current limit I is higher than I . R is

LM

LMcp s3

deleted from Figure 14. Rs2 is given by

Rs2Cs

=

L

R eq

,

Rs is obtained from

ILMR eq

Rs2

Rs

= 50mV,

Rs1 is then computed from

R s1

=

R s 2R s

Rs − Rs2

.

If the current limit is to be set to ILM=15A in the example

given in last section with Cs=33nF, then Rs2=4.12kΩ,

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 C32(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

 2005 Semtech Corp.

21

www.semtech.com

Share Link: