SC2446 datasheet(21/38 Pages) SEMTECH

21 / 38 page

 2004 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2446

Application Information (Cont.)

for R

s and RS3.

s2 is then obtained from

−

If the current limit is to be set to I

LM = 5A with the existing

power circuit parameter and C

s, it is calculated that

s=4.12 kΩ, Rs3=190 kΩ and Rs2=4.22 kΩ.

Similar steps and equations apply to the current limit

setting and scaling for current sinking mode.

Remark 4: When the current limit I

LM is lower than ILMcp,

the designer has the freedom of selecting higher Rds(ON)

MOSFETs to reduce cost. As a result, R

eg 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 MOSFETs.

Overload Protection and Hiccup

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 an 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 (75% for Ch1, 72%

for Ch2) of the reference voltage, the controller will shut

off both the top and the bottom MOSFET’s. Meanwhile

an internal 1.

4µA current source discharges the soft-start

capacitor C

32(C33) connected to the SS/EN pin.

When the capacitor is discharged to 0.5V, a 2

µA current

source recharges the SS/EN capacitor and converter

restarts. If overload persists, the controller will shut down

the converter when the soft start capacitor voltage

exceeds 3.2V. The converter will repeatedly start and shut

off until it is no longer overloaded. This hiccup mode of

overload protection is a form of foldback current limiting.

The following calculations estimate the average inductor

current when the converter output is shorted to the

ground.

a) The time taken to discharge the capacitor from 3.2V

to 0.5V

)

(

ssf

−

If C

32 = 0.1µF, tssf is calculated as 193ms.

b) The soft start time from 0.5V to 3.2V

)

(

ssr

−

When C

32 = 0.1µF, tssr is calculated as 135ms. Note that

during soft start, the converter only starts switching when

the voltage at SS/EN exceeds 1.2V.

c) The effective start-up time is

)

(

sso

−

The average inductor current is then

ssr

ssf

sso

LMcp

Leff

≈ 0.30 I

LMcp and is independent of the soft start

capacitor value. The converter will not overheat in hiccup.

Setting the Output Voltage

The non-inverting input of the channel-one error amplifier

is internally tied the 0.5V voltage reference output (Pin

8). The non-inverting input of the channel-two error

amplifier is brought out as a device pin (Pin 10) to which

the user can connect Pin 8 or an external voltage

reference. A simple voltage divider (R

o1 at top and Ro2 at

bottom) sets the converter output voltage. The voltage

feedback gain h=0.5/V

o is related to the divider resistors

value as

−

SC2446 Datasheet(PDF) 21 Page - Semtech Corporation

SC2446 Datasheet(HTML) 21 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Datasheet Download

Link URL