SC2447 datasheet(21/26 Pages) SEMTECH

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POWER MANAGEMENT

SC2447

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. There is an internal net 9.5

µA current source

charging the soft start capacitor C

32(C33), connected to

the SS/EN pin. The soft-start voltage V

SS/EN will reach its

final value if no current limit occurs. As the load current

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

first limit the inductor current. If V

SS/EN is higher than

3.2V, an internal net 37

µA current source will discharge

the soft-start capacitor C

32(C33)

during the off time after

tripping the over-current comparator. If V

SS/EN falls to

2.85V, the controller will shut off both the top and the

bottom MOSFETs by pulling down the GDL and tri-stating

the GDH output (PWM). An internal net 7.5

µA current

source discharges C

32(C33) .

When the capacitor is discharged below 0.5V, the overload

hiccup latch is reset, the 9.5

µA current source recharges

the SS/EN capacitor and converter restarts. The overload

hiccup function is enabled when the soft start capacitor

voltage exceeds 3.2V. The converter will repeatedly start

and shut off until it is no longer overloaded when the

soft-start voltage exceeds 3.2V. 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 2.85V is

ssf

)

(

−

If C

32 = 0.1µF, tssf1 is calculated as 0.945ms.

b) The time taken to discharge the capacitor from 2.85V

to 0.5V is

ssf

)

(

−

If C

32 = 0.1µF, tssf2 is calculated as 31.3ms.

c) The soft start time from 0.5V to 3.2V is

ssr

)

(

−

When C

32 = 0.1µF, tssr is

calculated as 28.4ms. Note

that during soft start, the converter only starts switching

when the voltage at SS/EN exceeds 1.25V.

d) The effective start-up time is

sso

)

(

−

The average inductor current is then

ssr

ssf

sso

LMcp

Leff

≈ 0.34 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 inputs of channel 1 and channel 2 error

amplifiers are brought out as device pins (Pin 10 and Pin

8). These pins can be tied to the precision 0.5V reference

output (Pin 9) of the SC2447. A simple voltage divider

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 as follows:

−

Once either R

o1 or Ro2 is chosen, the other can be

calculated for the desired output voltage V

Since the

number of standard resistance values is limited, the

calculated resistance may not be available as a standard

value resistor. As a result, there will be a set error in the

converter output voltage. This non-random error is

caused by the feedback voltage divider ratio. It cannot

be corrected by the feedback loop.

The following table lists a few standard resistor

combinations for realizing some commonly used output

voltages.

)

(

)

(

)

(

)

(

Application Information (Cont.)

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