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 2006 Semtech Corp.

www.semtech.com

SC2620

POWER MANAGEMENT

Applications Information

Since the inductor current charges C

circuit requires some minimum load current to get going.

Figures 8(a) and 8(b) show the dependence of the

minimum input voltage required to properly bootstrap a

5V and a 3.3V converters on the load current. Once started

the bootstrap circuit is able to sustain itself down to zero

load.

Shutdown and Soft-Start

Each regulating channel of the SC2620 has its own soft-

start circuit. Pulling its soft-start pin below 0.8V with an

open-collector NPN or an open-drain NMOS transistor turns

off the corresponding regulator. The other regulator

continues to operate. With one channel turned off, the

internal bias circuit is kept alive. In the “Typical

Characteristics”, the soft-start pin current is plotted against

the soft-start voltage with V

start pins is pulled low, 105µA flows out of that pin. Pulling

both soft-start pins below 0.2V shuts off the internal bias

circuit of the SC2620. The total V

40µA. In shutdown either SS pin sources only 2µA. A fast

charging circuit (enabled by the internal bias circuit), which

charges the soft-start capacitor below 1V, causes the

difference in the soft-start pin currents.

If either SS pin is released in shutdown, the internal current

source pulls up on the SS pin. When this SS voltage reaches

0.3V, the SC2620 turns on and the V

Figure 9(a). Normal Soft-start.

Fast

Charge

Output must be at

least 70% of its set

voltage in this

interval or the

regulator will

undergo shutdown

and restart (hiccup).

Hiccup

Enabled

0.3V

1V

0

2V

2.4V

Switching Starts

VSS

1V

0.7V

VFB

0

1V

1V

2V

0.3V

0

Switching

Not Switching

Switching

Not Switching

0

0.7V

1V

VFB

VSS

VCOMP

Figure 9(b).

Start-up Fails due to (i) Short Soft-start Duration or (ii) Output Overload or (iii)

Output Short-circuited.