SC2620 Datasheet, PDF (15/26 Pages) Semtech Corporation – Dual 2A, 30V Step-down Regulator with Programmable Frequency up to 1.4MHz

SC2620
POWER MANAGEMENT
Applications Information
Since the inductor current charges C_BST, the bootstrap
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_IN = 5V. When one of the soft-
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_IN current decreases to
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_IN quiescent current
2.4V
2V
VSS
1V
0.3V
0
Fast
Charge
1V
0.7V
Switching Starts
Output must be at
0
least 70% of its set
voltage in this
interval or the
regulator will
undergo shutdown
and restart (hiccup).
Hiccup
Enabled
VFB
Figure 9(a). Normal Soft-start.
2V
VSS
1V
0.3V
0
Swi tch in g
VCOM _P
Not Switching
Swi tch in g
Not Switching
1V
0.7V
V _FB
0
Figure 9(b).
 2006 Semtech Corp.
Start-up Fails due to (i) Short Soft-start Duration or (ii) Output Overload or (iii)
Output Short-circuited.
15
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