SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 16

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

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 SC2440. The total V
decreases to 38µA. 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 SC2440 turns on and the V
quiescent current increases to 3.3mA. The current
flowing out of the other SS pin (which is still pulled low)
increases to 105µA. The fast charging circuit quickly pulls
the released soft-start capacitor to 1V (slightly below the
switching threshold). The fast charging circuit is then
POWER MANAGEMENT
Applications Information
0.74V
2005 Semtech Corp.
0.74V
0.3V
2.4V
0.3V
1V
2V
1V
1V
1V
2V
1V
0
0
0
0
V SS
V FB
Charge
Fast
V SS
Figure 10(b). Start-up Fails due to (i) Short Soft-start Duration or (ii) Output Overload or (iii)
least 74% of its set
undergo shutdown
Output must be at
Switc hing Starts
interval or the
voltage in this
regulator will
and restart
(hiccup).
Switc hing
Output Short-circuited.
Enabled
V COMP
Hiccup
V FB
Figure 10(a). Normal Soft-start.
Not Switchi ng
IN
current
IN
16
disabled. A 1.8µA current source continues to charge
the soft-start capacitor (Figure 3). The soft-start voltage
ramp at the SS pin clamps the error amplifier output
(Figure 2). During regulator start-up, COMP voltage follows
the SS voltage. The converter starts to switch when its
COMP voltage exceeds 1.1V. The peak inductor current
gradually increases until the converter output comes into
regulation. Proper soft-start prevents output overshoot
during start-up. Current drawn from the input supply is
also well controlled. Notice that the inductor current, not
the converter output voltage, is ramped during soft-start.
Both soft-start capacitors are charged to a final voltage
of about 2.4V.
Switc hing
Not Switchi ng
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SC2440