SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 15

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

loss of
Since V
>2.5V) to maximize efficiency. Measurement of the 2-
channel regulator in Figure 1 shows that the efficiency
penalties are about 1.3% (V
3.3V) with input bootstrapping. In general efficiency
penalty increases as D decreases.
Figure 8 summarizes various ways of bootstrapping the
SC2440. A fast switching PN diode (such as 1N4148 or
1N914) and a small (0.1µF – 0.47µF) ceramic capacitor
can be used. In Figure 8(a) the power switch is
bootstrapped from the output. This is the most efficient
configuration and it also results in the least voltage stress
at the BOOST pin. The maximum BOOST pin voltage is
about
will preferably be a small Schottky diode (such as BAT-
54) to maximize bootstrap voltage. A 0.33-0.47µF
bootstrap capacitor may be needed to reduce droop.
Bench measurement shows that using Schottky
bootstrapping diode has no noticeable efficiency benefit.
The SC2440 can also be bootstrapped from the input
(Figure 8(b)). This configuration is not as efficient as Figure
8(a). However this may be only option if the output
voltage is less than 2.5V and there is no other supply
with voltage higher than 2.5V. Voltage stress at the
POWER MANAGEMENT
Applications Information
2005 Semtech Corp.
V
DI
OUT
IN
SW
Figure 9. Minimum Input Voltage Required to Start and to Maintain Bootstrap.(T
< V
V
7.5
7.0
6.5
6.0
5.5
5.0
4.5
V
OUT
OUT
1
IN
. If the output is below 2.8V, then D
Sustaining V
, D
D
TO INPUT
.
BST
BST
TIED
Minimum Starting and
Load Current (mA)
should always be tied to V
10
D
TO OUTPUT
SUSTAINING
BST
IN
(a)
vs Load Current
TIED
OUT
= 5V) and 2.2% (V
100
STARTING
V
MA729
OUT
= 5V
1000
OUT
OUT
BST T
(if
=
15
BOOST pin can be somewhat higher than 2V
pin voltage should not exceed its absolute maximum
rating of 40V.
Figures 8(c) and (d) show how to bootstrap the SC2440
from a second independent power supply V
> 2.5V. D
V
Since the inductor current charges C
circuit requires some minimum load current to get going.
Figures 9(a) and 9(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 SC2440 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
comes up before V
BST
5.5
5.0
4.5
4.0
3.5
in Figure 8(d) prevents start up difficulty if
0.1
Sustaining V
SUSTAINING
Minimum Starting and
1.0
Load Current (mA)
S
.
D
TO OUTPUT
STARTING
BST
IN
TIED
D
TO INPUT
10.0
vs Load Current
(b)
BST
TIED
A
100.0
= 25 C).
V
MA729
IN
OUT
= 5V. When one of
= 3.3V
BST
1000.0
, the bootstrap
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SC2440
IN
S
. The BOOST
with voltage