SC2620 Semtech Corporation, SC2620 Datasheet - Page 14

• Manufacturer Part Number: SC2620
• Description: 30V Step-down Regulator
• Manufacturer: Semtech Corporation

input power supply will be
switch). The energy loss due to base charge per cycle is
If
the output capacitor will still be
due to base charge per cycle is
loss of
Since V
to maximize efficiency. In general efficiency penalty
increases as D decreases.
Figure 7 summarizes various ways of bootstrapping the
SC2620. A fast switching PN diode (such as 1N4148 or
1N914) and a small (0.1µF – 0.47µF) ceramic capacitor
can be used. In Figure 7(a) the power switch is
bootstrapped from the output. This is the most efficient
configuration and it also results in the least voltage stress
POWER MANAGEMENT
Applications Information
I
SW
2008 Semtech Corp.
D
V
BST
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IN
T
is tied to the output, then the charge drawn from
ON
OUT
DI
SW
< V
for a power loss of
V
Figure 8. Minimum Input Voltage Required to Start and to Maintain Bootstrap.(T
OUT
IN
7.5
7.0
6.5
6.0
5.5
5.0
4.5
, D
1
.
BST
Sustaining V
D
TO INPUT
BST
should always be tied to V
TIED
Minimum Starting and
Load Current (mA)
10
I
D
TO OUTPUT
SUSTAINING
SW
BST
IN
(a)
DI
T
vs Load Current
TIED
ON
SW
I
I
SW
(the base charge of the
V
100
SW
STARTING
IN
T
V
V
MA729
ON
OUT
OUT
I
. The energy loss
SW
= 5V
T
ON
V
1000
OUT
OUT
for a power
.
(if >2.5V)
14
at the BOOST pin. The maximum BOOST pin voltage is
about
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 SC2620 can also be bootstrapped from the input
(Figure 7(b)). This configuration is not as efficient as Figure
7(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 BOOST pin can be
somewhat higher than 2V
reduces the maximum BOOST pin voltage. The BOOST pin
voltage should not exceed its absolute maximum rating of
42V.
Figures 7(d) and (e) show how to bootstrap the SC2620
from a second power supply V
Figure 7(d) can be the output of the other channel. Figures
1(a), 17(a) and 18(a) show this bootstrapping method. If
Channel 1 fails in these converters, Channel 2 will be shut
off (See Sequencing the Outputs). Proper bootstrapping
of Channel 2 therefore depends on the readiness of V
This may be a drawback in some applications. D
7(e) prevents start up difficulty if V
V +
IN
V
OUT
5.5
5.0
4.5
4.0
3.5
. If the output is below 2.8V, then D
0.1
Sustaining V
SUSTAINING
Minimum Starting and
1.0
Load Current (mA)
IN
D
TO OUTPUT
STARTING
BST
. The Zener diode in Figure 7(c)
IN
TIED
D
TO INPUT
10.0
vs Load Current
(b)
BST
S
with voltage > 2.5V. V
TIED
A
IN
= 25 C).
100.0
comes up before V
V
MA729
OUT
= 3.3V
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SC2620
1000.0
BST
in Figure
BST
OUT1
S
will
in
S
.
.