SC4524
POWER MANAGEMENT
Applications Information
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
at the BST pin. The maximum BST pin voltage is about
V
IN
+
V
OUT
. If the output is below 2.8V, then D
BST
will
T
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 SC4524 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 BST
pin can be somewhat higher than 2V
IN
. The Zener diode
in Figure 7(c) reduces the maximum BST pin voltage. The
BST pin voltage should not exceed its absolute maximum
rating of 42V.
Figures 7(d) and (e) show how to bootstrap the SC4524
from a second power supply V
S
with voltage > 2.5V.
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
Minimum Starting and
Sustaining V
I N
vs Load Current
7.5
Minimum Input Voltage (V)
D
B S T
TIED
TO OUTPUT
V
OUT
= 5V
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
Pulling the soft-start pin below 0.8V with an open-
collector NPN or an open-drain NMOS transistor turns
off the regulator. In “Typical Characteristics”, the soft-
start pin current is plotted against the soft-start voltage
with V
IN
= 5V. When the soft-start pin is pulled below 1V,
105µA current flows out of the pin. Pulling the soft-start
pin below 0.2V shuts off the internal bias circuit of the
SC4524. The total V
IN
current decreases to 40µA. In
shutdown the 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 the SS pin is released in shutdown, the internal current
source pulls up on the SS pin. When this SS voltage
reaches 0.4V, the SC4524 turns on and the V
IN
quiescent
current increases to 3.5mA. The fast charging circuit
quickly pulls the released soft-start capacitor to 1V
(slightly below the switching threshold). The fast charging
circuit is then disabled. A 1.8µA current source continues
to charge the soft-start capacitor (Figure 3). The soft-
Minimum Starting and
Sustaining V
I N
vs Load Current
5.5
Minimum Input Voltage (V)
D
BST
TIED
TO OUTPUT
V
OUT
= 3.3V
MA729
7.0
6.5
MA729
5.0
STARTING
STARTING
6.0
5.5
5.0
4.5
1
10
100
1000
Load Current (mA)
D
BST
TIED
TO INPUT
SUSTAINING
4.5
D
BST
TIED
TO INPUT
4.0
SUSTAINING
3.5
0.1
1.0
10.0
100.0
1000.0
Load Current (mA)
(a)
(b)
Figure 8. Minimum Input Voltage Required to Start and to Maintain Bootstrap.(T
A
= 25°C).
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2006 Semtech Corp.
14
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