SC4524SETRT 查看數據表（PDF） - Semtech Corporation

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SC4524SETRT

Programmable Frequency, 2A Output 30V Step-Down Switching Regulator

Semtech Corporation 

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

VIN +VOUT . If the output is below 2.8V, then DBST 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 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 2VIN. 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 VS with voltage > 2.5V.

Since

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bootstrap

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

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 VIN = 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 VIN 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 VINquiescent

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 VIN vs Load Current

7.5

DBST TIED

VOUT = 5V

TO OUTPUT

MA729

7.0

6.5

STARTING

6.0

5.5

5.0 DBST TIED

TO INPUT

SUSTAINING

4.5

1

10

100

Load Current (mA)

(a)

1000

Minimum Starting and

Sustaining VIN vs Load Current

5.5

DBST TIED

TO OUTPUT

VOUT = 3.3V

MA729

5.0

STARTING

4.5

DBST TIED

TO INPUT

4.0

SUSTAINING

3.5

0.1

1.0

10.0 100.0

Load Current (mA)

(b)

1000.0

Figure 8. Minimum Input Voltage Required to Start and to Maintain Bootstrap.(TA = 25°C).

© 2006 Semtech Corp.

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