SC4524B 查看數據表（PDF） - Semtech Corporation

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SC4524B

18V 2A Step-Down Switching Regulator

Semtech Corporation 

Fig.5

SC4524B

Applications Information (Cont.)

diodes should have an average forward current rating

at least 2A and a reverse blocking voltage of at least a

few volts higher than the input voltage. For switching

regulators operating at low duty cycles (i.e. low output

voltage to input voltage conversion ratios), it is beneficial

to use freewheeling diodes with somewhat higher

average current ratings (thus lower forward voltages). This

is because the diode conduction interval is much longer

than that of the transistor. Converter efficiency will be

improved if the voltage drop across the diode is lower.

The freewheeling diode should be placed close to the

SW pin of the SC4524B to minimize ringing due to trace

inductance. 10BQ015, 20BQ030 (International Rectifier),

B220A (Diodes Inc.), SS13, SS22 (Vishay), CMSH1-20M,

CMSH1-20ML and CMSH2-20M (Central-Semi.) are all

suitable.

SS270 REV 6-7

2.2

Minimum Bootstrap Voltage

vs Temperature

2.1

2.0

1.9

1.8

1.7

ISW = -2.6A

1.6

-50 -25

0 25 50 75 100 125

Temperature (oC)

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate Transistor (ISW= -2.6A).

The freewheeling diode should be placed close to the SW

pin of the SC4524B on the PCB to minimize ringing due to

trace inductance.

Bootstrapping the Power Transistor

The minimum BST-SW voltage required to fully saturate

the power transistor is shown in Figure 5, which is about

1.96V at room temperature.

D1

BST

C1

VIN

IN

SW

SC4524B

GND

D2

VOUT

VIN

(a)

The BST-SW voltage is supplied by a bootstrap Dc1ircuit

D1

powered from either the input or the output of the

converter (Figure 6). To maximBSTize efficiencyC, 1 tie the

BST

C1

bootstrap

diode

toVINthe

converter

IN

output

SW

if

VO>2.5V. VOUT

VIN

IN

SW

VOUT

Since the bootstrap supply current is proportional to the

SC4524B

SC4524B

converter

load

current,

using

a

lower

GND

voltagDe2 to

power

GND

D2

the bootstrap circuit reduces driving loss and improves

efficiency.

(a)

(b)

For the bootstrap circuit, a fast switching PN diode (such

Figure 6. Methods of Bootstrapping the SC4524B

as 1N4148 or 1N914) and a small (0.1µF – 0.47µF) ceramic

capacitor is sufficient for most applications. When

bootstrapping from 2.5V to 3.0V output voltages, use a Loop Compensation

low forward drop Schottky diode (BAT-54 or similar) for

D1.

The goal of compensation is to shape the frequency

response of the converter so as to achieve high DC

accuracy and fast transient response while maintaining

loop stability.

12

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