SC4525AEVB View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC4525AEVB

28V 3A Step-Down Switching Regulator

Semtech Corporation 

Fig 5

SC4525A

Applications Information (Cont.)

diodes should have an average forward current rating

at least 3A 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.

SS270 REV 6-7

2.2

Minimum Bootstrap Voltage

vs Temperature

2.1

2.0

1.9

1.8

1.7

ISW =-3.9A

The freewheeling diode should be placed close to the

1.6

SW pin of the SC4525A to minimize ringing due to trace

inductance. 20BQ030 (International Rectifier), B320A,

-50 -25 0 25 50 75 100 125

Temperature (oC)

B330A (Diodes Inc.), SS33 (Vishay), CMSH3-20MA and Figure 5. Typical Minimum Bootstrap Voltage required

CMSH3-40MA (Central-Semi.) are all suitable.

to Saturate Transistor (ISW= -3.9A)

D1

D3

The freewheeling diode should be placed close to the SW

pin of the SC4525A on the PCB to minimize ringing due to

BST

C1

trace inductance.

VIN

IN

SW

VOUT

VIN

Bootstrapping the Power Transistor

SC4525A

GND

D2

The minimum BST-SW voltage required to fully saturate

the power transistor is shown in Figure 4, which is about

1.98V at room temperature.

D1

(a)

D3

D1

The BST-SW voltage is supplied by a bootstrap circuit

powered from either the inputBSoT r the outpuC1t of the

BST

C1

converter (Figure 5V)I.N To maximize efficiency, tie theVOUT

VIN

VOUT

IN

SW

IN

SW

bootstrap diode to the convSeCr4te52r5Aoutput if VO>2.5V.

SC4525A

Since

the

bootstrap

supply

current is

GND

proportiDo2nal

to

the

GND

D2

converter load current (Equation (10), page 14), using a

lower voltage to power the bootstrap circuit reduces

driving loss and improves efficiency. (a)

(b)

Figure 6. Methods of Bootstrapping the SC4525A

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

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

capacitor is sufficient for most applications. When Loop Compensation

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

low forward drop Schottky diode (BAT-54 or similar) for The goal of compensation is to shape the frequency

D1. When bootstrapping from high input voltages (>20V), response of the converter so as to achieve high DC

reduce the maximum BST voltage by connecting a Zener accuracy and fast transient response while maintaining

diode (D3) in series with D1.

loop stability.

12

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