SC4525ASETRT datasheet(12/19 Pages) SEMTECH

12 / 19 page

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.

The freewheeling diode should be placed close to the

SW pin of the SC4525A to minimize ringing due to trace

inductance. 20BQ030 (International Rectifier), B320A,

B330A (Diodes Inc.), SS33 (Vishay), CMSH3-20MA and

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

The freewheeling diode should be placed close to the SW

pin of the SC4525A 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 4, which is about

.98V at room temperature.

The BST-SW voltage is supplied by a bootstrap circuit

powered from either the input or the output of the

converter (Figure 5). To maximize efficiency, tie the

bootstrap diode to the converter output if V

O>2.5V.

Since the bootstrap supply current is proportional to the

converter load current (Equation (0), page 4), using a

lower voltage to power the bootstrap circuit reduces

driving loss and improves efficiency.

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

as N448 or N94) and a small (0.µF – 0.47µF) ceramic

capacitor is sufficient for most applications. When

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

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

. When bootstrapping from high input voltages (>20V),

reduce the maximum BST voltage by connecting a Zener

diode (D

3) in series with D.

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate Transistor (I

SW= -3.9A)

Figure 6. Methods of Bootstrapping the SC4525A

Loop Compensation

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.

Fig 5

Minimum Bootstrap Voltage

vs Temperature

1.6

1.7

1.8

1.9

2.0

2.1

2.2

-50 -25

75 100 125

Temperature (oC)

SS270 REV 6-7

ISW =-3.9A

Fig 5

Minimum Bootstrap Voltage

vs Temperature

1.6

1.7

1.8

1.9

2.0

2.1

2.2

-50 -25

75 100 125

Temperature (oC)

SS270 REV 6-7

ISW =-3.9A

(a)

SC4525A

BST

GND

VOUT

VIN

(b)

SC4525A

BST

GND

VOUT

VIN

(a)

SC4525A

BST

GND

VOUT

VIN

(b)

SC4525A

BST

GND

VOUT

VIN

(a)

SC4525A

BST

GND

VOUT

VIN

(b)

SC4525A

BST

GND

VOUT

VIN

(a)

SC4525A

BST

GND

VOUT

VIN

(b)

SC4525A

BST

GND

VOUT

VIN

SC4525A

Applications Information (Cont.)

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