SC4525DEVB datasheet(12/21 Pages) SEMTECH

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SC4525D

Applications Information (Cont.)

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 SC4525D 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 SC4525D 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

2V 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 6(a), 6(b) and 6(c)). To maximize

efficiency, tie the bootstrap diode to the converter output

if V

O>2.5V as shown in Figure 6 (a). Since the bootstrap

supply current is proportional to the converter load

current, using a lower voltage to power the bootstrap

circuit reduces driving loss (Equation (11), page 14) and

improves efficiency.

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate the Transistor (I

SW= -3.9A)

Fig.5: Min BST vs Temp

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: Min BST vs Temp

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

For the bootstrap circuit, a fast switching PN diode

(such as 1N4148 or 1N914) and a small (0.33µF – 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

1. If VOUT > 8V, then a protection diode D4 between the

SW and the BST pins will be required as shown in Figure 6

(c). D

4 can be a small PN diode such as 1N4148 or 1N914 if

the operating temperature does not exceed 85 ºC. Use a

small Schottky diode (BAT54 or similar) if the converter is

to operate up to 125 ºC.

Figure 6. Methods of Bootstrapping the SC4525D

Fig.6: Methods of BST

(a)

SC4525D

BST

GND

VOUT

VIN

(b)

SC4525D

BST

GND

VOUT

VIN

Fig.6: Methods of BST

(a)

SC4525D

BST

GND

VOUT

VIN

(b)

SC4525D

BST

GND

VOUT

VIN

(C)

SC4525D

BST

GND

VOUT>8V

VIN

D4 is either a pn juntion diode or a Schottky diode

depending on the operating temperature.

(a)

SC4525D

BST

GND

VOUT

VIN

(a)

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