SC4524D datasheet(12/21 Pages) SEMTECH

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SC4524D

Applications Information (Cont.)

inductor current of a buck converter directly feeds C

resulting in very low ripple current. Avoid using Z5U

and Y5V ceramic capacitors for output filtering because

these types of capacitors have high temperature and high

voltage coefficients.

Freewheeling Diode

Use of Schottky barrier diodes as freewheeling rectifiers

reduces diode reverse recovery input current spikes,

easing high-side current sensing in the SC4524D. These

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 SC4524D to minimize ringing due to trace

inductance. 20BQ030 (International Rectifier), B230A

(Diodes Inc.), SS13, SS23 (Vishay), CMSH1-40M, CMSH1-

40ML and CMSH2-40M (Central-Semi.) are all suitable.

The freewheeling diode should be placed close to the SW

pin of the SC4524D on the PCB to minimize ringing due to

trace inductance.

Bootstrapping the Power Transistor

The typical minimum BST-SW voltage required to fully

saturate the power transistor is shown in Figure 5, which

is about 1.96V 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 and improves efficiency.

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). D4 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 5. Typical Minimum Bootstrap Voltage required

to Saturate Transistor (I

SW= -2.6A).

Figure 6(a). Bootstrapping the SC4524D from the

converter output

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 = -2.6A

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 = -2.6A

(a)

SC4524D

BST

GND

VOUT

VIN

(a)

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