SC4525CEVB datasheet(12/22 Pages) SEMTECH

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SC4525C

Applications Information (Cont.)

Output Capacitor

The output ripple voltage DV

O of a buck converter can be

expressed as













ESR

where C

O is the output capacitance.

Since the inductor ripple current DI

L increases as D

decreases (see first inductor selection capacitor equation),

the output ripple voltage is therefore the highest when

IN is at its maximum.

A 22µF to 47µF X5R ceramic capacitor is found adequate for

output filtering in most applications. Ripple current in the

outputcapacitorisnotaconcernbecausetheinductorcurrent

of a buck converter directly feeds C

O, 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 SC4525C. These

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 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 SC4525C 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.

Minimum Bootstrap Voltage

vs Temperature

1.6

1.7

1.8

1.9

2.0

2.1

2.2

-50

-25

100 125

Temperature (oC)

ISW =-3.9A

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate the Transistor (I

SW= -3.9A)

The BST-SW voltage is supplied by a bootstrap circuit

powered from either the input or the output of the

converter (Figures 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 Figures 6(a) and 6(c). 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.

The bootstrap diode D

can be a fast switching PN diode

(N448 or N94) if V

O falls between 3V and 8V as shown

in Figure 6(a). If the converter output voltage is between

2.5V and 3V or higher than 8V, then use a low forward drop

Schottky diode (BAT54 or similar) for D

(Figure 6(c)). If VO

is less than 2.5V, then it will be necessary to bootstrap the

SC4525C from V

IN (Figure 6(b)). If bootstrapping from VIN

> 20V, then connect a Zener diode D

3 in series with D to

reduce the voltage stress at the BST pin. Figure 6(b) shows

this configuration for V

IN > 20V. If bootstrapping from VIN <

20V, then D

alone will suffice.

A small ceramic capacitor (0.33uF - 0.47uF) is adequate for

bootstrapping.

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