SC4524C Просмотр технического описания (PDF) - Semtech Corporation

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SC4524C

28V 2A Step-Down Switching Regulator

Semtech Corporation 

SC4524C

Applications Information (Cont.)

Output Capacitor

The output ripple voltage DVO of a buck converter can be

expressed as

DVO

=

DIL

x

 ESR +



8 x FSW x

CO



where CO is the output capacitance.

Since the inductor ripple current DIL increases as D

decreases (see first Inductor selection equation), the

output ripple voltage is therefore the highest when VIN is

at its maximum.

A 10µF to 47µF X5R ceramic capacitor is found adequate

for output filtering in most applications. Ripple current

in the output capacitor is not a concern because the

inductor current of a buck converter directly feeds CO,

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.

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.

Minimum Bootstrap Voltage

vs Temperature

2.2

2.1

2.0

1.9

1.8

1.7

ISW = -2.6A

1.6

-50 -25 0 25 50 75 100 125

Temperature (o C)

Figure 5 — Typical Minimum Bootstrap Voltage

required to Saturate Transistor (ISW= -2.6A)

Freewheeling Diode

Use of Schottky barrier diodes as freewheeling rectifiers

reduces diode reverse recovery input current spikes,

easing high-side current sensing in the SC4524C. 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 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 SC4524C on the PCB to minimize ringing due to

trace inductance.

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 VO > 2.5V as shown in Figure 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 D1 can be a fast switching PN diode

(1N4148 or 1N914) if VO 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 D1 (Figure 6(c)). If VO

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

SC4524C from VIN (Figure 6(b)). If bootstrapping from VIN

> 20V, then connect a Zener diode D3 in series with D1 to

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

this configuration for VIN > 20V. If bootstrapping from VIN <

20V, then D1 alone will suffice. D1 is a PN junction diode as

in Figure 6(a).

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

bootstrapping.

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