SC4525CEVB View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC4525CEVB

28V 3A Step-Down Switching Regulator

Semtech Corporation 

SC4525C

Applications Information (Cont.)

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,

Fig 5 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 freewheeling diode should be placed close to the

SW pin of the SC4525C 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 SC4525C on the PCB to minimize ringing due to

trace inductance.

Bootstrapping the Power Transistor

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

D1. When bootstrapping from high input voltages (>20V),

reduce the maximum BST voltage by connecting a Zener

diode (D3) in series with D1 as shown in Figure 6 (b). 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

SoSp2e70raRtEeVu6p-7 to 125 ºC.

Minimum Bootstrap Voltage

vs Temperature

2.2

2.1

2.0

1.9

1.8

1.7

ISW =-3.9A

1.6

-50 -25

0 25 50 75 100 125

Temperature (oC)

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate the Transistor (ISW= -3.9A)

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

supply current is proportional to the converter load

current (Equation (10), page 14), using a lower voltage

to power the bootstrap circuit reduces driving loss and

improves efficiency.

D1

BST

C1

VIN

IN

SW

SC4525C

GND

D2

VOUT

(a)

Figure 6(a). Bootstrapping the SC4525C from the

Converter Output

12

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