SC4530 datasheet(16/23 Pages) SEMTECH

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SC4530

Reverse Input Protection

Consider a circuit board where the input power source

supplies several DC-DC converters, including an SC4530

regulator with a large output capacitor. During power-

off, the SC4530 regulator output may be held high by its

output capacitor, while V

IN is discharged rapidly by other

DC-DC converters. If V

IN falls to two diode voltages below

OUT, then the parasitic junction diodes inside the SC4530

(see Figure 2, page 5) will draw current from the output

through the SW pin to the input. If the load is light and

the output capacitor is large, then high reverse current

will flow, or even damage the internal circuits.

Figure 0 shows two protection schemes. In Figure 0(a),

a Schottky diode D

4 placed at the input blocks the reverse

current. This method has the disadvantage that it lowers

the converter efficiency. A PN junction diode placed from

the converter output to the input [(as shown in Figure

0(b)] shunts the reverse current away from the part, thus

protecting the part. This scheme is not suitable in a power

supply system where a backup battery is diode OR-ed

with the SC4530 regulator output and with the SC4530

input grounded.

Board Layout Considerations

In a step-down switching regulator, the input bypass

capacitor, the main power switch and the freewheeling

power diode carry pulse current with high di/dt (Figure

). To minimize jittering, the size of the loop formed by

these components must be minimized. Since the main

power switch and the freewheeling diode are already

integrated inside the part, connecting the input bypass

capacitor close to the ground pin minimizes size of the

switched current loop.

OUT

Heavy Lines Show the Fast Switching

Current Paths in a Step-down Converter.

The Input Capacitor Should be Placed

Close to the Part for Improved Switching

Performance.

Figure 11.

OUT

Heavy Lines Show the Fast Switching

Current Paths in a Step-down Converter.

The Input Capacitor Should be Placed

Close to the Part for Improved Switching

Performance.

Figure 11.

Applications Information (Continued)

SC4530

BST

BIAS

OUT

VIN

GND

OFF ON

(a)

Reversed Input Protection Schemes

(a) D

4 Blocks the Reverse Current

(b) D

4 Shunts the Reverse Current

from the Part During Power-off.

Figure 10.

(b)

SC4530

BST

BIAS

OUT

VIN

GND

OFF ON

1N4148

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