SC4525EEVB datasheet(12/23 Pages) SEMTECH

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SC4525E

Applications Information (Cont.)













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 output capacitor is not a concern because the inductor

current of a buck converter directly feeds C

O, resulting in

very low capacitor 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 SC4525E. 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 SC4525E on the PCB to minimize ringing due to

trace inductance.

Bootstrapping the Power Transistor

To maximize efficiency, the turn-on voltage across the

internal power NPN transistors should be minimized. If

these transistors are to be driven into saturation, then

their bases will have to be driven from a power supply

higher in voltage than V

IN. The required driver supply volt-

age (at least 2.3V higher than the SW voltage) is gener-

ated with a bootstrap circuit (the diode D

and the capaci-

tor C

in Figure 6). The bootstrapped output (the common

node between D

and C) is connected to the BST pin of

the SC4525E.

The minimum BST to SW voltage required to fully satu-

rate the power transistor is shown in Figure 5. The mini-

mum required V

C increases as temperature decreases.

The bootstrap circuit reaches equilibrium when the base

charge drawn from C

during transistor on time is equal to

the charge replenished during the off interval.

Figure 5. Typical Minimum Bootstrap Voltage required

to Saturate the Transistor (I

SW= -3.9A)

Figure 6 summarizes various ways of bootstrapping the

SC4525E. A fast switching PN diode (such as N448 or

N94) and a small (0.33μF – 0.47μF) ceramic capacitor

can be used for D

and C, respectively.

In Figure 6(a) the power switch is bootstrapped from the

output. This is the most efficient configuration and it also

results in the least voltage stress at the BST pin. The maxi-

mum BST pin voltage is about V

IN + VOUT. The minimum

OUT required for this bootstrap configuration is 2.5V. If the

output voltage is between 2.5V and 3V, then use a small

Schottky diode (such as BAt54) for D

to maximize the

bootstrap voltage.

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

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