SC4524FSETRT datasheet(12/23 Pages) SEMTECH

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SC4524F

Applications Information (Cont.)

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 SC4524F 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

1 and the capaci-

tor C

1 in Figure 6). The bootstrapped output (the common

node between D

1 and C1) is connected to the BST pin of

the SC4524F.

The minimum BST to SW voltage required to fully satu-

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

mum required V

C1 increases as temperature decreases.

The bootstrap circuit reaches equilibrium when the base

charge drawn from C

1 during transistor on time is equal to

the charge replenished during the off interval.

Figure 5 — Typical Minimum Bootstrap Voltage

required to Saturate Transistor (I

SW= -2.6A).

Figure 6 summarizes various ways of bootstrapping the

SC4524F. A fast switching PN diode (such as 1N4148 or

1N914) and a small (0.33μF – 0.47μF) ceramic capacitor

can be used for D

1 and C1, 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

1 to maximize the

bootstrap voltage.

The SC4524F can also be bootstrapped from the input

[Figure 6(b)]. This configuration is not as efficient as Figure

6(a). However this may be the only option if the output

voltage is less than 2.5V and there is no other supply with

voltage higher than 2.5V. Voltage stress at the BST pin can

be somewhat higher than 2V

IN.

Figures 6(c) and (d) show how to bootstrap the SC4524F

from a second independent power supply V

The minimum bootstrap capacitance C

1 can be estimated

as:

where V

S is the voltage applied to the anode of D1.

The inductor current charges the bootstrap capacitor

when it pulls the SW node low during the switch off time.

If D

1 is connected to the converter input, then C1 will be

charged as soon as V

IN is applied.

If the bootstrap diode is tied to the converter output [Fig

ures 6(a)], then C

1 can only be charged from the regulator

output through the inductor. Before the converter starts,

there is no output voltage or inductor current. Hence it

is necessary for the regulator to deliver some inductor

current to the output before C

1 can be charged. If VIN is

not much higher than the programmed V

OUT and it ramps

up very slowly, then the inductor current will not be high

enough for the bootstrap circuit to run, especially at light

loads. In order to have some inductor current to charge C

Minimum Bootstrap Voltage

vs Temperature

1.6

1.7

1.8

1.9

2.0

2.1

2.2

-50

-25

100 125

Temperature (o C)

ISW = -2.6A

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