SC4525EEVB datasheet(14/23 Pages) SEMTECH

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SC4525E

Applications Information (Cont.)

If V

IN(MAX) + VOUT > 42V, then a Zener diode D3 can be used

in series with D

to lower the BST voltage [Figure 6(c)]. The

following inequality gives a suitable range for the Zener

voltage V

The SC4525E 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. The BST pin voltage should

not exceed its absolute maximum rating of 42V.

To reduce BST voltage stress when stepping down from

high V

IN (>20V) to low VOUT (<2.5V), a Zener diode can be

added in series with D

. This is shown in Figure 6(d). The

Zener voltage can be selected as follows:

Figures 6(e) and (f) show how to bootstrap the SC4525E

from a second independent power supply V

The minimum bootstrap capacitance C

can be estimated

as:

where V

S is the voltage applied to the anode of D.

The inductor current charges the bootstrap capacitor

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

If D

is connected to the converter input, then C will be

charged as soon as V

IN is applied.

If the bootstrap diode is tied to the converter output

[Figures 6(a) and 6(c)], then C

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 deliv-

er some inductor current to the output before C

can be

charged. If V

IN is not much higher than the programmed

OUT and it ramps up very slowly, then the inductor cur-

rent will not be high enough for the bootstrap circuit to

run, especially at light loads. In order to have some induc-

tor current to charge C

, the converter output needs to be

loaded or V

IN needs to be increased. Using larger soft-start

capacitor C

SS will also help the bootstrap circuit to run be-

cause there will be current in the inductor over a longer

period of time. Figures 7(a) and 7(b) show the minimum

input voltage required to start bootstrap and to run be-

fore dropping out as a function of the load current. The

minimum start-up V

IN decreases with higher dVIN/dt or

larger soft-start capacitor C

SS. The lines labeled “dropout”

in these graphs show that once started, the bootstrap cir-

cuit is able to sustain itself down to zero load.

(a)

(b)

Figure 7. The Minimum Input Voltage to Start and

to Run Before Dropout. The Regulator is

Bootstrapped from its Output [Figure 6(a)].

1 is 1N4148. (a) VOUT = 5V (b) VOUT = 3.3V

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