SC4524EEVB datasheet(14/23 Pages) SEMTECH

14 / 23 page

SC4524E

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 Ze-

ner voltage V

The SC4524E can also be bootstrapped from the input

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

ure 6(a). However this may be the only option if the out-

put 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 volt-

age 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 SC4524E

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 [Fig-

ures 6(a) and 6(c)], then C

can only be charged from the

regulator output through the inductor. Before the con-

verter starts, there is no output voltage or inductor cur-

rent. Hence it is necessary for the regulator to deliver some

inductor current to the output before C

can be charged.

If V

IN is not much higher than the programmed VOUT 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

, the converter output needs to be loaded or VIN

needs to be increased. Using larger soft-start capacitor C

will also help the bootstrap circuit to run because there

will be current in the inductor over a longer period of time.

Figures 7(a) and 7(b) show the minimum input voltage re-

quired to start bootstrap and to run before dropping out

as a function of the load current. The minimum start-up V

decreases with higher dV

IN/dt or larger soft-start capacitor

SS. The lines labeled “dropout” in these graphs show that

once started, the bootstrap circuit 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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