SC4524FEVB datasheet(14/23 Pages) SEMTECH

14 / 23 page

SC4524F

Applications Information (Cont.)

the converter output needs to be loaded or V

IN needs to

be increased. Using larger soft-start capacitor C

SS will also

help in starting bootstrap 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 required to start

bootstrap and to run before dropping out as a function of

the load current. The minimum start-up V

IN decreases with

higher dV

IN/dt or larger soft-start capacitor CSS. The lines

labeled “dropout”in these graphs show that once started,

the bootstrap circuit is able to sustain itself down to zero

load.

Minimum Soft-start Capacitance Css

To ensure normal operation, the minimum soft-start

capacitance C

SS can be calculated in terms of the output

capacitance C

O and output load current IO according to

the following equations.

Substituting the first equation into the second equation,

where V

SS is the soft-start capacitor voltage and ISS is the

soft-start charging current. V

1 is the voltage defined in

Figure 2.

To ensure successful startup, the total current drawn

from the output must be less than the maximum output

capability of the part,

2.3A

≤

Substituting the third equation of this section into the

previous equation,

2.3A

≤

Rearranging,

Therefore the minimum C

SS depends on the output

capacitance and the load current. Larger C

SS is necessary

when starting into a heavy load (small R).

(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

dV =

(

)

[

]

1.2V

−













−

≥

3.5A

SS(MAX)

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