SC4525C datasheet(13/22 Pages) SEMTECH

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SC4525C

Applications Information (Cont.)

SC4525C

BST

GND

VIN

(a)

3V < VO < 8V

(b)

SC4525C

BST

GND

VIN

VO < 2.5V

(C)

SC4525C

BST

GND

VIN

2.5V < VO < 3V

or VO > 8V

Figures 6 — Methods of Bootstrapping the SC4525C

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

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,

Substituting the third equation of this section into the

previous equation,

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). If V

IN rise time

exceeds 00ms, then a larger C

SS may be necessary

to produce a monotonic rising output. Always verify

converter start-up on the bench.

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