SC4525FEVB datasheet(14/23 Pages) SEMTECH

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SC4525F

Applications Information (Cont.)

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.

dV =

(

)

[

]

1.2V

−

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,

3.5A

≤

Substituting the third equation of this section into the

previous equation,

3.5A

≤

Rearranging,













−

≥

3.5A

SS(MAX)

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 the regulator is to be started by turning on a bench

power supply, then C

SS will be best determined empirical-

ly because the rise time of a power supply can range from

a few milliseconds to a few hundred milliseconds. With

the maximum load applied, the output rise is observed

using a 22nF for C

SS. Adjust CSS until a linear VOUT ramp is

achieved.

Loop Compensation

The goal of compensation is to shape the frequency

response of the converter so as to achieve high DC

accuracy and fast transient response while maintaining

loop stability.

(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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