SC2440AEVB datasheet(19/28 Pages) SEMTECH

19 / 28 page

Applications Information (Continued)

SC2440A

Figure 8. The Minimum Input Voltage to Start and

to Run Before Dropout. The regulator is

bootstrapped from its output [Figure 7(a)]

with 1N4148. The minimum starting V

decreases when C

SS or dVIN/dt increases.

(a) V

OUT = 5V

(b) V

OUT = 3.3V

(a)

(b)

The minimum C

BST value can be estimated as follows:

(0)

where V

S is the voltage applied to the anode of DBST.

The inductor current charges the bootstrap capacitor

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

If D

BST is connected to the converter input, then CBST will be

charged as soon as V

IN is applied.

If the bootstrap diode is tied to the converter output [Fig-

ures 7(a), 7(c) and 7(g)], then C

BST can only be charged

from the regulator output through the inductor. Before

the converter starts, there is neither output voltage nor

inductor current. Hence it is necessary for the regulator

to deliver some inductor current to the output before C

BST

can be charged. If V

IN is not much higher than the pro-

grammed V

OUT and it ramps up very slowly, then the in-

ductor 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

BST, the converter out-

put needs to be loaded or V

IN needs to be increased. Using

a larger soft-start capacitor C

SS will also help the bootstrap

circuit to run because there will be current in the inductor

over a longer period of time. Figures 8(a) and 8(b) show

the minimum input voltage required to start and 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.

Soft-Start

Each regulating channel of the SC2440A has its own soft-

start circuit. Pulling its soft-start pin below 0.8V with an

open-collector NPN or an open-drain NMOS transistor

turns off the corresponding regulator. The other regula-

tor continues to run. During startup the soft-start capaci-

tors are charged as soon as V

IN exceeds its start threshold

(2.7V). The converter remains off until I

C (see Figure 3)

charges the soft-start capacitor above .3V. One of the

non-inverting inputs of the error amplifier EA is connect-

ed to the output of amplifier A

(Figure 2). The voltage V

at this non-inverting input rises at twice the soft-start ca-

pacitor charging rate.

If the converter is to start into a constant current load I

OUT

by releasing its SS pin with the input power supply al-

ready applied, then the sum of I

OUT and the COUT charging

current will have to be less than the minimum switch cur-

SC2440AEVB Datasheet(PDF) 19 Page - Semtech Corporation

SC2440AEVB Datasheet(HTML) 19 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL