L4990D datasheet(13/24 Pages) STMICROELECTRONICS

13 / 24 page

V13pk = IQpk⋅ Rsense =

(VCOMP − 1.4)

(8)

the conduction of the switch is terminated.

To increase the noise immunity, a ”Leading Edge

Blanking” of about 100ns is internally realized as

shown in fig. 27. Because of that, the smoothing

RC filter between this pin and Rsense could be re-

moved or, at least, considerably reduced.

D97IN503

ISEN

CLK

1.2V

FROM E/A

OVERCURRENT

COMPARATOR

PWM

COMPARATOR

TO PWM

LOGIC

TO FAULT

LOGIC

Figure 27. Internal LEB

Pin 14. DIS (Device Disable). When the voltage

on pin 14 rises above 2.5V the IC is shut down

and it is necessary to pull VCC (IC supply volt-

age, pin 8) below the UVLO threshold to allow the

device to restart. When disabled, the current con-

sumption of the IC is as low as before start-up.

The pin can be driven by an external logic signal

in case of power management, as shown in fig.

28. It is also possible to realize an overvoltage

protection, as shown in the section ” Application

Ideas”.

If used, bypass this pin to ground with a filter ca-

pacitor to avoid spurious activation due to noise

spikes. If not, it is advisable to connect the pin to

SGND, even though it might be left floating.

Pin 15. DC-LIM (Maximum Duty Cycle Limit). The

upper extreme, Dx, of the duty cycle range de-

pends on the voltage applied to this pin. Approxi-

mately,

≅

+ 230

)

if DC-LIM is grounded or left floating. Instead,

connecting DC-LIM to VREF (half duty cycle op-

tion), Dx will be set approximately to:

≅

⋅ RT + 260

(10)

and the output switching frequency will be halved

with respect to the oscillator one because an in-

ternal T flip-flop (see block diagram, fig. 1) is acti-

vated. Fig. 29 shows the operation.

The half duty cycle option speeds up the dis-

charge of the timing capacitor CT (in order to get

duty cycles as close as possible to 50%) so the

oscillator frequency - with the same RT and CT -

will be slightly higher.

The halving of frequency can be used to reduce

losses at light load in all those systems that must

comply with requirements regarding energy con-

sumption (e.g. monitor displays).

D97IN502

DIS

DISABLE

UVLO

2.5V

DISABLE

SIGNAL

Figure 28. Disable (Latched)

L4990 - L4990A

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