L4992 Просмотр технического описания (PDF) - STMicroelectronics

Номер в каталоге

Компоненты Описание

производитель

L4992

TRIPLE OUTPUT POWER SUPPLY CONTROLLER

STMicroelectronics 

L4992

DETAILED FUNCTIONAL DESCRIPTION (continued)

MOSFET’s (the so-called "logic-level" devices) is highly recommended.

The drivers are of "dynamic" type, which means they do not give origin to current consumption when

they are in static conditions (ON or OFF), but only during transitions. This feature is aimed at minimizing

the power consumption of the device even during stand-by when both low-side MOSFET’s are ON.

Protections

Each converter is fully protected against fault conditions. A monitoring system checks for overvoltages of

the output, quickly disabling both converters in case such an event occurs. This condition is latched and

to allow the device to start again either the supply voltage has to be removed or both RUN3 and RUN5

pins have to be driven low.

Undervoltage conditions are detected as well but do not cause interruption of the operation of the convert-

ers. Only PWROK signal (at pin 10) reveals the anomaly with a low output level.

If the chip overheats (above 135 °C typ.) the device stops operating as long as the temperature falls below a

safe value (105 °C typ.). The overtemperature condition is signalled by a low level on PWROK as well.

A current limitation comparator prevents from excessive current in case of overload or short-circuit. It in-

tervenes as the voltage VRsense exceeds 100 mV, turning off the high-side switch before the error sum-

ming does. By the way, this also gives the designer the ability to program the maximum operating current

by selecting an appropriate sense resistor.

This pulse-by-pulse limitation gives a quasi-constant current characteristic. If a "folded back" charac-

teristic, like the one shown in Fig. 4a, is desired the external circuit of Fig. 4b can be used. The circuits

acts on the current limitation and is extremely simple and cheap. The advantage of such a technique i

that a short circuit will cause a current much lower than the maximum to flow. Th e stress of the power

components will be very little and no overheating will occur. The part values shown in Fig. 4b produce

IFOLD = 1A in the Demo Board (see the relevant section).

Inrush current at start-up is reduced with soft-start. An external capacitor (one for each converter) is

charged by an internal 4µA current generator and its linearly ramping voltage increases the setpoint of

the current limit comparator, starting from zero up to the final value of 100 mV. Thus duty cycle reaches

gradually its steady-state value and dangerous current peaks as well as overshoots of the output voltage

are avoided.

+12 V LINEAR REGULATOR

The +12V Linear regulator is capable of delivering up to 120 mA to an external load through pin REG12.

It is supplied from pin V13IN which accepts voltages included in the range of 13 to 20V.

If the application works with input voltages included between 14 and 20V, the supply for the regulator

can be obtained directly from the input source. If such is not the case, the most convenient way to get

the supply is to use an auxiliary winding on the 3.3 V section inductor with a catch diode, Ds, and a filter

capacitor, Cs, as shown in fig. 5. This winding delivers energy to pin V13IN during the recirculation pe-

riod of each switching cycle with a voltage determined by the turns ratio n and little dependent on the in-

put voltage.

Figure 4.

VIN

VO

VREF-VF-0.1

IFOLD Imax

IO

(a)

32(25)

L

31(26)

30(27)

L4992

12

6(19)

1N4148

6.8KΩ

(3.9KΩ)

5(20)

(b)

Rsense 5V(3.3V)

CO

100Ω

D98IN815

9/26

Share Link: