L5991A(1999) Ver la hoja de datos (PDF) - STMicroelectronics

Número de pieza

componentes Descripción

Fabricante

L5991A
(Rev.:1999)

PRIMARY CONTROLLER WITH STANDBY

STMicroelectronics 

L5991 - L5991A

STANDBY FUNCTION

The standby function, optimized for flyback topol-

ogy, automatically detects a light load condition

for the converter and decreases the oscillator fre-

quency on that occurrence. The normal oscillation

frequency is automatically resumed when the out-

put load builds up and exceeds a defined thresh-

old.

This function allows to minimize power losses re-

lated to switching frequency, which represent the

majority of losses in a lightly loaded flyback, with-

out giving up the advantages of a higher switching

frequency at heavy load.

This is accomplished by monitoring the output of

the Error Amplifier (VCOMP) that depends linearly

on the peak primary current, except for an offset.

If the the peak primary current decreases (as a re-

sult of a decrease of the power demanded by the

load) and VCOMP falls below a fixed threshold

(VT1), the oscillator frequency will be set to a

lower value (fSB). When the peak primary current

increases and VCOMP exceeds a second threshold

(VT2) the oscillator frequency is set to the normal

value (fosc). An appropriate hysteresis (VT2-VT1)

prevents undesired frequency change when

power is such that VCOMP moves close to the

threshold. This operation is shown in fig. 20.

Both the normal and the standby frequency are

externally programmable. VT1 and VT2 are inter-

nally fixed but it is possible to adjust the thresh-

olds in terms of input power level.

APPLICATION INFORMATION

Detailed Pin Function Description

Pin 1. SYNC (In/Out Synchronization). This func-

tion allows the IC’s oscillator either to synchronize

other controllers (master) or to be synchronized to

an external frequency (slave).

As a master, the pin delivers positive pulses dur-

ing the falling edge of the oscillator (see pin 2). In

slave operation the circuit is edge triggered. Refer

to fig. 22 to see how it works. When several IC

work in parallel no master-slave designation is

needed because the fastest one becomes auto-

Figure 21. Synchronizing the L5991.

Figure 20. Standby dynamic operation.

Pin

PNO

Normal operation

fos c

fSB

PSB

Stand-by

1

2

VT1

3

VT2 4

VCOMP

matically the master.

During the ramp-up of the oscillator the pin is

pulled low by a 600µA internal sink current gener-

ator. During the falling edge, that is when the

pulse is released, the 600µA pull-down is discon-

nected. The pin becomes a generator whose

source capability is typically 7mA (with a voltage

still higher than 3.5V).

In fig. 21, some practical examples of synchroniz-

ing the L5991 are given.

Since the device automatically diminishes its op-

erating frequency under light load conditions, it is

reasonable to suppose that synchronization will

refer to normal operation and not to standby.

Pin 2. RCT (Oscillator). Two resistors (RA and RB)

and one capacitor (CT), connected as shown in

fig. 22, allow to set separately the operating fre-

quency of the oscillator in normal operation (fosc)

and in standby mode (fSB).

CT is charged from Vref through RA and RB in nor-

mal operation (STANDBY = HIGH), through RA

only in standby ( STANDBY = LOW). See pin 16

description to see how the STANDBY signal is gen-

erated.

When the voltage on CT reaches 3V, the capaci-

tor is quickly internally discharged. As the voltage

has dropped to 1V it starts being charged again.

RB

SYNC

ST-BY

1

16

L5991

VREF

4

2

RA

RCT

CT

(a)

SYNC

1

L5991

2

RCT

ST-BY

L4981A

(MASTER)

16

SYNC

16

L5991

1 (SLAVE) 4 VREF

RB

17 18

2

RA

RCT

ROSC

COSC

CT

(b)

RA

VREF

RCT

RB

4

2 L5991 1

(MASTER)

16

ST-BY

SYNC

CT

SYNC

L4981A

(SLAVE)

16 17 18

ROSC

COSC

D97IN728A

(c)

8/23

Share Link: