L5993D datasheet(8/22 Pages) STMICROELECTRONICS

Part #	L5993D

Description	CONSTANT POWER CONTROLLER

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Manufacturer	STMICROELECTRONICS [STMicroelectronics]

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CONSTANT POWER FUNCTION

Pulse-by-pulse current limitation prevents peak

primary current from exceeding a given level.

This, in turn, limits the maximum power deliver-

able to the output or, in other words, the power

capability of a converter. The capability, however,

depends on switching frequency: for example, in

a discontinuous current mode flyback they are just

proportional.

In SMPS’ of raster-scanned CRT displays the

switching frequency is usually synchronized to the

raster line scan signal of the display in order to in-

crease noise immunity. More and more often,

CRT displays are required to operate within a

range of different video frequencies (e.g. from 31

kHz to 64 kHz), thus also the switching frequency

of the SMPS will vary in that range.

In case of some failure, the power throughput may

be excessive without necessarily tripping the

pulse-by-pulse current limitation circuit because of

a high operating frequency.

For the sake of safety, it would be then desirable

to design the power stage of a converter (power

MOSFET, transformer, catch diode) so as to be

able to withstand the maximum power throughput

under failure conditions. However, this is a con-

siderable increase of size and cost.

The ”Constant Power” function of the L5993 al-

lows to overcome this problem. The device

changes the threshold of its pulse-by-pulse cur-

rent limitation circuit so as to maintain fairly con-

stant the power capability of a flyback converter

despite the changes of the switching frequency.

This is accomplished by clamping the output of

the error amplifier (VCOMP) to a value which de-

creases as the frequency of the signal fed into pin

1 (SYNC) builds up.

The frequency-to-voltage conversion needed to

achieve this functionality is performed by detect-

ing the peak voltage of the (synchronized) oscilla-

tor with a peak-holding circuit. One external ca-

pacitor only is required.

It is important to point out that shape, amplitude

and duration of the synchronization pulses are of

no concern with this technique.

APPLICATION INFORMATION

Detailed Pin Functions 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. 21 to see how it works. When several IC

work in parallel no master-slave designation is

needed because the fastest one becomes auto-

matically the master.

During the ramp-up of the oscillator the pin is

pulled low by a 600

µA internal sink current gener-

ator. During the falling edge, that is when the

pulse is released, the 600

µA 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. 20, some practical examples of synchroniz-

ing the L5993 are given.

Pin 2. RCT (Oscillator). A resistor (RT) and a ca-

pacitor (CT), connected as shown in fig. 21 set the

operating frequency fosc of the oscillator.

CT is charged through RT until its voltage reaches

3V, then is quickly internally discharged. As the

voltage has dropped to 1V it starts being charged

again.

The frequency can be established with the aid of

fig. 13 diagrams or considering the approximate

relationship:

fosc

≅

1

CT

⋅ (0.693

⋅ RT + KT)

(1)

where KT is defined as:

KT

= 



90, V15 = VREF

160 V15 = GND/OPEN

(2)

and is linked to the duration of the falling edge of

the sawtooth:

Td

≅ 30 ⋅ 10-9 +KT ⋅ CT (3)

Td is also the duration of the sync pulses deliv-

L5993

RT

VREF

SYNC

RCT

L4981A

(MASTER)

L5993

(SLAVE)

RT

VREF

SYNC

RCT

ROSC

COSC

CT

L5993

(MASTER)

L4981A

(SLAVE)

SYNC

ROSC

CT

COSC

SYNC

(a)

(b)

(c)

RT

D97IN766B

CT

VREF

4

1

2

1

2

16

18

17

4

2

1

RCT

1

2

4

16

17

18

Figure 20. Sinchronizing the L5993.

L5993

8/22

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