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參數(shù)資料
| 型號： | L5993D |
| 廠商： | 意法半導(dǎo)體 |
| 英文描述： | CONSTANT POWER CONTROLLER |
| 中文描述： | 恒功率控制器 |
| 文件頁數(shù)： | 8/22頁 |
| 文件大?。?/td> | 200K |
| 代理商： | L5993D |
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 discontinuouscurrent mode flyback they are just
proportional.
In SMPS’ of raster-scanned CRT displays the
switching frequencyis usually synchronized to the
raster line scan signal of the displayin 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 theSMPSwill vary in thatrange.
In case of some failure,the power throughputmay
be excessive without necessarily tripping the
pulse-by-pulsecurrent limitation circuit becauseof
a highoperating 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-
siderableincrease 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
despitethe changesof the switching frequency.
This is accomplished by clamping the output of
the error amplifier (VCOMP) to a value which de-
creases as the frequencyof thesignal fed into pin
1 (SYNC)buildsup.
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 concernwith this technique.
APPLICATION INFORMATION
DetailedPinFunctions Description
Pin 1.
SYNC (In/Out Synchronization). This func-
tion allows the IC’s oscillator either to synchronize
other controllers(master) or to be synchronizedto
an externalfrequency(slave).
As a master, the pin delivers positive pulses dur-
ing the falling edge of the oscillator (see pin 2). In
slave operationthe 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-
maticallythe master.
During the ramp-up of the oscillator the pin is
pulled low by a 600
μ
A internal sink current gener-
ator. During the falling edge, that is when the
pulse is released, the 600
μ
A pull-down is discon-
nected. The pin becomes a generator whose
source capability is typically 7mA (with a voltage
still higher than3.5V).
In fig. 20, some practical examples of synchroniz-
ing the L5993are given.
Pin 2.
RCT (Oscillator). A resistor (R
T
) and a ca-
pacitor (C
T
), connectedas shown in fig.21 set the
operatingfrequencyf
osc
of the oscillator.
C
T
is charged through R
T
untilits 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:
f
osc
1
C
T
(
0.693
R
T
+
K
T
)
(
1
)
where K
T
is definedas:
K
T
=
90, V
15
=
VREF
160 V
15
=
GND/OPEN
(
2
)
and is linked to the duration of the falling edge of
the sawtooth:
T
d
30
10
-9
+ K
T
C
T
(3)
T
d
is also the duration of the sync pulses deliv-
L5993
L5993
R
T
VREF
SYNC
SYNC
RCT
RCT
L4981A
(MASTER)
L5993
(SLAVE)
R
T
VREF
SYNC
RCT
R
OSC
C
OSC
C
T
L5993
(MASTER)
L4981A
(SLAVE)
SYNC
R
OSC
C
T
C
OSC
SYNC
(a)
(b)
(c)
R
T
D97IN766B
C
T
VREF
4
1
2
1
2
16
18
17
4
2
1
RCT
1
2
4
16
17
18
Figure 20. Sinchronizingthe L5993.
L5993
8/22
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