now disconnected and C
T
is charged through R
A
only. In this way the oscillator frequency (f
SB
) will
be lower. Refer to pin 2 description to see how to
calculate the timing components.
Typical values for V
T1
and V
T2
are 2.5 V and 4V
respectively. This 1.5V hysteresis is enough to
prevent undesired frequency change up to a 5.5
to 1 f
osc
/ f
SB
ratio.
The value of V
T1
is such that in a discontinuous
flyback the standby frequency is activated when
the input power is about 13% of the maximum. If
necessary, it is possible to decrease the power
threshold below 13% by adding a DC offset (V
o
)
on the current sense pin (13, ISEN). This will also
allow a frequency change greater than 5.5 to 1.
The following equations, useful for design, apply:
P
inSB
=
1
2
⋅
L
P
⋅
ƒ
osc
⋅
0.367
−
V
o
R
sense
2
(
12
)
,
P
inNO
=
1
2
⋅
L
P
⋅
ƒ
SB
⋅
0.867
−
V
o
R
sense
2
(
13
)
,
ƒ
osc
ƒ
SB
<
0.867
−
V
o
0.367
−
V
o
2
(
14
)
,
where P
inSB
is the input power below which the
L5991 recognizes a light load and switches the
oscillator frequency from
ƒ
osc
to f
SB
, P
inNO
is the
input power above which the L5991 switches
back from
ƒ
SB
to
ƒ
osc
and L
p
the primary induc-
tance of the flyback transformer.
Connect to Vref or leave open this pin when
stand-by function is not used.
Layout hints
Generally speaking a proper circuitboard layout is
vital for correct operation but is not an easy task.
Careful component placing, correct traces routing,
appropriate traces widths and, in case of high
voltages, compliance with isolation distances are
the major issues. The L5991 eases this task by
putting two pins at disposal for separate current
returns of bias (SGND) and switch drive currents
(PGND) The matter is complex and only few im-
portant points will be here reminded.
1) All current returns (signal ground, power
ground, shielding, etc.) should be routed sepa-
rately and should be connected only at a single
ground point.
2) Noise coupling can be reduced by minimizing
the area circumscribed by current loops. This
applies particularly to loops where high pulsed
currents flow.
3) For high current paths, the traces should be
doubled on the other side of the PCB whenever
possible: this will reduce both the resistance
and the inductance of the wiring.
4) Magnetic field radiation (and stray inductance)
can be reduced by keeping all traces carrying
switched currents as short as possible.
5) In general, traces carrying signal currents
should run far from traces carrying pulsed cur-
rents or with quickly swinging voltages. From
this viewpoint, particular care should be taken
of the high impedance points (current sense in-
put, feedback input, ...). It could be a good idea
to route signal traces on one PCB side and
power traces on the other side.
6) Provide adequate filtering of some crucial
points of the circuit, such as voltage references,
IC’s supply pins, etc.
L5991 - L5991A
14/23
