Datasheet

Operation at no load or very light load L6699
22/38 Doc ID 022835 Rev 2
Equation 6
Note that, unlike the f
max
considered in the Section 6.1: Oscillator , here f
max
is associated to
some load Pout
B
greater than the minimum one. Pout
B
is normally such that the
transformer's peak currents are low enough not to cause audible noise.
The resonant converter's switching frequency, however, depends also on the input voltage;
so, in case there is quite a large input voltage range, with the circuit of
Figure 14 the value of
Pout
B
would change considerably. In this case it is recommended to use the arrangement
shown in
Figure 15 , where the information on the converter's input voltage is added to the
voltage applied to the STBY pin. Due to the strongly non-linear relationship between
switching frequency and input voltage, it is more practical to find empirically the right amount
of correction R
A
/ (R
A
+ R
B
) needed to minimize the change in Pout
B
. Take care in choosing
the total value R
A
+ R
B
much greater than R
L
to minimize the effect on the LINE pin voltage
(see
Section 10: Line sensing function ).
Figure 16. Load-dependent operating modes: timing diagram
Whichever circuit is in use, its operation can be described as follows. As the load falls below
the value Pout
B
the frequency tries to exceed the maximum programmed value f
max
and the
voltage on the STBY pin (
V
STBY
) goes below 1.26 V. The IC then stops with both gate-drive
outputs low, so that both MOSFETs of the half bridge leg are in the OFF-state. The voltage
V
STBY
now increases as a result of the feedback reaction to the energy delivery stop and, as
it exceeds 1.29 V, the IC restarts switching. After a while,
V
STBY
goes down again in
response to the energy burst and stops the IC. In this way the converter works in a burst-
mode fashion with a nearly constant switching frequency. A further load decrease then
increases the time between consecutive bursts and/or reduces the duration of each burst.
This reduces the average switching frequency, which can go down even to few hundred
hertz. The timing diagram of
Figure 16 illustrates this kind of operation, showing the most
significant signals. A small capacitor (typically in the hundred pF) from the STBY pin to
ground, placed as close to the IC as possible to reduce switching noise pick-up, helps obtain
clean operation.
1
f
f
RF
·
8
3
=RF
-
min
max
min
max
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