AN2067 Datasheet, PDF (5/25 Pages) STMicroelectronics – LED manufacturers propose white LEDs inside a monolithic chip

AN2067
Block diagram
C_OVL > 12.5 × 10^–6 ⋅ T_SS
C_VDD
>
8
×
10⁴
⋅


-_D----_R-¹---_S---_T-
–
1_
⋅
-^C----^O---_V-^V---^LD----_D^â‹---^I_h--^D-_y--^D-_s--_t-^c---^h--²-
C_VDD
>


I_{--VD----D-D--1-D---}⋅_-h--yt_{--S-s--t-S-}
Where t_ss and D_RST are respectively the time needed for the output voltages to pass from 0
V to their nominal values at startup, and the restart duty cycle in overload or short-circuit
condition. A typical value of 10% is generally set for this last parameter, as it insures that the
output diodes and the transformer don't overheat. The other parameters can be found in the
datasheet of the device. As the VDD capacitor has to respect two conditions, the maximum
value defines its value.
1.2
Standby operation
On the opposite load configuration, the converter is lightly loaded and the COMP voltage
decreases until the shutdown threshold is reached at typically 0.5 V. At this point, the
switching is disabled and energy doesn’t pass to secondary side. So, the output voltage
decreases and the regulation loop rises again above the shutdown threshold, thus the
normal switching operation is resumed. A burst mode with pulse skipping takes place, as
long as the output power is below the one corresponding to the minimum turn-on of the
device. As the COMP voltage works at 0.5 V, the peak drain current is very low (it is defined
by the minimum turn-on time of the device, and by the primary winding of the transformer)
and no audible noise is generated.
In addition, the minimum turn-on time depends on the COMP voltage. Below 1 V (V_COMPbl),
the blanking time increases till 400 ns, whereas it is 150 ns for higher voltages. The
minimum turn-on time, resulting from these values, is respectively 600 ns and 350 ns, when
the internal propagation time is taken into account. This feature brings the following benefits:
– This change induces the hysteresis between the normal operation and burst mode
which is reached when the output power decreases.
– A short value in normal operation assures a good drain current control in case of
short-circuit on secondary side.
– A long value in standby operation reinforces the burst mode by skipping the
switching cycles, thus decreasing the switching losses.
Further details regarding the standby operation can be found in the datasheet.
DocID10973 Rev 2
5/25
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