AN2521

Functional check

19/33

exceeds a preset value, usually larger than the maximum Vo that can be expected, including

also overshoots due to worst-case load/line transients.

In this case, VO = 400 V, Vox = 460 V. Select: R3 + R5 + R11 = 6.6 M

Ω. Three resistors in

series have been chosen according to their voltage rating.

Thus: R19 = 6.6 M

Ω · 2.5 / (460-2.5) = 36 kΩ.

When this function is triggered, the gate drive activity is immediately stopped, the device is

shut down, its quiescent consumption is reduced below 250 µA and the condition is latched

as long as the supply voltage of the IC is above the UVLO threshold. At the same time the

pin PWM_LATCH (pin #8) is asserted high. The PWM_LATCH is an open source output

capable of delivering 3.7 V minimum with a 0.5 mA load, intended for tripping a latched

shutdown function of the PWM controller IC in the cascaded dc-dc converter, so that the

entire unit is latched off. To restart the system it is necessary to recycle the input power, so

UVLO thresholds.

The PFC_OK pin doubles its function as a not-latched IC disable: a voltage below 0.2 V will

shut down the IC, reducing its consumption below 1 mA. In this case both PWM_STOP and

PWM_LATCH keep their high impedance status. To restart the IC simply let the voltage at

the pin go above 0.26 V.

Note that this function offers complete protection against not only feedback loop failures or

erroneous settings, but also against a failure of the protection itself. If a resistor in the

PFC_OK divider fails short or open, or the PFC_OK (#7) pin is floating, it will result in the

shutting down of the L6563 and stopping of controller operation of the flyback stage.

Figure 26.

EVAL6668-75W Open loop at 115 Vac-60 Hz - full load

An open loop event is captured in Figure 26. Note the protection intervention stopping the

operation of the L6563 and the activation of the PWM_LATCH pin that is connected to the

L6668 pin #7 (DIS). This function of the L6668 is a latched device shutdown. Internally the

pin connects a comparator which shuts the IC down and brings its consumption to a value

just higher than before start-up, when the voltage on the pin exceeds 2.2 V. The information

is latched and it is necessary to recycle the input power to restart the IC. The latch is