Brownout protection

DAP005

30/39

7

Brownout protection

As already seen, the DAP005 is provided with an ON/OFF control pin (AC_OK, 10)

internally connected to the inverting input of a comparator. The non-inverting input is

internally referenced to 0.52V, so that the IC is disabled if the voltage applied at the AC_OK

pin is below the internal reference. In this case, the voltage on the PWM_STOP pin is

asserted low and the consumption of the IC is reduced below 1 mA as well. For good noise

immunity, hysteresis is also provided, so that the IC is re-enabled as the voltage on the pin

exceeds 0.6V. This function, in systems where the PFC pre-regulator acts as the master

stage, can be used to implement what is commonly called “brownout protection”.

Brownout Protection is basically a not-latched device shutdown function that must be

activated when a condition of mains undervoltage is detected. This condition may cause

overheating of the primary power section due to an excess of RMS current. Brownout can

also cause the PFC pre-regulator to work open loop and this could be dangerous to the PFC

stage itself and the downstream converter, should the input voltage return abruptly to its

rated value. Another problem is the spurious restarts that may occur during converter power

down and that cause the output voltage of the converter not to decay to zero monotonically.

For these reasons it is usually preferable to shutdown the unit in case of brownout.

IC shutdown upon brownout can be easily realized as shown in Figure 46. The scheme on

the left is of general use, the one on the right can be used if the bias levels of the multiplier

and the RFF·CFF time constant are compatible with the specified brownout level and with the

specified holdup time respectively.

It is worth noticing one point: this brownout protection works correctly only in systems where

the PFC stage is the master and the cascaded DC-DC converter is the slave or, in other

words, where the PFC stage starts first and enables/disables the operation of the DC-DC

stage. One reason is that in case of brownout the whole converter must be switched off, not

operate as long as brownout lasts and restart as brownout disappears.

This kind of operation cannot be achieved if the PFC stage is slave to the DC-DC converter

(that is, the DC-DC converter starts first and determines start-up and shutdown of the PFC

stage) because the DAP005 could detect brownout only after it has started, that is after the

DC-DC converter has started. This would result in an intermittent operation, not in a

complete shutdown.

The second reason is that in case of a master DC-DC stage a turn-off signal coming from

the slave PFC may interfere during converter power-off and give origin to a not well-defined

power-off sequence or even to spurious restarts. Therefore, while the AC_OK pin can be

used to switch off the PFC stage only, if acceptable, the use of the PWM_STOP pin is not

recommended in systems with a master DC-DC stage.

This function is quite flexible and can be used for different purposes. In systems comprising

an auxiliary converter and a main converter (e.g. desktop PC’s silver box or hi-end LCD-TV),

where the auxiliary converter also powers the controllers of the main converter, the pin

AC_OK can be used to start and stop the main converter. In the simplest case, to

enable/disable the PWM controller the PWM_STOP pin can be connected to either the

output of the error amplifier (Figure 44 a) or, if the chip is provided with it, to its soft-start pin

(Figure 44 b). The use of the soft-start pin allows the designer to delay the start-up of the

DC-DC stage with respect to that of the PFC stage, which is often desired. An underlying

assumption in order for that to work properly is that the UVLO thresholds of the PWM

controller are certainly higher than those of the DAP005.