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NCP1399 Series

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restart in such cases. The controller blanks the SKIP/REM

input information and pulls down the SKIP/REM input for

tREM_TIMER time during controller restart so that the

secondary side bias can be restored and the secondary

off−mode controller can activate the REM optocoupler. This

REM TIMER blank sequence is activated each time the

VCC pin voltage reaches VCC_ON threshold – except in the

situation when after IC left off−mode operation by standard

way and VCC is restored – i.e. when the REM optocoupler

is activated by the secondary off−mode controller.

The SKIP/REM input blanking is activated in following

cases:

• VCC pin voltage reaches VCC_ON threshold during first

start−up phase (i.e. when VCC was below VCC_RESET

threshold before)

• Auto−recovery timer elapsed and new start is initiated

The REM TIMER helps to assure fast application re−start

from fault conditions by forcing controller operation after

tREM_TIMER . However, the secondary controller drives the

remote pin via REM optocoupler during normal operating

conditions in order to switch between ON and OFF

operating modes. The controller is active for very short time

during no−load conditions − just during the time needed to

re−fill the secondary side capacitors to the nominal output

voltage level. In this case we do not use REM TIMER

because it would increase the no−load power consumption

by forcing the application to run for a longer time than

necessary. The REM TIMER blank period is thus not

activated in no−load conditions.

The bias on VCC pin needs to be assured when off−mode

operation takes place. The auxiliary winding is no more able

to provide any bias thus the HV startup current source is

operated in DSS mode – i.e. the VCC pin voltage is cycling

between VCC_ON and VCC_OFF thresholds. This approach

keeps IC biasing in order to memorize the current operation

sate.

Please refer to Figure 64 for an illustration on how the

NCP1399 Active ON off−mode system works under all

operating conditions/modes.

Active OFF Off−mode Control – NCP1399A Device

Family

The NCP1399A device family uses LLC FB pin voltage

information for off−mode operation detection − refer to

Figure 41. The SKIP/REM pin is internally connected to the

skip mode block in this case and serves as a VFB_SKIP_IN

threshold voltage adjust pin. The secondary off−mode

controller reuses the LLC stage regulation optocoupler in

order to reduce total system cost. The off−mode operation is

initiated once the LLC FB pin is pulled down below

VREM_ON threshold and the VCC pin voltage drops below

VCC_OFF threshold in the same time. The optocoupler has to

be active at all time the application is held in off−mode. No

biased is then provided by the secondary off−mode

controller during normal operation – this is why this

approach is called Active OFF off−mode operation. The

application no−load input power consumption is slightly

higher compared to Active ON off−mode solution,

previously described, because the optocoupler needs to be

biased during off mode operation

Figure 41. Active OFF Off−mode Internal Detection Based on the LLC FB Pin Voltage

The controller monitors the LLC FB pin voltage level and

restarts via regular startup sequence (including VCC pin

voltage ramp−up to VCC_ON level and soft−start) once the

FB pin is released by the secondary off−mode controller.

The HV startup current source is working in DSS mode

during application off−mode operation – i.e. the VCC pin

voltage is cycling between VCC_ON and VCC_OFF

thresholds. This approach keeps IC biased so that the actual

operation sate is memorized. The LLC FB pin pull−up

resistor is disconnected when off−mode operation is

activated in order to reduce IC power consumption and also

needed current for optocoupler driving from secondary side.

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