5.4.3

Fixed frequency DCM operation

In fixed frequency circuits designed to operate always in DCM in all line and load conditions, the SRK1001

controller operates in a similar way to QR operation: the turn-off is accomplished by adaptive mechanism in

steady state operation, while timer turn-off is invoked to protect against current inversion during load transitions

and in CC regulation. In this case, the timer mode can be either the one for FF CCM or the one for QR

application, which is the cheaper option as no capacitor on TON pin is required.

5.4.4

Variable frequency CCM operation

Some advanced flyback controllers provide multimode operation in order to maximize performance and optimize

transformer design. Some of these controllers also provide variable frequency CCM operation to facilitate

overload management, while helping to reduce the transformer size (on overpower request, the operating

frequency is increased). The SRK1001 controller is suitable for operation in these kinds of applications, allowing

SR MOSFET turn-off through a synchronization signal (provided from primary side through a pulse transformer)

applied to the SYNC pin. Refer to Section 5.7 Low consumption mode operation: sleep-mode, burst mode,

disable state for a detailed operation description.

5.5

Minimum TOFF programming

In flyback applications operating in DCM and QR with valley skipping, resonance occurs across transformer

windings after demagnetization, whose period TRES depends on the transformer primary inductance and on the

total capacitance across primary switch. In order to prevent this ringing from affecting the SRK1001 internal

timings, a blanking time after turn-off needs to be programmed. The circuit is based on an internal timing

capacitance and an external resistor RTOFF connected from TOFF pin to ground and allows fixing a TOFF_MIN time

according to the following expression:

TOFF_MIN=30*10−12*RTOFF

RTOFF is expressed in Ohms and TOFF_MIN in seconds. The TOFF_MIN time must be set slightly longer than the

ringing period TRES: in this way, referring to Figure 9, the circuit provides a blanking time from the falling edge of

the driving signal to the time instant occurring after the DVS pin voltage is permanently higher than VTH_A for

TOFF_MIN.

Furthermore, referring to Figure 10, an internal comparator referenced to a voltage VR higher than 2 times VOUT

senses the DVS pin voltage (where VR and VDS are conveniently scaled). When VDS voltage rises above VR, the

comparator triggers and the blanking time is terminated. This helps during constant voltage regulation operation,

at high input voltage levels (where typically the conduction time of the primary MOSFET is short), preventing the

blanking time determined by TOFF_min from delaying the SR MOSFET turn-on. The internal threshold VR is fixed

to 2.83 VCC (where VCC equals VOUT or VOUT-VF, depending on whether VAUX functionality is used or not; see

Section 5.9 VAUX pin operation in CC regulation).

Figure 9. Blanking time after turn-off

TB_OFF

ISR

IP

VDS

TRES

TOFF_min

GD

blanking time

after turn-off

VTH_A

VZCD_OFF

adapted

ZCD_OFF

threshold

SRK1001

Minimum TOFF programming

DS13155 - Rev 1

page 13/27