SC1933C datasheet(5/28 Pages) POWERINT

5 / 28 page

Rev. C 11/18

SC1933C/SC1936C

www.power.com

Figure 8. Primary-Secondary Handshake Flowchart.

The second is included to ensure that if communication is lost, the

primary tries to restart. Although this should never be the case in

normal operation, it can be useful when system ESD events (for

example) causes a loss of communication due to noise disturbing the

secondary controller. The issue is resolved when the primary restarts

after an auto-restart off-time.

The first auto-restart off-time is short (t

AR(OFF)SH). This short auto-

restart time is to provide quick recovery under fast reset conditions.

The short auto-restart off-time allows the controller to quickly check to

determine whether the auto-restart condition is maintained beyond

AR(OFF)SH. If so, it will resort to a full auto-restart off-time.

The auto-restart is reset as soon as an AC reset occurs.

SOA Protection

In the event that there are two consecutive cycles where the drain

current is reached 110% of I

LIM within ~500 ns, the controller will skip

2.5 cycles or ~25 ms (based on full frequency of 100 kHz). This

provides sufficient time for the transformer to reset with large

capacitive loads without extending the start-up time.

Input Line Voltage Monitoring

The UNDER/OVER INPUT VOLTAGE pin is used for input undervoltage

and overvoltage sensing and protection.

A 4 MΩ resistor is tied between the high-voltage DC bulk capacitor

after the bridge (or to the AC side of the bridge rectifier for fast AC

reset) and the UNDER/OVER INPUT VOLTAGE pin to enable this

functionality. This function can be disabled by shorting the UNDER/

OVER INPUT VOLTAGE pin to SOURCE pin.

At power-up, after the primary bypass capacitor is charged and the

ILIM state is latched, and prior to switching, the state of the UNDER/

OVER INPUT VOLTAGE pin is checked to confirm that it is above the

brown-in and below the overvoltage shutdown thresholds.

In normal operation, if the UNDER/OVER INPUT VOLTAGE pin current

falls below the brown-out threshold and remains below brown-in for

longer than t

UV-, the controller enters auto-restart. Switching will only

resume once the UNDER/OVER INPUT VOLTAGE pin current is above

the brown-in threshold.

In the event that the UNDER/OVER INPUT VOLTAGE pin current is

above the overvoltage threshold, the controller will also enter

auto-restart. Again, switching will only resume once the UNDER/

OVER INPUT VOLTAGE pin current has returned to within its normal

operating range.

The input line UV/OV function makes use of an internal high-voltage

switch on the UNDER/OVER INPUT VOLTAGE pin to reduce power

consumption. If the cycle off-time t

OFF is greater than 50 ms, the

internal high-voltage switch will disconnect the external 4 MΩ resistor

from the internal IC to eliminate current drawn through the

4 MΩ resistor. The line sensing function will activate again at the

beginning of the next switching cycle.

Primary-Secondary Handshake

At start-up, the primary-side initially switches without any feedback

information (this is very similar to the operation of a standard

TOPSwitch™, TinySwitch™ or LinkSwitch™ controllers).

If no feedback signals are received during the auto-restart on-time

AR), the primary goes into auto-restart mode. Under normal

conditions, the secondary controller will power-up via the FORWARD

pin or from the OUTPUT VOLTAGE pin and take over control. From

this point onwards the secondary controls switching.

If the primary controller stops switching or does not respond to cycle

requests from the secondary during normal operation (when the

secondary has control), the handshake protocol is initiated to ensure

that the secondary is ready to assume control once the primary

begins to switch again. An additional handshake is also triggered if

the secondary detects that the primary is providing more cycles than

were requested.

PI-8876-110818

Yes

tAR(OFF)

tAR

Yes

S: Has powered

up within tAR

P: Has Received

Handshaking

Pulses

S: Has Taken

Control?

P: Stops Switching, Hands

Over Control to Secondary

P: Not Switching

S: Doesn’t Take Control

P: Continuous Switching

S: Doesn’t Take Control

P: Goes to Auto-Restart Off

S: Bypass Discharging

P: Auto-Restart

S: Powering Up

P: Primary Chip

S: Secondary Chip

End of Handshaking,

Secondary Control Mode

Start

P: Powered Up, Switching

S: Powering Up

P: Switching

S: Sends Handshaking Pulses

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