8.5.4

Special modes for voltage transitions

Protocols like USB-PD require to change the output voltage while the converter is operating following specific

timing requirements. ST-ONEMP provides special operating modes to Manage negative transition while the

applied load to the converter is very low.

8.5.4.1

Bleeding

The device integrates bleeding current sinks to discharge the output during negative voltage transitions which

may happen at light load.

Bleeding current IBLEED is initially set to 10 mA, then increased to 20 mA.

8.5.4.2

Forced burst

In order to maintain the primary side PVCC powered, the IC forces bursts at regular intervals to recharge the

primary side through its auxiliary winding.

8.5.4.3

Active bleeding mode

The IC can be configured to operate in a reverse flyback mode transferring energy from the output capacitor to

the input bulk capacitor by switching the SR MOSFET.

This function is useful when the 20 mA bleeding current is not enough to meet the negative transition timing

specifications, for example in applications with a big SVCC capacitor.

8.5.5

CCM mode

USB-PD converters allow a wide range of output voltage variations. When the output voltage is low, it is desirable

to provide a higher current than what is allowed at maximum output voltage. In normal operating mode the peak

current in the transformer is roughly proportional to the output current and loosely related to the voltage. For this

reason, an active clamp flyback transformer designed for 45 W at 15 V would require a much higher saturation

current to be able to provide the same power at 10 V or 5 V. To help overcome this issue, the device can operate

in CCM mode, extending the output current that can be provided for the same transformer saturation level.

While in normal mode, the converter can limit the duration of the secondary side conduction (phase 4 in Figure 8)

to a programmable time, forcing the system to enter CCM mode. The CCM is enabled only if two conditions are

met:

•

The output voltage is below a programmable threshold

•

The peak current requested by the control loop is higher than a programmable threshold

The duration of the off-time in CCM mode is equal to the time programmed in the specific parameter, multiplied by

5V/VoutwhereVoutistheprogrammedoutputvoltage.

8.6

Control loop

The controller includes both a voltage regulation loop, which operates normally, and a current regulation loop

which is used in constant current mode, which is used to manage the CC/CV mode as required by USB-PD PPS

mode.

8.6.1

Voltage sensing

The IC senses the output voltage through a programmable resistor divider connected to the internal 12 bit ADC.

The resistor divider rescales the SVCC voltage so that the full scale can be configured as 6 V, 12 V or 24 V. The

IC selects the scale depending on the target output voltage.

The thresholds for switching scales are 5.8 V and 11.8 V.

During output voltage transitions, the greater scale between the initial and final voltages is used. At the end of the

transition the scale is changed according to the final voltage.

For example in a transition from 5 V to 9 V the scale is initially switched to 12 V, at the end of the transition it is

kept at 12 V scale. For a transition 9 V to 5 V, the scale is kept at 12 V and at the end of transition is changed to 6

V scale.

8.6.2

Peak current mode unit

The primary side low-side switch is controlled in peak current mode.

ST-ONEMP

Control loop

DS13989 - Rev 2

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