PM8801 datasheet(26/37 Pages) STMICROELECTRONICS

26 / 37 page

PD interface

PM8801

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DocID018560 Rev 2

4.8

Wakeup from sleep mode

The PM8801 pin WKUP is a logic-level active-low input, internally pulled high. It can be

connected to VSS ground through a switch. The closure of this switch wakes up the device

from sleep mode, performing a soft-start of the DC/DC converter.

If the switch is closed during normal operations, no action is taken by the PM8801.

Other conditions to wake up the device are a voltage drop below UVLO threshold or the

insertion of a wall adapter connected to the SA pin.

Figure 12. Wakeup sequence

Ch1: WKUP, Ch2: VSS-RTN, Ch3: Iinput, Ch4: 5 Vout

4.9

High-voltage startup regulator

The PM8801 embeds a high-voltage startup regulator to provide a controlled reference

voltage of 8.0 V to the internal current mode PWM controller during its startup phase.

The regulator output is connected to the VC pin as well as to the DC/DC section.

In standard isolated topology, the VC pin is diode connected to the auxiliary winding of the

transformer used for the flyback or forward configuration. When the voltage from the

transformer exceeds the regulated voltage, the high-voltage regulator is shut off, reducing

the amount of power dissipated inside the PM8801.

In more detail, when the voltage from the auxiliary winding exceeds 8.0 V, the regulator

resets its intervention threshold to 7 V. In this way, a loosely regulated voltage from the

auxiliary winding is allowed without current-sharing with the internal regulator.

PM8801 Datasheet(PDF) 26 Page - STMicroelectronics