PD interface

PM8803

22/34

Doc ID 018559 Rev 2

This feature is active only when working from an input voltage with a "frontal" connection,

that must use the hot-swap MOSFET; this voltage could be from the PoE interface or from

an external auxiliary adapter connected before the internal hot-swap MOSFET.

If the auxiliary source is connected after the hot-swap MOSFET, it will be opened and this

feature is disabled, allowing the converter to work with a low-voltage auxiliary source. The

PGOOD comparator includes hysteresis to allow the PM8803 to operate near the current

limit point without inadvertently disabling it. The MOSFET voltage must increase to 12 V

before PGOOD is deasserted.

This feature will also allow withstanding positive line transients of up to 12 V without

stopping DC/DC normal operations as shown in

Figure 9. The line transient is managed by

the PWM section, adjusting its operating parameters accordingly without shutting down the

output voltage. The input current during the transient is controlled by the hot-swap MOSFET

at the DC current limit.

After PGOOD assertion, a comparator on the gate of the hot-swap MOSFET controls the

transition between the 140 mA to the programmed DC current limit, with a 2 V threshold.

The comparator is needed to ensure that the charge of the DC/DC input capacitor is

completed, avoiding current spikes on the last portion of the charge.

The PM8803 provides a default continuous current limitation of 640 mA. This is achieved by

leaving the pin DCCL floating. A different DC current limit can be set by connecting a

equation:

Figure 10.

R

[]

22400

I

[]

-----------------------

=

20

120

220

320

420

520

620

720

820

920

1020

1120

20

40

60

80100

120

140

160

AM045100v1