QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1335A-B
HIGH POWER, HIGH EFFICIENCY POE PD INTERFACE WITH INTEGRATED SWITCHING
REGULATOR
2
LTC4269IDKD-1 switches the port voltage over to the
power supply controller through its internal MOSFET
which lies between the V
PORTN
and V
NEG
pins. This volt-
age charges C18/19 through a trickle charge resistor,
R9 to power the bias pin, V
CC
, of the power supply
controller. Once the bias power gets to its V
CC(ON)
threshold, the IC begins a controlled soft-start of the
output. As the output voltage rises, bias power is
taken over by the bias supply made up of T1’s bias
winding and D11.
When the soft-start period is over, the output voltage
is regulated by observing the pulses across the bias
winding during the flyback time. The Primary Gate
drive (PG) and Synchronous Gate (SG) drive is then
Pulse Width Modulated (PWM) in order to keep the
output voltage constant. The synchronous gate drive
signal is transmitted to the secondary via the small
signal transformer, T2. The output of T2 then drives
a discrete gate drive buffer, R22 and Q6/7 in order to
achieve fast gate transition times, hence a higher effi-
ciency.
The two-stage input filter, C5, L2, and C6 and output
filter, C1/3, L1, and C10 are the reasons that this PoE
flyback supply has exceptionally low differential mode
conducted emissions.
QUICK START PROCEDURE
Demonstration circuit 1335A-B is easy to set up to
evaluate the performance of the LTC4269IDKD-1 in a
PoE+ PD application. Refer to Figure 1 for proper
equipment setup and follow the procedure below:
NOTE:
When measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the output (or
input) voltage ripple by touching the probe tip and
probe ground directly across the +VOUT and –VOUT
(or VPORT_P and VPORT_N) terminals. See Figure 2
for proper scope probe technique.
1.
Place test equipment (voltmeter, ammeter, and
electronic load) across output.
2.
Input supplies:
a.
Connect a PoE+ capable PSE with a CAT-5 cable
to the RJ45 connector, J1. See Figure 1.
b.
Or, connect a 37V to 57V capable power supply
(‘‘Power Supply’’ in Figure 1) across VPORT_P and
VPORT_N.
c.
If evaluating the auxiliary power supply (‘‘Auxil-
iary Supply’’ in Figure 1) capability, connect a 44V
to 57V capable power supply across AUX+ to AUX-.
3.
Check for the proper output voltage of 5V.
4.
Once the proper output voltage is confirmed, adjust
the load within the operating range and observe the
output voltage regulation, ripple voltage, efficiency
and other parameters.
