QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 959
HIGH-POWER MULTI-OUTPUT SUPPLY WITH POE CONTROLLER
LTC4257CDD-1/LTC3825EFE
DESCRIPTION
Demonstration circuit 959 is a high-power triple out-
put (37.4W) supply featuring the LTC3825 with the
LTC4257-1. This board acts as a pre-standard high
power Power-over-Ethernet (PoE) Powered Device
(PD) and connects at the RJ45 to a compatible high
power Power Sourcing Equipment (PSE) device. The
LTC4257-1 provides IEEE802.3af standard PoE PD
interfacing. When the PD is fully powered, the
LTC4257-1 switches power over to the LTC3825. The
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER
Input Voltage
CONDITION
In from PSE or auxiliary
VALUE
37V to 57V
5V ±2.5% @ 3A
Output Voltage V
OUT
V
IN
= 37V to 57 from PSE
11.8V ±2% @ 0.5A
3.3V ±3% @ 5A
5V: < 20mVpp @ 3A
Typical Output Ripple V
OUT
V
IN
= 48V
11.8V: < 120mVpp @ 0.5A
3.3V: < 25mVpp @ 5A
Nominal Switching Frequency
Efficiency
Isolation Voltage
V
IN
= 48V, full output current
250kHz
85.3% Typical
1500VDC
highly-integrated LTC3825, small supply utilizes an
isolated flyback topology with synchronous rectifica-
tion that requires no opto-isolator allowing for low-
parts count. The DC959 output supplies are 5V @
3A, 11.8V @ 0.5A, and 3.3V @ 5A.
Design files for this circuit board and PSE solution
are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
QUICK START PROCEDURE
DC959 is easy to set up to evaluate the performance
of the LTC3825 with the LTC4257-1 in a high power
PD application. Refer to Figure 1 for proper meas-
urement equipment setup and follow the procedure
below:
1.
2.
a.
b.
3.
4.
Connect a high power PSE with a CAT5 cable to
RJ45 connector J1.
37V to 57V option applied across VIN+ and VIN-
if PSE is not available.
Place test equipment (voltmeter, ammeter, and
load) across outputs 5V, 11.8V, and 3.3V.
Input supplies:
Check for the proper output voltages 5V, 11.8V,
and 3.3V.
Once the proper output voltages are established,
adjust the loads within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other parameters.
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 959
HIGH-POWER MULTI-OUTPUT SUPPLY WITH POE CONTROLLER
OPERATING PRINCIPLES
A compatible high power PSE is connected to the
DC959 at the RJ45 connector J1. As required by
IEEE802.3af, a bridge is used across the data pairs
and signal pairs. Schottky diodes are used at the
input to improve efficiency over standard diode
bridges. The LTC4257-1 provides IEEE802.3af stan-
dard PoE 25k signature resistance and is set for a
power class 4. When the PD is powered and voltage
is above the turn on UVLO, the LTC4257-1 signals a
power good to the LTC3825 to begin operation and
switches power over through MOSFET Q8 to allow for
currents higher than standard PoE.
The LTC3825 regulates the output voltages by sens-
ing the average of all the output voltages via a trans-
former winding during the flyback time. This allows
for tight output regulation without the use of an opto-
isolator, providing improved dynamic response and
reliability. Synchronous rectification increases the
conversion efficiency and cross-regulation effective-
ness above a conventional flyback topology. No ex-
ternal driver ICs or delay circuits are needed to
achieve synchronous rectification; a single resistor is
all that is needed to program the synchronous recti-
fier’s timing.
Figure 1. Proper measurement equipment setup
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