Datasheet
RTN
D
1
58V
C
1
0.1uF
R
DEN
R
CLS
From Ethernet
Transformers
V
DD
V
SS
CLS
From Spare
Pairs or
Transformers
DEN
D
A
Adapter
PPD
R
PPD2
R
PPD1
V
DD1
COM
ARTN
APD
C
1
0.1uF
R
DEN
R
CLS
From Ethernet
Transformers
V
DD
V
SS
CLS
From Spare
Pairs or
Transformers
DEN
PPD
D
VDD
V
DD1
RTN
COM
ARTN
C
IN
D
A
Adapter
R
APD2
R
APD1
APD
1.8kW
24V
D
1
58V
R
DEN
R
CLS
V
DD
V
SS
CLS
DEN
PPD
D
VDD
V
DD1
RTN
COM
ARTN
C
IN
D
A
Adapter
APD
R
HLD
1.8KW
D
HLD
24V
D
APD
30V
R
APD1
26.7kW
R
APD2
3.01kW
R
VDD1
3.3MW
For48V
Adapter
D
RTN
58V
C
VDD
10nF
TPS23754
TPS23754-1
TPS23756
www.ti.com
SLVS885G –OCTOBER 2008–REVISED OCTOBER 2013
Figure 34. Option 2 PoE Backup ORing
Option 1 ORing of a low voltage adapter (for example, 24 V) is possible by connecting a resistor divider to PPD
as in Figure 35. When 1.55 V ≤ V
PPD
≤ 8.3 V, the hotswap MOSFET is enabled, T2P is activated, and the class
feature is disabled. The hotswap current limit is unaffected, limiting the available power. For example, the
maximum input power from a 24-V adapter would be 19.3 W [(24 V – 0.6 V) × 0.825 A].
Figure 35. Low-Voltage Option 1 ORing
Softstart
Converters require a softstart on the voltage error amplifier to prevent output overshoot on startup. Figure 36
shows a common implementation of a secondary-side softstart that works with the typical TL431 error amplifier.
The softstart components consist of D
SS
, R
SS
, and C
SS
. They serve to control the output rate-of-rise by pulling
V
CTL
down as C
SS
charges through R
OB
, the optocoupler, and D
SS
. This has the added advantage that the TL431
output and C
IZ
are preset to the proper value as the output voltage reaches the regulated value, preventing
voltage overshoot due to the error amplifier recovery. The secondary-side error amplifier will not become active
until there is sufficient voltage on the secondary. The TPS23754 device provides a primary-side softstart which
persists long enough (about 4 ms) for secondary side voltage-loop softstart to take over. The primary-side
current-loop softstart controls the switching MOSFET peak current by applying a slowly rising ramp voltage to a
second PWM control input. The PWM is controlled by the lower of the softstart ramp or the CTL-derived current
demand. The actual output voltage rise time is usually much shorter than the internal softstart period. Initially the
internal softstart ramp limits the maximum current demand as a function of time. Either the current limit,
secondary-side softstart, or output regulation assume control of the PWM before the internal softstart period is
over. Figure 25 shows a smooth handoff between the primary and secondary-side softstart with minimal output
voltage overshoot.
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