Datasheet

R
FBU
R
FBL
TLV431
R
OB
C
IZ
R
SS
C
SS
D
SS
FromRegulated
OutputVoltage
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
TPS23757
SLVS948D –JULY 2009–REVISED NOVEMBER 2013 www.ti.com
Option 1 ORing of a low voltage adapter (e.g., 24 V) is possible by connecting a resistor divider to PPD as in
Figure 30. When 1.55 V ≤ V
PPD
≤ 8.3 V, the hotswap MOSFET is enabled, APb 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 9.3 W [(24 V – 0.6 V) × 0.4 A].
Figure 30. Low-Voltage Option 1 ORing
Softstart
Converters require a softstart on the voltage error amplifier to prevent output overshoot on startup. Figure 31
shows a common implementation of a secondary-side softstart that works with the typical TLV431 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
TLV431 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 TPS23757 provides a primary-side softstart
which persists long enough (~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 21 shows a smooth handoff between the primary and secondary-side softstart with minimal output
voltage overshoot.
Figure 31. Error Amplifier Soft Start
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