V

SS

CLS

DEN

D

A

Adapter

PPD

R

PPD1

APD

R

FBU

R

FBL

TLV431

R

OB

C

IZ

R

SS

C

SS

D

SS

FromRegulated

OutputVoltage

TPS23754

TPS23754-1

TPS23756

SLVS885D – OCTOBER 2008 – REVISED DECEMBER 2009

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 35. When 1.55 V

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.

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 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 25

shows a smooth handoff between the primary and secondary-side softstart with minimal output voltage

overshoot.

Figure 36. Error Amplifier Soft Start

32

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