AN3198

AN3198
Maxim > Design Support > Technical Documents > Application Notes > Power-Supply Circuits > APP 3198
Keywords: positive emitter-coupled logic, PECL, low-dropout regulators, LDO, line termination, linear
regulators
APPLICATION NOTE 3198
Modified LDO Sinks PECL-Termination Current
Jun 23, 2004
Abstract: By configuring a current-sinking negative LDO to operate as a positive-voltage sink, you can
construct a low-voltage supply (1.3V) for terminating PECL-logic lines.
Also See: Modified LDO Regulator Sinks PECL-Termination Current
The positive emitter-coupled logic (PECL) used in high-speed telecommunications requires odd supply
voltages: a positive VCC of +3.3V, and a termination voltage (VTT ) equal to VCC -2V = +1.3V. The VTT
supply is regulated with respect to VCC, and must be able to sink current.
Most positive low-dropout (LDO) regulators cannot sink current. Negative LDOs are designed for that
purpose but normally deliver a negative voltage.
Figure 1 shows a current-sinking negative LDO modified for positive-voltage operation. The GND pin
connects to VCC, and IN connects to ground. Those connections allow the negative LDO to operate as a
positive-voltage sink in which the voltage at VSET equals VCC -1.25V:
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Figure 1. The connections shown enable a negative-output LDO (with its inherent current-sinking ability)
to generate a positive output voltage.
The output voltage is regulated with respect to VCC. That feature is perfect for PECL-termination
applications, because it requires the output voltage to track the VCC supply. Maximum output current is
limited by the internal protection circuitry (to about 400mA) and by the package power-dissipation rating
(about 550mW). For applications that require higher output voltage or higher current (or both), you can
add series diodes to dissipate some of the power (Figure 2). You can add as many diodes as needed to
dissipate power, but the voltage at OUT (Pin 5) must remain at least 300mV above ground (IN, pin 2).
Figure 2. To operate the Figure 1 circuit at a voltage or current that would otherwise exceed IC1's
internal or package power-dissipation ratings, add power-dissipating diodes as shown.
Related Parts
MAX1735
200mA, Negative-Output, Low-Dropout Linear Regulator
in SOT23
Free Samples Page 2 of 3
More Information
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Application Note 3198: http://www.maximintegrated.com/an3198
APPLICATION NOTE 3198, AN3198, AN 3198, APP3198, Appnote3198, Appnote 3198
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