Electronic Design

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 +3.3 V, and a termination voltage (VTT) equal to VCC − 2 V = +1.3 V. The VTT supply is regulated with respect to VCC and must be able to sink current.

Most positive low-dropout (LDO) regulators can't sink current, but negative LDOs are designed for that purpose. 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 let the negative LDO operate as a positive-voltage sink in which the voltage at VSET equals VCC − 1.25 V:

VSET = VCC − 1.25 V
VCC − VSET = 1.25 V
\[(VCC − VOUT)/(R1 + R2)\]R2 = 1.25 V
VOUT = VCC − (R1 + R2)(1.25 V/R2)
VOUT = VTT = VCC − 2 V (For PECL)
(R1 + R2)(1.25 V/R2) = 2 V (For PECL)
R1 = 0.6R2 (For PECL)

The output voltage is regulated with respect to VCC. This 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 400 mA) and by the package power-dissipation rating (about 550 mW). For applications that require higher output voltage or higher current (or both), you can add series diodes to dissipate some of the power (Fig. 2). You can add as many diodes as needed to dissipate power, but the voltage at OUT (pin 5) must remain at least 300 mV above ground (IN, pin 2). 