Digital current source is nonvolatile

Digitally programmable current sources that feature automatic trimming and retain the setting despite power-down cycles are useful in applications such as RF- and laser-communications drivers. The circuit in Figure 1, for example, is particularly suited for setting the drive current for the optical pump in widely tunable VCSELs (vertical-cavity surface-emitting lasers). These lasers are suitable in systems using wavelength-agile DWDM fiber-optic communication links. The circuit in Figure 1 delivers a stable drive current that derives its control from DPP (digitally programmed potentiometer), a Catalyst Semiconductor (www.catsemi.com) CAT5512. For the circuit values shown, you can set the output current to 500 mA to 1A; however, you can alter this span over a wide range by the judicious selection of sense resistors R₁ and R₂.

The unique features of the CAT5512 make possible the low component count (two chips and two resistors). The device combines a 5-bit-resolution, 100-kΩ DPP, a nonvolatile EEPROM for long-term storage of the DPP setting, and a unity-gain analog-wiper-buffer amplifier. The DPP provides a complete digital interface of the LT317 precision regulator chip to the R₁, R₂ split current-sense-resistor network. The result is a robust, precision programmable current source with 5-bit resolution over a flexible I_MIN to I_MAX range. The basis of circuit operation is the fact that the LT317 regulator generates the current necessary to maintain a constant 1.25V across the effective sense resistance: R₁+(1–p)R₂, where p is the DPP setting: 0, 0.032, 0.064, 0.097, ..., 0.98, 1. In this way, I₁=1.25V/(R₁+(1–p)R₂) over the range of I_MIN=1.25V/(R₁+R₂) for p=0 to I_MAX=1.25V/R₁ for p=1. The pertinent design equations are R₁=1.25V/I_MAX, and R₂=1.25V/I_MIN–R₁.

Note that the R₂ equation is an approximation, based on the assumption that R₂ is much lower than 100 kΩ, the parallel DPP resistance. The full expression for R₂ is:

You exert control of the DPP setting "p"—and storage of the setting in nonvolatile EEPROM—via the three-wire digital interface, as described in the CAT5512 data sheet. The load-voltage- compliance limit is a function of the W₁ and W₂V+ supply jumpers, connected to the LT317. The maximum output voltage is the difference between the LT317's input voltage and the sum of the LT317's dropout voltage (approximately 2V) and the voltage drop across the R₁+R₂ series resistance: V_MAX=V+–2V–I_MAX(R₁+R₂). In the circuit in Figure 1, this voltage is V+–4.5V. This arithmetic leads to a V_MAX of only 500 mV if you use the W₁ option and V+=5V. This compliance figure may be insufficient for some applications. If, by contrast, you choose the W₂ option and V+ is greater than 7.5V (not necessarily regulated), V_MAX increases to a much more adequate 2.5V.

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