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BIAS CURRENT GENERATION AND APC LOOP

ANALOG REFERENCE AND TEMPERATURE SENSOR

POWER-ON RESET

ONET8501V

SLLS837B – JUNE 2007 – REVISED SEPTEMBER 2007

The bias current generation and APC loop are controlled by means of the 2-wire interface. In open loop

operation, selected by setting OLENA = 1 (bit 4 of register 0) the bias current is set directly by the 8-bit wide

control word BIASC[0..7] (register 2). In automatic power control mode, selected by setting OLENA = 0, the bias

current depends on the register settings BIASC[0..7] and the coupling ratio (CR) between the VCSEL bias

Three photodiode current ranges can be selected by means of the PDRNG[1..0] bits (register 0). The photodiode

range should be chosen to keep the laser bias control DAC, BIASC[0..7], close to the center of its range. This

keeps the laser bias current set point resolution high.

For details regarding the bias current setting in open- as well as in closed-loop mode, see Table 16.

In closed-loop mode, the photodiode polarity bit, PDPOL (bit 0 of register 0), must be set for common-anode or

common-cathode configuration to ensure proper operation.

The ONET8501V VCSEL driver is supplied by a single 3.3V10% supply voltage connected to the VCC pins. This

voltage is referred to ground (GND).

On-chip bandgap voltage circuitry generates a reference voltage, independent of the supply voltage, from which

all other internally required voltages and bias currents are derived.

An external zero temperature coefficient resistor must be connected from the RZTC pin of the device to ground

(GND). This resistor is used to generate a precise, zero-TC current which is required as a reference current for

the on-chip DACs.

In order to minimize the module component count, the ONET8501V provides an on-chip temperature sensor.

The output voltage of the temperature sensor is available at the TS pin. Due to the die temperature of the 8501V

and for high accuracy applications, the use of an external temperature sensor may be required. However, in

order to improve the part-to-part accuracy of the sensor, the offset voltage and temperature slope can be

adjusted through the 2-wire interface. The offset voltage can be adjusted by means of the TSSH[0..3] bits

(register 10) and the direction of the offset can be set by the sign bit TSHSGN (bit 4 of register 10). The

temperature slope can be adjusted by means of the TSSL[0..3] bits (register 11) and the sign bit TSLSGN (bit 4

of register 11).

The temperature sensor can be disabled by setting TSDIS = 1 (bit 1 of register 9).

The ONET8501V has power on reset circuitry which ensures that all registers are reset to zero during startup.

disable pin DIS is low.

The ONET8501V can be disabled using either the ENA control register bit or the disable pin DIS. In both cases

the internal registers are not reset. After the disable pin DIS is set low and/or the enable bit ENA is set back to 1,

the part returns to its prior output settings.

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