DRV593

DRV594

SLOS401A - SEPTEMBER 2002 REVISED - OCTOBER 2002

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13

HEAT/COOL TRANSITION

As the device transitions from cooling to heating, the duty cycle of the PWM outputs decrease to a small value and

the H/C outputs remains at ground. When the device transitions to heating mode, the H/C outputs change from zero

volts to VDD and the PWM outputs change to a high duty cycle. The direction of the current flow is reversed, but a

low voltage is maintained across the load. The duty cycle decreases as the part is put further into heating mode to

drive more current through the load. Figure 20 illustrates the transition from cooling to heating.

ZERO-CROSSING REGION

When the differential output voltage is near zero, the control logic in the DRV593 and DRV594 causes the outputs

to change between heating and cooling modes. There are two possible states for the PWM and H/C outputs to obtain

zero volts differentially: both outputs can be at VDD or both outputs can be at ground. Therefore, random noise causes

the outputs to change between the two states when the two input voltages are equal. The outputs switch from zero

to VDD, although not at a fixed frequency rate. Some of the pulses may be wider than others, but the two outputs

(PWM and H/C) track each other to provide zero differential voltage. These uneven pulse widths can increase the

switching noise during the zero-crossing condition.

To avoid this phenomenon, hysteresis should be implemented in the control loop to prevent the device from operating

within this region. Although planning for operation during the zero-crossing is important, the normal operating points

for the DRV593 and DRV594 are outside of this region. For laser temperature/wavelength regulation, the zero volts

output condition is only a concern when the laser temperature or wavelength, relative to the ambient temperature,

requires no heating or cooling from the TEC element.

PWM

VDD

VDD

0

IN -

IN +

0

H/C

VDD

0

Figure 20. Transition From Cooling to Heating