V

Load +

–(1–D)

V

DD

H/C

PWM

Load

Voltage

−VDD

VDD

VDD

−VDD/2

0

0

0

DRV593

DRV594

SLOS401C – OCTOBER 2002 – REVISED JULY 2010

www.ti.com

HEATING MODE

Figure 19 shows the DRV593 and DRV594 in heating mode. The H/C output is at VDD and the PWM output is

proportional to the voltage across the load.

The differential voltage across the load is determined using Equation 3. The variables are the same as used

previously for Equation 1 and Equation 2.

(3)

For example, a 50% duty cycle, shown in Figure 19, results in –2.5 V across the load for VDD = 5 V. The

differential voltage across the load is defined as the voltage measured after the filter on the PWM output relative

to the H/C output.

Figure 19. Heating Mode

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.

12

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