DRV593_15 Datasheet, PDF (12/27 Pages) Texas Instruments – 3−A HIGH−EFFICIENCY PWM POWER DRIVER

DRV593
DRV594
SLOS401C – OCTOBER 2002 – REVISED JULY 2010
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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.
VLoad + –(1–D) V_DD
(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.
VDD
PWM
0
VDD
H/C
0
Load
Voltage
0
−VDD/2
−VDD
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.
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