The ON_OFF_CONFIG command controls the conditions which TPS53676 requires to enable power conversion. By default only the AVR_EN (active high) pin enables channel A, and only the BVR_EN pin (active high) enables channel B. This command can program the controller ignore the VR_EN pins and require the OPERATION command to be sent to enable power conversion, or even require a combination of the two.

When enabled, first the controller waits for a delay time given by TON_DELAY, then ramps the output voltage at a controlled slew rate SR_BOOT. The device requires 750 μs typically (up to 1.0 ms), to begin ramping the output voltage, after being enabled. Turn-on delay added by the TON_DELAY is in addition to this delay.

The ON_OFF_CONFIG command also controls the turn-off behavior. When configured for immediate-off, the controller immediately tri-states all PWM pins assigned to that channel and stops transferring power immediately. When configured for soft-off the controller first waits for the TOFF_DELAY time, then actively ramps down the output voltage at a controlled slew rate.

Figure 7-5 Soft-start and immediate-off (decay)

Figure 7-6 Soft-start and soft-off

The TON_RISE and TOFF_FALL commands are used to calculate the turn-on and turn-off (in the case of soft-off) slew rates. While these commands are numerically programmable from 0 to 31.75 ms, only a limited set of slew rates are supported. During the enable time period, the device calculates the target rising and falling slew rates according to Equation 1 and Equation 2, then selects the nearest available value from Table 7-4.

Example: VOUT_COMMAND = 0.88 V, TON_RISE = 1.0 ms

The target slew rate is calculated as SR_BOOT = LOOKUP(880 mV/1000 μs) = 0.88 mV/μs. The nearest supported value of 0.9375 mV/μs is selected.

The expected rise time is approximately (880 mV / 0.9375 mV/μs) ≈ 940 μs.