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[FAQ] TPS7H5001-SP: Duty Cycle Limits for a TPS7H500X-SP Controller

Part Number: TPS7H5001-SP

The TPS7H500X-SP family of controllers have both minimum and maximum duty cycle limits that have to be taken into account when using the controller

Maximum Duty Cycle

The maximum duty cycle is easier to take into account as there is an adjustable limit set by the DCL pin:

When designing for the maximum duty cycle there should be room between the operation duty cycle and the actual limit.
This is so the controller has room during transients and to grow due to losses in the converter
This number changes depending on many factors, but 10% for a 50% DCL limit, 20% for a 75% DCL limit, and 30% for a 100% DCL limit can be a good starting point.

The TPS7H500X-SP line of controllers can work with no minimum off-time when using the 100% DCL limit.
While this is fine for the controller, make sure any high side drivers you use in tandem with the controller can support the maximum duty cycle that you want.
The high side driver needs to charge the boot strap voltage it has in order to properly run the high side FET.
This can cause a minimum off-time based on the FET you pick.

Minimum Duty Cycle

The minimum duty cycle of the TPS7H500X-SP family is determined by the minimum on time of the controller.
In the electrical specifications the minimum on-time of the controller is specified with a 10kOhm LEB (Leading Edge Blanking) resistor

Note that the maximum number of 85 ns is a combination of two different parts.
The minimum on-time of the device is a combination of the actual minimum on-time of the device plus the leading edge blanking.

A 10 kOhm LEB resistor corresponds to a leading edge blanking time of 19 ns maximum.
This means the 66 ns of the controller is added to the 19 ns of the LEB to get 85 ns for this electrical specification.
If the LEB resistor is increased, the maximum minimum on-time will increase as well.

One should be careful when picking the switching frequency of their converter to not violate the minimum on-time of the controller
Since the minimum on-time of the controller is a static value, often decreasing frequency will allow for the voltage drop needed.

There should be room between the on-time the converter is run at and the minimum on-time of the controller
Failure to allow for sufficient room will cause the controller to have a non-monotonic start-up or not have room during output current transients of the converter
While what head room needed changes with many factors, a good starting number will be that the min on-time combination should not be more than half of the duty cycle of your converter

Violating the minimum on-time of the controller will lead to the controller working in pulse skipping mode.
Details on pulse skipping mode are listed in the datasheet 8.3.12
While useful, it is not suggested to design a converter to only work in this mode, and other options should be considered.

Thanks,
Daniel