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Pulse-width modulation (PWM) is an indispensable technique for controlling load power when driving off-board loads with high-side switches (HSS). By adjusting PWM duty cycle, average load power can be controlled with accuracy and much more efficiently than designs which rely on linear regulation of voltage or current to the load. PWM control can be used to increase product functionality — for example, to enable a vehicle owner to choose the heat level in a heated seat or adjust the brightness of footwell lighting. Figure 1-1 compares PWM and linear control schemes which result in the same load power.
PWM duty cycle may be software or hardware defined and can be varied in real-time. Apart from the limitations of the high-side switch or nuances of the load, the average power delivered to the load is virtually independent of PWM frequency. This allows for flexibility in the system design.
Thermal limits of any IC should always be kept in mind, and this is especially true for high-side switches which are often asked to switch large currents to the load.
We can estimate the total power dissipated by the device itself by summing power dissipation due to the FET RON, switching losses, and quiescent power.
We can calculate power dissipated in the HSS FET as in Equation 1. This power is proportional to the on-resistance (specified in the device datasheet) and the average current through the FET.
RΘJA | Junction-to-ambient thermal resistance |
RΘJB | Junction-to-board thermal resistance |
RΘJC | Junction-to-case thermal resistance. |
TI HSSs utilize a thermal pad which is recommended to be soldered directly to the PCB. This benefits devices thermals by utilizing the large surface area of the attached thermal plane as a heatsink, which closely couples with board temperature. Board temperature, however, is difficult to measure accurately and is highly dependent on board construction and solder coverage/quality on the high side switch. Ambient temperature, on the other hand, is easier to measure and straightforward to use in junction temperature estimation.
For a given ambient temperature the operating junction temperature may be estimated as below.
In a PWM application, it is important to know the limits of timing that the HSS imposes on the final system design. Timing limitations affect how accurately load power can be controlled over the range of duty cycles. Since resistive and LED loads that convert load power to energy are intended to be detected by humans, some variation from ideal load power can often be tolerated. In cases where load power has to be more precisely controlled, such as in automotive lighting where luminous power is strictly regulated by governmental agencies, a designer may choose to use a lower PWM frequency in order to guarantee compliance with lightning standards regardless of HSS parameter skew.
The principal phenomena that limit the pulse width a HSS can drive accurately are limited output slew-rate and propagation delay mismatch, also known as pulse-width distortion.