Pulse-skipping for automotive apps (Part 1)
This article discusses under which circumstances pulse-skipping mode may occur, details what implications this involves and if it is of concern in normal operation. Furthermore, solutions to avoid pulse-skipping mode are introduced.
There are three main scenarios that could lead to pulse-skipping:
1. Low power mode (LPM): if LPM is enabled, the device will not switch continuously at low load conditions, but skip pulses to reduce the switching losses.
2. Another occasion is often referred to as "Drop-Out" mode. If the input-voltage is close to the desired output-voltage, the part attempts 100% duty-cycle on the high-side-FET. However, in case of boot-strap-supply, a recharge of the boot-strap-capacitor is required to drive it.
3. A further reason for pulse-skipping, not further covered herein, is driven by minimum on-time requirements: Any switch-mode-power-supply has a minimum-on-time of the FETs or gate-drivers. In particular with high input and low output voltages at high switching frequencies, this could lead to pulse-skipping.
In the following, only scenarios 1 and 2 are discussed.
There are various ways to implement pulse-skipping into an IC:
Hysteretic-mode pulse-skipping leaves out pulses that are not required, and will initiate a single pulse once the output voltage has fallen below a threshold. This allows for a relatively low output ripple.
Another implementation is Burst-mode, which, as the name says, send a burst of pulses if the output-voltage drops below a threshold. This tends to introduce a higher output ripple.
Further options include constant on-time or constant off-time, both coming with the disadvantage that even during normal operation the switching frequency varies with load and is difficult to compensate and filter. Those variants will not be discussed in more detail in this report.
As an example for hysteretic mode let's use the TPS43340-Q1 multi-rail supply.
Figure 1 shows the output voltage (AC-coupled) of a Pulse-Skipping device, TPS43340-Q1, (Channel1, yellow trace), and the corresponding switch-node voltage (CH2, red trace). The part is supplied with 14 V, and set to switch at 400kHz. The output voltage is 5 V:
600kHz, VIN=14V, Vout=5V, Iout=10mA, LPM enabled
Figure 1: Pulse-skipping in low-power mode CH1=Vout (AC), CH2=PhaseNode, bottom graph shows a zoom into the pulse.
Hysteretic pulse-skipping will generate a single pulse (red trace), where the cycle-time depends on the load, while the pulse-duration depends on the selected switching frequency (at lower frequencies the pulse would be prolonged). For some combinations of switching frequency and load-conditions, two or more consecutive pulses may be required to reach the upper voltage threshold. In LPM, the draining of the output capacitors is slow resulting in a small output voltage ripple, which is not a concern in usual applications. For TPS43340-Q1, it's about 30 mV.
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