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Simplify compensation with LED buck regulator

Posted: 03 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Synchronous buck converters? LEDs? current-mode-control? TPS54218? capacitor?

Synchronous buck converters are typically used to regulate the current in LEDs, often in applications such as automotive, medical, industrial, and even personal electronics. Most controllers regulate outputs with control schemes that can be generally categorised into constant on-time, voltage-mode or peak-current-mode. Peak current-mode controllers are arguably the majority, but how do you compensate the control loop to ensure stability if you are regulating current rather than the output voltage?

In peak-current-mode control, the control signal (or COMP voltage) controls the peak current in the inductor by means of an inner control loop, thereby simplifying the output voltage feedback loop. But what if you are regulating the current in an LED, in order to maintain constant brightness, rather than the output voltage? It is known that current-mode-control (CMC) can, for all practical purposes, eliminate the frequency response effect of the inductor itself when compensating the power supply for stability. As you will see, using the output current as the feedback signal can make "closing the loop" even simpler.

Figure 1 shows a step-down converter, TPS54218, synchronous buck controller directly driving the current in an LED through high-side sense resistor R3. This current sense voltage is amplified by a factor of 20 by the current sense monitor, INA193, which allows considerably less power dissipation in R3 and boosts efficiency. The current feedback signal out of the current shunt monitor feeds a resistive divider (R6/R8), which completes the feedback path to VSENSE.

Figure 1: Sync buck converter configured to regulate a constant current in an LED.

The operational amplifier (op amp) allows the LED current to be adjusted higher or lower by means of a control signal (VCNTL). By its very nature, the controller continuously adjusts the duty cycle and output current to maintain 0.8V at the VSENSE pin. If the op amp output voltage rises, it may raise the voltage on VSENSE, so the controller adjusts the LED current downward to prevent VSENSE from increasing.

Figure 2 is a simple SPICE model of figure 1 simulating the control loop. VC1 is the voltage at the COMP pin, which directly drives the power stage with a transconductance gain of 13. This current drives the LED directly through the inductor and sense resistor. Note that changes in the inductance value and LED values have no impact on the response as the current in the inductor is controlled.

Figure 2: Simplified AC model of control loop to measure gain and phase margin.

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