LED buck regulator with current-mode control simplifies compensation

LED buck regulator with current-mode control simplifies compensation

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By eeNews Europe

Synchronous buck converters are commonly 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 categorized 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 (see Figure 31 in the TPS54218 datasheet for more internal details of the controller). 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.

The current-shunt-monitor transfer function is simply a voltage-to-voltage gain of 20, with a high-frequency pole (and buffer) near 500 kHz. This output feeds the R6/R8 divider, which is grounded at the op amp U3 output because it is a dc voltage. The last portion to complete the feedback loop is the internal transconductance amp of the step-down converter, which has a voltage-to-current gain of 225 uA/V.

External compensation component C6 is connected from this point (V_COMP) to ground. Note that V_COMP is also the starting point (VC1) for our loop simulation. The loop gain is the voltage measured at the V_COMP pin divided by the injected perturbation at VC1. So by setting VC1 to a 1 Vac signal, the loop gain ends up being simply the voltage measured at V_COMP.

Figure 3 shows the responses measured at the VSENSE and V_COMP node. V_COMP is representative of the loop gain and phase margin while VSENSE is the power stage, which is simply the entire loop less the compensating op amp. The most notable point to make here is that VSENSE, which is the response up to and before COMP capacitor C6, is nearly flat. The power stage’s response is flat due to current-mode control and only the current-shunt-monitor response begins to decrease phase slightly at higher frequencies.

Figure 3
Simulation results show very benign responses, with the entire loop largely set by C6.

Adjusting the loop gain and the bandwidth of the converter is set by the value of C6 alone. A smaller value for C6 increases gain due to its higher impedance while a larger value decreases it. The gain should be set low enough to assure good stability. Try to avoid the temptation to push the gain up too high. There are additional second-order effects, such as slope compensation, that affect the gain and phase at higher frequencies that are not included in the model.

However, this model provides an excellent first-order approximation as well as insight into the loop gain of a current-mode synchronous buck LED regulator.

About the author

John Betten is an applications engineer and senior member of group technical staff at Texas Instruments. Betten has more than 29 years of AC/DC and DC/DC power conversion design experience. He received his BSEE from the University of Pittsburgh, Pennsylvania, and is a member of IEEE. Betten can be reached at Be sure to include the article title for reference.

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