Issue 129 April 2001
Have You Seen the Light?

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Slam-bang switching

Quick survey: Raise your hand if you’ve ever fried a transistor relay driver before you found out about resistor capacitor snubbers and clamp diodes. Hah! Thought so. And ever since then, you’ve regarded inductive kick as a really bad thing, right?

Boost mode DC/DC converters harness inductive current to a good cause. The MAX629 stores energy as current in an inductor, then routes that current into a capacitor to create a higher voltage. It’s the same principle as the relay coil frying your transistor but done deliberately.

The key components in Figure 2 are L1, a 47-µH inductor (the small black circle at the top of the circuit board in Photo 1); C2, a 10-µF tantalum capacitor; and D2, a Schottky diode.

The circuitry behind the MAX629’s LX pin includes a high-current, open-drain FET with a current monitor in the source. With that transistor off, C2 charges quickly through D2, and with the voltages equalized to about 12 V from the battery, the current through L1 drops to zero. When the transistor turns on, the LX pin sinks current from L1. D2 prevents current flow from C2, so the capacitor maintains its charge and voltage.

The current through L1 builds up from zero with a time constant determined by the parasitic resistance of the inductor plus the internal resistance behind the LX pin. According to the datasheets, the total resistance is about 1.5 W, giving an L/R time constant of 30 ms.

The MAX629 monitors the LX current and shuts off the FET at 500 mA (or 250 mA, depending on the ISET input pin). Figure 3 shows the voltage at LX, which starts at 12 V, drops to zero (actually, about 200 mV) when the FET goes on, then rises to 24 V when it turns off.

(Click here to enlarge.)

Figure 3—The AC-coupled top trace shows the output voltage across C2 and the bottom trace shows the voltage at the MAX629 LX pin. When the LEDs turn on at the trigger point, two divisions from the left, the output current jumps from 0 to 150 mA.

Where did the 24 V come from? You’ll recall that the current through an inductor cannot change instantaneously. When the FET turns off, no current flows into LX and the diode is still reverse biased. That’s the point when your relay driver transistor fried itself, but things are different here.

The voltage at LX rises rapidly, until D2 becomes forward biased and routes that half amp to C2. The inductor current then drops as the voltage on C2 rises. Over the course of several hundred cycles, the MAX629 pumps the initial 12 V on C2 up to 24 V, which is why we call the MAX629 a "boost converter."

The upper trace in Figure 3 shows the voltage across C2, AC-coupled at 200 mV per division. When the LED turns on at the second division from the left, the output voltage on C2 ramps down as the MAX629 is charging the inductor. The sudden 340-mV jump occurs as the inductor stuffs half an amp into the capacitor.

Perhaps this is the first time you’ve seen equivalent series resistance (ESR) in action. Only 0.7 W of ESR in C2 will account for that bump. This is why boost converters don’t often power sensitive analog circuitry, at least not without a post regulator to smooth things out.

The combined effects of declining inductor current and LED load reduce the voltage on C2. The MAX629 begins another cycle when the voltage falls below the minimum setpoint. The cycle repeats at about 330 kHz.

Because the L/R time constant for L1 is greater than the cycle time, the inductor current rises predictably. It also falls predictably as the capacitor absorbs the charge.

I picked 24 V because it was high enough to run the LEDs with just enough headroom for current regulation. The MAX629 is a mature device and you’ll find similar products from many vendors. One of them will produce the voltage you need, although some require an external switching FET to deliver higher power.

OK, pop quiz time. What happens when (not if) you accidentally short the positive terminal of C2 to the ground plane around it? Answer: Poof! L1 emits magic smoke!

That 30-ms L/R time constant may be large with respect to normal operation, but it’s small compared to human reaction times. The current through L1 rises past the inductor’s 500-mA maximum rating toward the 12-V/0.6-W DC limit. However, the AWG 37 wire in the inductor burns out long before the current stabilizes at 20 A. Consider yourself warned.