In most wireless systems, e.g., GPS, WiFi, Bluetooth, etc., carrier signal purity is of utmost importance. And, in order for the carrier signal to remain free of interference throughout the entire frequency range, the supply voltage ripple should generally be no higher than 10mV. But what exactly is meant by “ripple?”

When looking at the output voltage waveform of a typical DC/DC converter on an oscilloscope, one can detect what is referred to as “switching noise.” This noise is made up of interference voltage spikes and ripple, with the former being high-frequency interference caused by frequencies above several MHz, which occurs when power is turned on/off. High frequency interference is influenced by peripheral components, and by parasitic capacitance and inductance within the circuit.

Ripple, on the other hand, is the voltage change that occurs when the inductor coil current is smoothed by the CL capacitor. Its value can be roughly calculated by multiplying the change in inductor coil current by the equivalent series resistance of the CL capacitor. Observing the waveform of the DC/DC converter output voltage on an oscilloscope shows how ripple is dependent on inductor coil and equivalent series resistance values.

Let us first discuss how ripple is influenced by changes in the inductor coil’s current value. In Figure 1, where the input and output conditions are identical, we see that the change in current flowing through the coil decreases as the inductance value is increased. At an inductance value of 10µH the change in current is low, and the ripple is thus also lower.

In the figure, which shows the dependence of ripple on the equivalent series resistance value of the CL capacitor, it can be clearly seen that the ripple associated with ceramic capacitors is lowest. That’s because the equivalent series resistance of a ceramic capacitor is less than that of a tantalum capacitor. Or, in general terms: The lower the equivalent series resistance, the lower the ripple.

Synchronous converters with high efficiency, like TOREX’s new 3MHz XC9235/36/37 series, feature extremely low ripple – typically, less than 10mV. An additional advantage for equipment designers is that the output voltage remains stable, thus eliminating the need for external filters or regulator circuits. Given the high switching frequency of 3MHz, a very efficient step-down transformation circuit for handling currents of up to 600mA can be implemented using only a low-profile induction coil and two capacitors (externally connected).

Equipped with the ultra-compact USP6C package measuring a mere 1.8mm x 2.0mm x 0.6mm, the IC and its associated components require only 6mm2 of board space. The devices are thus ideally suited for space-critical applications. It’s one reason why leading manufacturers of RF chipsets choose these TOREX devices for their Bluetooth and Wireless-LAN applications, with another being that they meet the requirement of extremely low ripple.

TOREX’s XC9235/36/37 IC series offer step-down DC/DC converters with an integrated P-channel FET and N-channel FET. Input voltage ranges from 2.0V to 6.0V, while the output voltage can be internally set in 50mV increments within the range 0.8V to 4.0V – via laser trimming in a final fabrication step. The series will soon be expanded with new versions (XC9235/36/37 D/E/F/G) offering an even lower input voltage (1.8V instead of 2V) and the ability to externally select the output voltage.

When used with ceramic output capacitors, all devices in the XC9235/36/37 series achieve extremely low ripple over the entire range of load currents. And, as shown in Fig. 6, ripple for the XC9236 and XC9337 ICs remains less than 10mV even near their PFM/PWM switching point. This is in contrast to similar competing products, whose ripple value frequently climbs noticeably near the threshold point.



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Fig.1: A Typical Circuit Requires Only Three External Components
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Fig.2: The Ultra-Compact Package Helps To Optimize Board Space
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Fig.3: Efficiency Is Dependent Upon The Output Current (Vout = 1.8V)
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Fig.4: The XC9236 and XC9237 Series – Lower Ripple Across The Entire Load Range
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