The world of switching DC/DC converters is awash with a dizzying array of product offerings. For a given application, much of the power supply design effort can be spent simply searching for the optimum combination of package size, switching frequency, input and output voltage range, and desirable features. In many cases, though, the LT3580 offers an optimal solution. It is the right choice for many diverse applications because of its smart combination of features, performance and ease of use.
The LT3580 is a current control switching regulator available in tiny 8-lead packages (MSOP and 3mm × 3mm DFN). Operating from 200kHz–2.5MHz, it supports numerous configurations including boost, inverting, flyback and SEPIC. Inputs can be from 2.5V–32V, and an integrated 2A, 42V NPN power switch allows the LT3580 to provide efficient power from a fraction of a watt up to more than several watts.
Be Picky—Choose the Ideal Clock Frequency up to 2.5MHz
Choosing a converter switching frequency is often a compromise between several performance parameters such as physical size, output ripple, efficiency and spectral noise issues. While most converter ICs operate at a single fixed frequency, the LT3580 operates at any frequency from 200kHz–2.5MHz allowing you to choose the ideal frequency for any application.
The high frequency capability (up to 2.5MHz) of the LT3580 helps to reduce the overall size of the converter by permitting the use of smaller inductors and output capacitors. Small inductors, with correspondingly small inductances, work best at higher frequencies because they store and release less energy in each switching cycle. This can be seen by looking at the energy storage relationship for an inductor,
which shows that for a given peak inductor current (I), the stored energy is proportional to the inductance (L). Thus smaller inductances, storing less energy per cycle, switch at higher frequencies to deliver the same power as larger inductances. Also, smaller inductances reach their peak current (or energy) faster than large inductances as seen by rearranging the relationship
and solving for ΔT.
This shows that, for a given inductor voltage (V), a smaller inductor (L) will ramp to its peak current (I) in less time (T) than a larger inductance, again leading to higher frequency operation to make best use of the inductor.
Depending on the load requirements, high frequency operation also facilitates smaller output capacitors. Since charge is delivered to the output in smaller but more frequent packets, the voltage ripple is reduced for a given capacitance.
Figure 3 shows an example of reduced solution size at a higher switching frequency. The 5V to 12V boost converter operates at 2.5MHz and uses a smaller inductor and less output capacitance than the 1.2MHz solution in Figure 1. The tradeoff is slightly reduced efficiency due to the increased switching losses incurred at the higher switching frequency.
For large voltage gains, the LT3580’s low frequency capability (down to 200kHz) is very useful. Figure 5 shows a direct conversion from 5V to 40V running at 750kHz. Figure 6 shows a 5V to 350V flyback converter running at 200kHz.
Finally, the LT3580’s wide frequency range makes it easy to avoid sensitive frequency bands that can’t tolerate spectral noise. For example radio power supplies may operate at 2MHz or above to avoid the AM broadcast band. Also, some RF communications products are sensitive to noise at 455kHz, therefore switching above 600kHz is desired.
Accurate Clocking Options
The LT3580 provides two options for generating the clock. First, the integrated oscillator can be accurately set between 200kHz–2.5MHz by connecting a single resistor from the RT pin to ground, where
The boost converter in Figure 3, for example, uses a 35.7k RT resistor to set the switching frequency to 2.5MHz. The internal oscillator’s frequency is accurate to ±10% with little temperature variation as shown in Figure 4. The excellent frequency tolerance maximizes system performance by reducing necessary design margin.
The switching frequency can also be synchronized to an external clock source. The SYNC pin overrides the internal oscillator when toggled at frequencies greater than 75% of the internal oscillator’s set frequency. Simply connect a digital clock signal to the SYNC pin using VIH levels from 1.3V to 5.5V, VIL levels below 0.4V and any frequency between 200kHz and 2.5MHz. Using an external clock source is often helpful for several reasons, including…
- Synchronization of several switching regulators, often out of phase, to reduce switching current spikes
- Additional frequency precision yielding higher performance
- Precisely targeting the frequency out of sensitive bands for EMI benefits.
The LTC6908 resistor set oscillator is a nice choice for generating the SYNC clock due to its high precision, dual phase outputs, spread spectrum capabilities, small size and simple operation.
Single-Pin Feedback and Support for Multiple Configurations
The novel single-pin feedback of the LT3580 reduces external component count and allows it to be used in many different converter topologies. The output voltage is set by simply connecting a single external resistor from VOUT to the FB pin. The FB pin automatically servos to the correct reference voltage for a given topology (1.215V for positive VOUT and 5mV for negative VOUT).
Supported configurations include boost, SEPIC (Figure 10), and other topologies such as the flyback (Figure 6) and inverting (Figure 7).
Finally, to improve VOUT accuracy, the FB pin is factory trimmed to an accurate current, instead of trimming the resistance, which is typical of other parts. This eliminates multiplication of reference voltage errors to VOUT.
Soft-Start Feature Limits Start-Up Current
The LT3580 contains a soft-start circuit to limit peak switch currents during start-up. High start-up current is inherent in switching regulators since the feedback loop is saturated due to VOUT being far from its final value. The regulator tries to charge the output capacitors as quickly as possible, which results in large peak currents.
The start-up current can be limited by connecting an external capacitor (typically 100nF to 1µF) to the SS pin. This capacitor is slowly charged to ~2.2V by an internal 275k resistor once the part is activated. SS voltages below ~1.1V reduce the internal current limit. Thus, the gradual ramping of SS also gradually increases the current limit as the capacitor charges. This, in turn, allows the VOUT capacitor to charge gradually toward its final value while limiting the start-up current (see Figure 9).
Innovative SHDN Pin Resets Soft-Start and Serves as Undervoltage Lockout (UVLO)
The SHDN pin has threshold hysteresis to resist noise and tolerate slowly varying input voltages. Driving the SHDN pin to ground shuts down the LT3580 and reduces input current to less than 1µA. Driving SHDN above 1.38V enables the part and begins the soft-start sequence. A built in safety feature ensures that the SS capacitor is actively discharged before start-up begins. This allows for proper soft-start even in the event of short SHDN pulses or thermal lockout.
The LT3580 also features an integrated UVLO that shuts down the chip when the input voltage falls below ~2.3V. However, the SHDN pin can also be configured to disable the chip below even higher voltages as shown in Figure 8.
Typically, UVLO is needed in situations where the input supply is current-limited, has a relatively high source resistance, or ramps up/down slowly. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current-limit or latch low under low voltage conditions. The configurable UVLO prevents the regulator from operating at source voltages where these problems might occur.
The shutdown pin comparator has voltage hysteresis with typical thresholds of 1.32V (rising) and 1.29V (falling). Resistor RUVLO2 is optional but can be included to reduce overall UVLO voltage variation caused by variations in SHDN pin current. A good choice for RUVLO2 is 10k ±1%. After choosing a value for RUVLO2, RUVLO1 can be determined from either of the following:
where VIN+ and VIN– are the VIN voltages when rising or falling respectively.
The LT3580 is a smart choice for many DC/DC converter applications. It’s packed with features without compromising performance or ease of use and is available in tiny 8-lead packages.The accurate and adjustable clock, 2A/42V power switch, wide input voltage range, integrated soft-start and a configurable SHDN pin make the LT3580 an ideal choice for many DC power supply needs. For additional information and a complete data sheet visit www.analog.com.