8.2.1.2.1 Selecting the External Capacitors

The LM27313 requires ceramic capacitors at the input and output to accommodate the peak switching currents the part needs to operate. Electrolytic capacitors have resonant frequencies which are below the switching frequency of the device, and therefore can not provide the currents needed to operate. Electrolytics may be used in parallel with the ceramics for bulk charge storage which will improve transient response.

When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage, they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from Taiyo-Yuden, AVX, and Murata.

8.2.1.2.2 Selecting the Output Capacitor

A single ceramic capacitor of value 4.7 µF to 10 µF provides sufficient output capacitance for most applications. For output voltages below 10 V, a 10 µF capacitance is required. If larger amounts of capacitance are desired for improved line support and transient response, tantalum capacitors can be used in parallel with the ceramics. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used, but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies above
500 kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output capacitor with excessive ESR can also reduce phase margin and cause instability.

8.2.1.2.3 Selecting the Input Capacitor

An input capacitor is required to serve as an energy reservoir for the current which must flow into the inductor each time the switch turns ON. This capacitor must have extremely low ESR and ESL, so ceramic must be used. We recommend a nominal value of 2.2 µF, but larger values can be used. Because this capacitor reduces the amount of voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other circuitry.

8.2.1.2.4 Feed-Forward Compensation

Although internally compensated, the feed-forward capacitor Cf is required for stability (see Equation 1). Adding this capacitor puts a zero in the loop response of the converter. Without it, the regulator loop can oscillate. The recommended frequency for the zero fz should be approximately 8 kHz. Cf can be calculated using the formula:

8.2.1.2.5 Selecting Diodes

The external diode used in the typical application should be a Schottky diode. If the switch voltage is less than 15V, a 20V diode such as the MBR0520 is recommended. If the switch voltage is between 15 V and 25 V, a 30-V diode such as the MBR0530 is recommended. If the switch voltage exceeds 25V, a 40V diode such as the MBR0540 should be used.

The MBR05xx series of diodes are designed to handle a maximum average current of 500 mA. For applications with load currents to 800 mA, a Microsemi UPS5817 can be used.

8.2.1.2.6 Setting the Output Voltage

The output voltage is set using the external resistors R1 and R2 (see Equation 2). A value of 13.3 kΩ is recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula:

8.2.1.2.7 Duty Cycle

The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost application is defined as:

Equation 3.

This applies for continuous mode operation.

The equation shown for calculating duty cycle incorporates terms for the FET switch voltage and diode forward voltage. The actual duty cycle measured in operation will also be affected slightly by other power losses in the circuit such as wire losses in the inductor, switching losses, and capacitor ripple current losses from self-heating. Therefore, the actual (effective) duty cycle measured may be slightly higher than calculated to compensate for these power losses. A good approximation for effective duty cycle is:

8.2.1.2.8 Inductance Value

The first question we are usually asked is: “How small can I make the inductor?” (because they are the largest sized component and usually the most costly). The answer is not simple and involves trade-offs in performance. More inductance means less inductor ripple current and less output voltage ripple (for a given size of output capacitor). More inductance also means more load power can be delivered because the energy stored during each switching cycle is:

An important point to observe is that the LM27313 will limit its switch current based on peak current. This means that because lp(max) is fixed, increasing L will increase the maximum amount of power available to the load. Conversely, using too little inductance may limit the amount of load current which can be drawn from the output.

Best performance is usually obtained when the converter is operated in “continuous” mode at the load current range of interest, typically giving better load regulation and less output ripple. Continuous operation is defined as not allowing the inductor current to drop to zero during the cycle. It should be noted that all boost converters shift over to discontinuous operation as the output load is reduced far enough, but a larger inductor stays “continuous” over a wider load current range.

To better understand these tradeoffs, a typical application circuit (5V to 12V boost with a 10 µH inductor) will be analyzed.

Because the LM27313 typical switching frequency is 1.6 MHz, the typical period is equal to 1/f_SW(TYP), or approximately 0.625 µs.

We will assume: V_IN = 5 V, V_OUT = 12 V, V_DIODE = 0.5 V, V_SW = 0.5 V. The duty cycle is:

The typical ON time of the switch is:

It should be noted that when the switch is ON, the voltage across the inductor is approximately 4.5 V.

Use the equation:

Then, calculate the di/dt rate of the inductor which is found to be 0.45 A/µs during the ON time. Using these facts, we can then show what the inductor current will look like during operation:

Figure 14. 10 µH Inductor Current, 5 V – 12 V Boost

During the 0.390-µs ON time, the inductor current ramps up 0.176 A and ramps down an equal amount during the OFF time. This is defined as the inductor “ripple current”. It can also be seen that if the load current drops to about 33 mA, the inductor current will begin touching the zero axis which means it will be in discontinuous mode. A similar analysis can be performed on any boost converter, to make sure the ripple current is reasonable and continuous operation will be maintained at the typical load current values.

8.2.1.2.9 Maximum Switch Current

The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the application. This is illustrated in Figure 15 which shows typical values of switch current as a function of effective (actual) duty cycle:

Figure 15. Switch Current Limit vs Duty Cycle

8.2.1.2.10 Calculating Load Current

As shown in Figure 14 which depicts inductor current, the load current is related to the average inductor current by the relation:

The switch current can be found by:

Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:

Combining all terms, we can develop an expression which allows the maximum available load current to be calculated:

Equation 12.

The equation shown to calculate maximum load current takes into account the losses in the inductor or turn-OFF switching losses of the FET and diode. For actual load current in typical applications, we took bench data for various input and output voltages and displayed the maximum load current available for a typical device in graph form:

Figure 16. Max. Load Current vs V_IN

8.2.1.2.11 Design Parameters V_SW and I_SW

The value of the FET ON voltage (referred to as V_SW in the equations) is dependent on load current. A good approximation can be obtained by multiplying the "ON Resistance" of the FET times the average inductor current.

FET ON resistance increases at V_IN values below 5V, because the internal N-FET has less gate voltage in this input voltage range (see Typical Characteristics). Above V_IN = 5 V, the FET gate voltage is internally clamped to 5V.

The maximum peak switch current the device can deliver is dependent on duty cycle. The minimum switch current value (I_SW) is ensured to be at least 800 mA at duty cycles below 50%. For higher duty cycles, see Typical Characteristics.

8.2.1.2.12 Minimum Inductance

In some applications where the maximum load current is relatively small, it may be advantageous to use the smallest possible inductance value for cost and size savings. The converter will operate in discontinuous mode in such a case.

The minimum inductance should be selected such that the inductor (switch) current peak on each cycle does not reach the 800 mA current limit maximum. To understand how to do this, an example will be presented.

In this example, the LM27313 nominal switching frequency is 1.6 MHz, and the minimum switching frequency is 1.15 MHz. This means the maximum cycle period is the reciprocal of the minimum frequency:

Assume: V_IN = 5 V, V_OUT = 12 V, V_SW = 0.2 V, and V_DIODE = 0.3 V. The duty cycle is:

Therefore, the maximum switch ON time is:

An inductor should be selected with enough inductance to prevent the switch current from reaching 800 mA in the 0.524 µs ON time interval (see Figure 17):

Figure 17. Discontinuous Design, 5 V – 12 V Boost

The voltage across the inductor during ON time is 4.8 V. Minimum inductance value is found by:

In this case, a 3.3-µH inductor could be used, assuming it provided at least that much inductance up to the 800-mA current value. This same analysis can be used to find the minimum inductance for any boost application.

8.2.1.2.13 Inductor Suppliers

Some of the recommended suppliers of inductors for this product include, but are not limited to, Sumida, Coilcraft, Panasonic, TDK and Murata. When selecting an inductor, make certain that the continuous current rating is high enough to avoid saturation at peak currents. A suitable core type must be used to minimize core (switching) losses, and wire power losses must be considered when selecting the current rating.

8.2.1.2.14 Shutdown Pin Operation

The device is turned off by pulling the shutdown pin low. If this function is not going to be used, the pin should be tied directly to V_IN. If the SHDN function will be needed, a pullup resistor must be used to V_IN (50 kΩ to 100 kΩ is recommended), or the pin must be actively driven high and low. The SHDN pin must not be left unterminated.