Table 2. Design Parameters

Table 3. List of Components

11.2.2.1 Output Voltage Setting

The output voltage is calculated by Equation 5:

Equation 5.

To minimize the current through the feedback divider network, R2 should be between 180 kΩ and 360 kΩ. The sum of R1 and R2 should not exceed ~1 MΩ, to keep the network robust against noise. For the example, R1 is 768 kΩ and R2 is 243 kΩ.

An external feed forward capacitor C1 is required for optimum load transient response. The value of C1 should be 100 pF. The connection from the FB pin to the resistor divider should be kept short and away from noise sources, such as the inductor or the SW line.

11.2.2.2 Input Current Limit

The input current limit is set by selecting the correct external resistor value. Equation 6 is a guideline for selecting the correct resistor value:

Equation 6.

For a current limit of 1 A, the resistor value is 10 kΩ.

To allow maximum current limit the ILIM pin can be directly connected to V_IN.

11.2.2.3 Maximum Output Current

The maximum output current is set by R_ILIM and the input to output voltage ratio and can be calculated by Equation 7:

Equation 7.

Following the example I_OUT(max) is 648 mA at 3.6 V input voltage and decreases, with lower input voltage values.

11.2.2.4 Inductor Selection

As for all switching power supplies two main passive components are required for storing the energy during operation: an inductor and an output capacitor. The inductor must be connected between the VIN pin and SW pin to make sure that the TPS61252 device operates. To select the right inductor current rating, the programmed input current limit as well as the current ripple through the inductor should be calculated. An estimation of the maximum peak inductor current can be done using Equation 8.

Equation 8.

Regarding the above example the current ripple (ΔI_L) is 426 mA and therefore an inductor with a rated current of about 1.5 A should be used.

The TPS61252 is designed to work with inductor values between 1.0 µH and 2.2 µH. For typical applications, a 1.5 µH inductor is recommended. In space constrained applications, it might be possible to consider smaller inductor values depending on the targeted inductor ripple current. Therefore the inductor value can be reduced down to 1.0 µH without degrading the stability.

In regular boost converter designs the current through the inductor is defined by the fixed switch current limit of the converter's switches and therefore bigger inductors have to be chosen. The TPS61252 allows the design engineer to reduce the current limit to the needs of the application regardless the maximum switch current limit of the converter. Programming a lower current value allows the use of smaller inductors without the danger of saturation.

11.2.2.5 Output Capacitor

For the output capacitor, it is recommended to use small X5R or X7R ceramic capacitors placed as close as possible to the VOUT and GND pins of the IC. If, for any reason, the application requires the use of large capacitors which cannot be placed close to the IC, a smaller ceramic capacitor in parallel to the large one is required. This small capacitor should be placed as close as possible to the converter's VOUT and GND pins.

To maintain control loop stability of the boost converter, a minimum effective output capacitance of at least 8 µF is recommended. That means due to DC Bias effect (see NOTE) a 22 µF capacitor with 0805 case size and a voltage rating of 10 V is necessary. In height restricted application two 10 µF capacitors with 0603 case size and 6.3 V voltage rating can also be used.

In addition to the minimum C_OUT the application might need more capacitance. To get an estimate of the minimum output capacitance necessary for the application, Equation 9 is used:

Equation 9.

Where ΔV is the maximum allowed output ripple.

With a chosen ripple voltage of 10 mV, a minimum effective capacitance of 9.6 μF is needed regarding the example. The total ripple is larger due to the ESR of the output capacitor. This additional component of the ripple can be calculated using Equation 10:

Equation 10.

11.2.2.6 Input Capacitor

Multilayer X5R or X7R ceramic capacitors are an excellent choice for input decoupling of the step-up converter as they have extremely low ESR and are available in small form factors. The input capacitors should be located as close as possible to the device. While a 10μF input capacitor is sufficient for most applications, larger values may be used to reduce input current ripple. Also, low ESR tantalum capacitors may be used.

11.2.2.7 Checking Loop Stability

The first step of circuit and stability evaluation is to look from a steady-state perspective at the following signals:

• Switch node, SW
• Inductor current, I_L
• Output ripple voltage, V_OUT(AC)

These are the basic signals that need to be measured when evaluating a switching converter. When the switching waveform shows large duty cycle jitter or the output voltage or inductor current shows oscillations, the regulation loop may be unstable. This is often a result of board layout and/or L-C combination.

As the next step in the evaluation of the regulation loop, the load transient response is tested. During the time between when the load transient takes place and the turn on of the high side switch, the output capacitor must supply all of the current required by the load. V_OUT immediately shifts by an amount equal to ΔI_(LOAD) x ESR, where ESR is the effective series resistance of C_OUT. ΔI_(LOAD) begins to charge or discharge C_OUT, generating a feedback error signal used by the regulator to return V_OUT to its steady-state value. The results are most easily interpreted when the device operates in PWM mode.

During this recovery time, V_OUT can be monitored for settling time, overshoot or ringing that helps judge the converter’s stability. Without any ringing, the loop has usually more than 60° of phase margin. Because the damping factor of the circuitry is directly related to several resistive parameters (for example, MOSFET r_DS(on)) that are temperature dependant, the loop stability analysis has to be done over the input voltage range, load current range, and temperature range.