Table 2. Design Parameters

9.2.2.1 Step-by-Step Design Procedure

The following design procedure provides an example for selecting component values for the TPS2505.

The following design parameters are needed as inputs to the design process.

• Input voltage range
• Output voltage on AUX
• Input ripple voltage
• Output ripple voltage on AUX
• Output current rating of AUX rail
• Output current rating of USB rail
• Nominal efficiency target
• Operating frequency

A power inductor, input and output filter capacitors, and current-limit threshold resistor are the only external components required to complete the TPS2505 boost-converter design. The input ripple voltage, AUX ripple voltage, and total output current affect the selection of these components.

9.2.2.2 Switching Frequency

The switching frequency of the TPS2505 is internally fixed at 1 MHz for the specified V_IN range.

9.2.2.3 AUX Voltage

The AUX voltage of the TPS2505 is internally fixed at 5.1 V.

9.2.2.4 Determine Maximum Total Current (I_AUX + I_LDO + I_USB1 + I_USB2 )

Using Figure 1, the maximum total current at V_IN = 2.7 V is 1 A using the conservative line. The design requirements are met for this application.

9.2.2.5 Power Inductor

The inductor ripple current, Δi, should be at least 20% of the average inductor current to avoid erratic operation of the peak-current-mode PWM controller. Assume an inductor ripple current, Δi, which is 30% of the average inductor current and a power-converter efficiency, η, of 90%. Using the minimum input voltage, the average inductor current at V_IN = 2.7 V is:

Equation 4.

Figure 11. Waveform of Current in Boost Inductor

The corresponding inductor ripple current is:

Equation 5.

Verify that the peak inductor current is less than the 3-A peak switch current:

Equation 6.

The following equation estimates the duty cycle of the low-side SWN MOSFET:

Equation 7.

where

• R_SWN is the low-side control MOSFET ON-resistance
• R_SWP is the high-side synchronous MOSFET ON-resistance
• R_L is an estimate of the inductor DC resistance

The following equation calculates the recommended inductance for this design.

Equation 8.

The RMS inductor current is:

Equation 9.

Select a Coilcraft LPS4018-222ML inductor. This 2.2-µH inductor has a saturation current rating of 2.7 A and an RMS current rating of 2.3 A. See Component Recommendations for specific additional information.

9.2.2.6 Output AUX Capacitor Selection

The AUX output capacitor, C_AUX, discharges during the PWM MOSFET on-time, resulting in an output ripple voltage of ΔV_AUX. ΔV_AUX is largest at maximum load current.

Equation 10.

Equation 11.

Ceramic capacitors exhibit a DC bias effect, whereby the capacitance falls with increasing bias voltage. The effect is worse for capacitors in smaller case sizes and lower voltage ratings. X5R and X7R capacitors exhibit less DC bias effect than Y5V and Z5U capacitors.

Select a TDK C3225X5R1A226M 22-µF, 10-V X5R ceramic capacitor to allow for a 50% drop in capacitance due to the DC bias effect. See Component Recommendations for specific additional information.

9.2.2.7 Output USB1/2 Capacitor Selection

The USB1/2 output capacitors provide energy during a load step on the USB outputs. The TPS2505 does not require a USB output capacitor, but many USB applications require that downstream-facing ports be bypassed with a minimum of 120-μF, low-ESR capacitance.

Select a Panasonic EEVFK1A151P 150-μF, 10-V capacitor.

9.2.2.8 Input Capacitor Selection

The ripple current through the input filter capacitor is equal to the ripple current through the inductor. If the ESL and ESR of the input filter capacitor are ignored, then the required input filter capacitance is:

Equation 12.

Select a TDK C2012X5R1A106K 10-µF, 10-V, X5R, size 805 ceramic capacitor. The capacitance drops 20% at 3.3-V bias, resulting in an effective capacitance of 8 µF.

An additional 0.1-µF ceramic capacitor should be placed locally from IN to GND to prevent noise from coupling into the device if the input capacitor cannot be located physically near to the device.

In applications where long, inductive cables connect the input power supply to the device, additional bulk input capacitance may be necessary to minimize voltage overshoot. See Component Recommendations for specific additional information.

9.2.2.8.1 Current-Limit Threshold Resistor R_ILIM

The current-limit threshold I_OS of the power switches are externally adjustable by selecting the RILIM1/2 resistors. To eliminate the possibility of false tripping, RILIM1/2 should be selected so that the minimum tolerance of the current-limit threshold is greater than the maximum specified USB load, I_USB.

It is also important to account for I_OS shifts due to variation in VIN and I_AUX. This shift due to the additional loading in AUX can add up to ±75 mA of variation to the I_OS as calculated according to Programming the Current-Limit Threshold Resistor RILIM. Select RILIM1 so that the minimum current-limit threshold equals 600 mA to ensure a minimum I_USB1 current-limit threshold of 525 mA. In the same way, select RILIM2 so that the minimum current-limit threshold equals 200 mA to ensure a minimum I_USB2 current-limit threshold of 125 mA.

Equation 13.

Equation 14.

Equation 15.

Choose the next smaller 1% resistor, which are 34.8 kΩ for RLIM1 and 95.3 kΩ for RLIM2.