7.3.1.1 Setting the Boost Converter Output Voltage

The boost converter output voltage is set by the R1/R2 resistor divider, and is calculated using Equation 3.

Equation 3.

where

• V_ref = 1.146 V (the boost converter internal reference voltage).

To minimize quiescent current consumption, the value of R1 should be in the range of 100 kΩ to 1 MΩ.

7.3.1.2 Boost Converter Rectifier Diode

The reverse voltage rating of the diode must be higher than the maximum output voltage of the converter, and its average forward current rating must be higher than the output current of the boost converter. Use Equation 4 to calculate the rectifier diode repetitive peak forward current.

Equation 4.

Use Equation 5 to calculate the power dissipated in the rectifier diode.

Equation 5.

where

• V_F is the rectifier diode forward voltage.

The main diode parameters affecting converter efficiency are its forward voltage and reverse leakage current, and both should be as low as possible.

7.3.1.3 Choosing the Boost Converter Output Capacitance

The output capacitance of the boost converter smooths the output voltage and supplies transient output current demands that are outside the loop bandwidth of the converter. Generally speaking, larger output currents or smaller input supply voltages require larger output capacitances. Use Equation 6 to calculate the output voltage ripple of the boost converter.

Equation 6.

where

• C_O is the boost converter output capacitance.

7.3.1.4 Compensation

The boost converter requires a series R-C network connected between the COMP pin and ground to compensate its feedback loop. The COMP pin is the output of the boost converter's error amplifier, and the compensation capacitor determines the amplifier's low-frequency gain and the resistor its high-frequency gain. Because the converter gain changes with the input voltage, different compensation capacitors may be required: lower input voltages require a higher gain, and therefore a smaller compensation capacitor value. If an input supply voltage of the application changes (for example, if the TPS65150-Q1 device is supplied from a battery), choose compensation components suitable for a supply voltage midway between the minimum and maximum values. In all cases, verify that the values selected are suitable by performing transient tests over the full range of operating conditions.

A feed-forward capacitor C_FF in parallel with the upper feedback resistor R1 adds an additional zero to the loop response, which improves transient performance. Table 2 suggests suitable values for the cut-off frequency of the feedforward zero; however, these are only guidelines. In any application, variations in input supply voltage, inductance, and output capacitance all affect circuit operation, and the optimum value must be verified with transient tests before being finalized.

The cut-off frequency of the feed-forward zero is determined using Equation 7.

Equation 7.

where

• f_co is the cutoff frequency of the feedforward zero formed by R1 and C_FF.

7.3.1.5 Soft Start

The boost converter features a soft-start function that limits the current drawn from the input supply during start-up. During the first 2048 switching cycles, the switch current of the boost converter is limited to 40% of its maximum value; during the next 2048 cycles, it is limited to 60% of its maximum value; and after that it is as high as it must be to regulate the output voltage (up to 100% of the maximum). In typical applications, this results in a start-up time of about 5 ms (see Figure 9).

Figure 9. Boost Converter Switch Current Limit During Soft-Start

7.3.1.6 Gate Drive Signal

The GD pin provides a signal to control an external P-channel enhancement MOSFET, allowing the output of the boost converter to be isolated from its input when disabled (see Figure 36). The GD pin is an open-drain type whose output is latched low as soon as the output voltage of the boost converter reaches its power-good threshold. The GD pin goes high impedance whenever the input voltage falls below the undervoltage lockout threshold or the device shuts down as the result of a fault condition (see Adjustable Fault Delay).

7.3.2.1 Negative Charge Pump Output Voltage

The negative charge pump output voltage is set by resistors R1 and R2 and is given by Equation 8.

Equation 8.

where

• V_(REF) = 1.213 V (the voltage on the REF pin).

7.3.2.2 Negative Charge Pump Flying Capacitance

The flying capacitance transfers charge from the SUP pin to the negative charge pump output. TI recommends a flying capacitor of at least 100 nF for output currents up to 20 mA. Smaller values can be used with smaller output currents.

7.3.2.3 Negative Charge Pump Output Capacitance

The output capacitor smooths the discontinuous current delivered by the flying capacitor to generate a DC output voltage. In general, higher output currents require larger output capacitances. Use Equation 9 to calculate the negative charge pump output voltage ripple.

Equation 9.

where

• I_O is the negative charge pump output current,
• C_O is the negative charge pump output capacitance, and
• f = 1.2 MHz (the negative charge pump switching frequency).

7.3.2.4 Negative Charge Pump Diodes

The average forward current of both diodes is equal to the negative charge pump output current. If the recommended flying capacitor (or larger) is used, the repetitive peak forward current in D1 and D2 is equal to twice the output current.

7.3.3.1 Positive Charge Pump Output Voltage

The positive charge pump output voltage is set by resistors R1 and R2 and is calculated using Equation 10.

Equation 10.

where

• V_ref = 1.214 V (the positive charge pump reference voltage).

TI recommends choosing a value for R2 not greater than 1 MΩ.

7.3.3.2 Positive Charge Pump Flying Capacitance

The flying capacitance transfers charge from the SUP pin to the charge pump output. TI recommends a flying capacitor of at least 330 nF ^{The minimum recommended flying capacitance for the positive charge pump is larger than for the negative charge pump because the r} for output currents up to 20 mA. Smaller values can be used with smaller output currents.

7.3.3.3 Positive Charge Pump Output Capacitance

The output voltage ripple of the positive charge pump is given by Equation 11.

Equation 11.

where

• I_O is the output current of the positive charge pump,
• C_O is the output capacitance of the positive charge pump, and
• f = 1.2 MHz (the switching frequency of the positive charge pump).

7.3.3.4 Positive Charge Pump Diodes

The average forward current of both diodes is equal to the positive charge pump output current. If the recommended flying capacitance (or larger) is used, the repetitive peak forward current in D1 and D2 equal to twice the output current.

7.3.7.1 Boost Converter Overvoltage Protection

The boost converter features an overvoltage protection function that monitors the voltage on the SUP pin and forces the TPS65150-Q1 device to enter fault mode if the boost converter output voltage exceeds the overvoltage threshold.

7.3.7.2 Adjustable Fault Delay

The TPS65150-Q1 device detects a fault condition and shuts down if the boost converter output or either of the charge pump outputs falls out of regulation for longer than the fault delay time t_d(FDLY). Fault conditions are detected by comparing the voltage on the feedback pins with the internal power-good thresholds. Outputs that fall below their power-good threshold but recover within less than t_d(FDLY) seconds are not detected as faults and the device does not shut down in such cases. The output fault detection function is active during start-up, so the device shuts down if any of its outputs fails to reach its power-good threshold during start-up. Shut-down following an output voltage fault is a latched condition, and the input supply voltage must be cycled to recover normal operation after it occurs.

The fault detection delay time is set by the capacitor connected between the FDLY and VIN pins and is given by Equation 14.

Equation 14.

where

• R_(FDLY) = 450 kΩ (the internal resistance connected to the FDLY pin) and
• C_FDLY is the external capacitance connected to the FDLY pin.

Figure 15. Adjustable Fault Delay Time

7.3.7.3 Thermal Shutdown

A thermal shutdown is implemented to prevent damage because of excessive heat and power dissipation. Typically, the thermal shutdown threshold is 155°C. When this threshold is reached, the device enters shutdown. The device can be enabled again by cycling the input supply voltage.

7.3.7.4 Undervoltage Lockout

The TPS65150-Q1 device has an undervoltage lockout (UVLO) function. The UVLO function stops device operation if the voltage on the VIN pin is less than the UVLO threshold voltage. This makes sure that the device only operates when the supply voltage is high enough for correct operation.