Table 1. Design Parameters

8.2.1.2.1 Boost Converter Design Procedure

The first step in the design procedure is to calculate the maximum possible output current of the main boost converter under certain input and output voltage conditions. This example is for a 3.3-V to 10-V conversion:

1. Duty cycle
Equation 1.
2. Average inductor current
Equation 2.
3. Inductor peak-to-peak ripple current
Equation 3.
4. Peak switch current
Equation 4.

The integrated switch, the inductor, and the external Schottky diode must be able to handle the peak switch current. The calculated peak switch current must be equal to or lower than the minimum N-MOSFET switch current limit specified in Electrical Characteristics. If the peak switch current is higher, the converter cannot support the required load current. This calculation must be done for the minimum input voltage, where the peak switch current is highest. The calculation includes conduction losses like switch r_DS(ON) (0.5 V) and diode forward drop voltage losses (0.8 V). Additional switching losses and inductor core and winding losses require a slightly higher peak switch current in the actual application. This calculation still allows for good design and component selection.

8.2.1.2.1.2 Output Capacitor Selection

For the best output voltage filtering, TI recommends a low ESR output capacitor. Ceramic capacitors have a low ESR value, but depending on the application, tantalum capacitors can be used as well. A 22-μF ceramic output capacitor works for most of the applications. Higher capacitor values can be used to improve the load transient regulation. The output voltage ripple can be calculated as:

Equation 5.

where

• I_P = Peak switch current as calculated in the previous section with I_SW(peak)
• L = Selected inductor value
• I_O = Normal load current
• ƒ_S = Switching frequency
• V_D = Rectifier diode forward voltage (typical 0.3 V)
• C_O = Selected output capacitor
• ESR = Output capacitor ESR value

8.2.1.2.1.7 Setting the Output Voltage and Selecting the Feedforward Capacitor

The output voltage is set by the external resistor divider and is calculated as:

Equation 6.

Across the upper resistor a bypass capacitor is required to speed up the circuit during load transients as shown in Figure 5.

Figure 5. Feed-forward Capacitor

Together with R1 the bypass capacitor C8 sets a zero in the control loop at approximately 50 kHz:

Equation 7.

A value closest to the calculated value should be used. Larger feedforward capacitor values reduce the load regulation of the converter and cause load steps as shown in Figure 6.

Figure 6. Load Step Caused By A Too Large Feed-forward Capacitor Value

For more information on how to calculate a Boost Converter's Output Stage refer to Basic Calculation of a Boost Converter's Power Stage.

8.2.1.2.2 Negative Charge Pump

The negative charge pump provides a regulated output voltage by inverting the main output voltage V_O1. The negative charge pump output voltage within its output voltage range is set with external feedback resistors.

The maximum load current of the negative charge pump depends on the voltage drop across the external Schottky diodes and the internal on resistance of the charge pump MOSFETS. When the voltage drop across these components is larger than the voltage difference from V_O1 to V_O2, the charge pump is in dropout, providing the maximum possible output current. Therefore, the higher the voltage difference between V_O1 and V_O2, the higher the possible load current. See Figure 16 for the possible output current versus boost converter voltage V_O1.

Estimating the maximum output voltage range of a single negative charge-pump stage:

Setting the output voltage:

Equation 9.

Equation 10.

The lower feedback resistor value R4 should be in a range between 40 kΩ to 120 kΩ or the overall feedback resistance should be within 500 kΩ to 1 MΩ. Smaller values load the reference too heavily and larger values may cause stability problems. For this design, 619 kΩ and 150 kΩ were chosen, delivering –5 V. The negative charge pump requires two external-Schottky diodes. The peak current rating of the Schottky diode has to be twice the load current of the output. For a 20-mA output current, the dual Schottky diode BAT54 or similar is a good choice.

8.2.1.2.3 Positive Charge Pump

The positive charge pump can be operated in a voltage doubler mode or a voltage tripler mode.

The output voltage needs to be within the voltage ranges of the configuration, see Voltage Doubler Mode and Voltage Tripler Mode. The output voltage within its limitation is set by the external resistor divider and is calculated as:

Equation 11.

Equation 12.

The maximum load current of the positive charge pump depends on the voltage drop across the internal Schottky diodes, the internal ON-resistance of the charge pump MOSFETS, and the impedance of the flying capacitor. When the voltage drop across these components is larger than the voltage difference V_O1 × 2 to V_O3 (doubler mode) or V_O1 × 3 to V_O3 (tripler mode), then the charge pump is in dropout, providing the maximum possible output current. Therefore, the higher the voltage difference between V_O1 x 2 (doubler) or V_O1 × 3 (tripler) to V_O3, the higher the possible load current. See Figure 17 and Figure 18 for output current versus boost converter voltage, V_O1, and the following calculations.

8.2.1.2.4 VCOM Buffer

The VCOM buffer is typically used to drive the backplane of a TFT panel. The VCOM output voltage is typically set to half of the main output voltage V_O1 plus a small shift to implement the specific compensation voltage. The TFT video signal gets coupled through the TFT storage capacitor plus the LCD cell capacitance to the output of the VCOM buffer. Because of these, short current pulses in the positive and negative direction appear at the output of the VCOM buffer. To minimize the output voltage ripple caused by the current pulses, a transconductance amplifier having a current source output and an output capacitor is used. The output capacitor supports the high frequency part of the current pulses drawn from the LCD panel. The VCOM buffer only needs to handle the low frequency portion of the current pulses. A 1-μF ceramic output capacitor is sufficient for most of the applications. When using other output capacitor values it is important to keep in mind that the output capacitor is part of the VCOM buffer loop stabilization.

The VCOM buffer has an integrated soft start to avoid voltage drops on V_O1 during start-up. The soft start is implemented as such that the VCOMIN is held low until the VCOM buffer is fully biased and the common mode range is reached. Then the positive input is released and the VCOM buffer output slowly comes up. Usually a 1-nF capacitor on VCOMIN to GND is used to filter high frequency noise coupled in from V_O1. The size of this capacitor together with the upper feedback resistor value determines the start-up time. The larger the capacitor from VCOMIN to GND, the slower the soft start.

8.2.1.2.5 Linear Regulator Controller

The TPS65100-Q1 device includes a linear regulator controller to generate a 3.3-V rail when the system is powered from a 5-V supply. Because an external NPN transistor is required, the input voltage of the TPS65100-Q1 device applied to VIN needs to be higher than the output voltage of the regulator. To provide a minimum base drive current of 13.5 mA, a minimum internal voltage drop of 500 mV from V_I to V_base is required. This can be translated into a minimum input voltage on V_I for a certain output voltage as Equation 13 shows:

The current design operating from V_I = 3.3 V cannot support this and the linear regulator is not used. The second example operating from V_I = 5 V has the 3.3-V regulator implemented.

The base drive current together with the h_FE of the external transistor determines the possible output current. External transistors are selected depending on the output current, power dissipation, and PCB space requirements of the application. The device is stable with a 4.7-μF ceramic output capacitor. Larger output capacitor values can be used to improve the load transient response when higher load currents are required.

Table 2. Table Of Graphs