7.3.1 Main Boost Converter

The main boost converter operates with PWM and a fixed switching frequency of 1.6 MHz. The converter uses a unique fast-response voltage-mode controller scheme with input voltage feedforward. This achieves excellent line and load regulation (0.2%/A load regulation typical) and allows the use of small external components. For higher flexibility to the selection of external component values the device uses external loop compensation. The TPS65100-Q1 device maintains continuous conduction even at light load currents because of an internal PMOS in parallel connected between SW and SUP pin. When the inductor current is positive, the external schottky diode with the lower forward voltage conducts the current. This causes the converter to operate with a fixed frequency in continuous conduction mode over the entire load current range. This avoids the ringing on the switch pin as seen with a standard nonsynchronous boost converter and allows a simpler compensation for the boost converter.

7.3.2 VCOM Buffer

VCOMIN is the input of the VCOM buffer. If VCOM is not required connect VCOMIN pin to Ground and thereby reduce the overall quiescent current.

The VCOM buffer features soft start to avoid a large voltage drop at V_O1. During operation the VCOMIN pin cannot be pulled dynamically to ground.

7.3.3 Positive Charge Pump

The TPS65100-Q1 device has a fully regulated integrated positive charge pump generating V_O3. The input voltage for the charge pump is applied to the SUP pin that is equal to the output of the main boost converter V_O1. The charge pump is capable of supplying a minimum load current of 20 mA. Depending on the voltage difference between V_O1 and V_O3 higher load currents are possible (see Figure 17 and Figure 18).

7.3.4 Negative Charge Pump

The TPS65100-Q1 device has a regulated negative charge pump using two external Schottky diodes. The input voltage for the charge pump is applied to the SUP pin that is connected to the output of the main boost converter V_O1. The charge pump inverts the main boost converter output voltage and is capable of supplying a minimum load current of 20 mA. Depending on the voltage difference between V_O1 and V_O2, higher load currents are possible (see Figure 16).

7.3.5 Linear Regulator Controller

The TPS65100-Q1 device includes a linear regulator controller to generate a 3.3-V rail which is useful when the system is powered from a 5-V supply. The regulator is independent from the other voltage rails of the device and has its own enable (ENR). Since most of the systems require this voltage rail to come up first it is recommended to use a R-C delay on EN. This delays the start-up of the main boost converter which will reduce the inrush current as well.

7.4.1 Enable and Power-ON Sequencing (EN, ENR)

The device has two enable pins. These pins should be terminated and should not be left floating to prevent unpredictable operation. Pulling the enable pin (EN) high enables the device and starts the power-on sequencing with the main boost converter V_O1 coming up first, then the negative and positive charge pump and the VCOM buffer. If the VCOMIN pin is held low, the VCOM buffer remains disabled. The linear regulator has an independent enable pin (ENR). Pulling this pin low disables the regulator, and pulling this pin high enables this regulator.

If the enable pin EN is pulled high, the device starts its power-ON sequencing. The main boost converter starts up first with its soft start. If the output voltage has reached 91.25% of its output voltage, the negative charge pump comes up next. The negative charge pump starts with a soft start and when the output voltage has reached 91% of the nominal value, the positive charge pump comes up with a soft start. The VCOM buffer is enabled as soon as the positive charge pump has reached its nominal value and VCOMIN is greater than typically 1 V. Pulling the enable pin low shuts down the device. Depended on load current and output capacitance, each of the outputs goes down.

7.4.2 Soft Start

The main boost converter as well as the charge pumps, linear regulator, and VCOM buffer have an internal soft start. This avoids heavy voltage drops at the input voltage rail or at the output of the main boost converter V_O1 during start-up caused by high inrush currents (see Figure 14 and Figure 15). During soft start of the main boost converter V_O1, the internal current limit threshold is increased in three steps. The device starts with the first step, where the current limit is set to 2/5 of the typical current limit (2/5 of 2.3 A) for 1024 clock cycles, then increased to 3/5 of the current limit for 1024 clock cycles, and finally raised to the full current limit.

7.4.3 Fault Protection

All the outputs of the TPS65100-Q1 device have short-circuit detection that can force the device into shutdown. The main boost converter has overvoltage and undervoltage protection. If the output voltage V_O1 rises above the overvoltage protection threshold of 105% of V_O1 (typical), the device stops switching but remains operational. When the output voltage falls below this threshold again, the converter continues operation. When the output voltage falls below power good threshold of 91.25% of V_O1 (typical), in case of a short-circuit condition, then the TPS65100-Q1 device goes into shutdown. Because there is a direct pass from the input to the output through the diode, the short-circuit condition remains. If this condition needs to be avoided, a fuse at the input or an output disconnect using a single transistor and resistor is required. The negative and positive charge pumps have an undervoltage lockout to protect the LCD panel from possible latchup conditions in the event of a short-circuit condition or faulty operation. When the negative output voltage is less than 90.5% (typical) of its output voltage (closer to ground), the device enters shutdown. When the positive charge pump output voltage V_O3 is below 92% (typical) of its output voltage, the device goes into shutdown as well. See the electrical characteristics table under fault protection thresholds. The device can be enabled again by toggling the enable pin (EN) below 0.4 V or by cycling the input voltage below the UVLO of 1.7 V. The linear regulator reduces the output current to typical 20 mA under a short-circuit condition when the output voltage is < 1 V (typical). See the functional block diagram. The linear regulator does not go into shutdown under a short-circuit condition.

7.4.4 Thermal Shutdown

A thermal shutdown is implemented to prevent damage due to excessive heat and power dissipation. Typically, the thermal shutdown threshold is 160°C. If this temperature is reached, the device goes into shutdown. The device can be enabled by toggling the enable pin to low and back to high or by cycling the input voltage to GND and back to V_I again.