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. To add higher flexibility to the selection of external component values, the device uses external loop compensation. Although the boost converter looks like a nonsynchronous boost converter topology operating in discontinuous mode at light load, the TPS6514x series maintains continuous conduction even at light load currents. This is accomplished using the Virtual Synchronous Converter Technology for improved load transient response. This architecture uses an external Schottky diode and an integrated MOSFET in parallel connected between SW and SUP (see Functional Block Diagram). The integrated MOSFET Q2 allows the inductor current to become negative at light load conditions. For this purpose, a small integrated P-channel MOSFET with typically 10-Ω r_DS(on) is sufficient. 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 Power Good Output

The TPS6514x series has an open-drain power good output with a maximum sink capability of 1 mA. The power good output goes high as soon as the main boost converter V_O1 and the negative and the positive charge pumps are within regulation. The power good output goes low as soon as one of the outputs is out of regulation. In this case, the device goes into shutdown at the same time. See Electrical Characteristics for the power good thresholds.

7.3.3 Enable and Power-On Sequencing (EN, ENR)

The device has two enable pins. These pins must be terminated and not left floating to prevent faulty 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. 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 reaches 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 reaches 91% of the nominal value, the positive charge pump comes up with the soft start. Pulling the enable pin low shuts down the device. Dependent on load current and output capacitance, each of the outputs comes down.

7.3.4 Positive Charge Pump

The TPS6514x series 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. Higher load currents are possible depending on the voltage difference between V_O1 and V_O3. See Figure 22 and Figure 23.

7.3.5 Negative Charge Pump

The TPS6514x series 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. Higher load currents are possible depending on the voltage difference between V_O1 and V_O2. See Figure 21.

7.3.6 Linear Regulator Controller

The TPS6514x series 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). Because most of the systems require this voltage rail to come up first, TI recommends using a R-C delay on EN. This delays the start-up of the main boost converter which reduces the inrush current as well.

7.3.7 Soft Start

The main boost converter as well as the charge pumps and linear regulator 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. See Figure 19 and Figure 20. 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 the 3rd step is the full current limit. The TPS65141 has an extended soft-start time where each step is 2048 clock cycles.

7.3.8 Fault Protection

All of the outputs of the TPS65140 and TPS65145 have short-circuit detection and cause the device to go into shutdown. The TPS65141, as an exception, does not enter shutdown in case one of the outputs falls below its power good threshold. The main boost converter has overvoltage and undervoltage protection. If the output voltage V_O1 rises above the overvoltage protection threshold of typically 5% of V_O1, then the device stops switching, but remains operational. When the output voltage falls below this threshold, the converter continues operation. When the output voltage falls below the undervoltage protection threshold of typically 8.75% of V_O1, because of a short-circuit condition, the TPS65140 and TPS65145 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 must 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 (UVLO) to protect the LCD panel of possible latch-up conditions due to a short-circuit condition or faulty operation. When the negative output voltage is typically above 9.5% of its output voltage (closer to ground), then the device enters shutdown. When the positive charge pump output voltage, V_O3, is below 8% typical of its output voltage, the device goes into shutdown. See the fault protection thresholds in Electrical Characteristics. The device is enabled 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 20 mA typical under a short-circuit condition when the output voltage is typically < 1 V. See Functional Block Diagram. The linear regulator does not go into shutdown under a short-circuit condition.

7.3.9 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.

7.4.1 Enabling and Disabling the Device

The TPS6514x turns on when the input voltage is higher than V_UVLO and the enable pin EN is pulled to HIGH. The device goes into shutdown and all its function apart from the linear regulator are disabled if one of these conditions is present:

• enable pin EN is pulled to LOW
• V_O1, V_O2, or V_O3 is out of regulation (only for TPS65140 and TPS65145)