8.3.1 Boost Converter

The boost converter uses a fixed-frequency current-mode topology, and its output voltage (V_POS) is fixed at 4.6 V

For the highest output voltage accuracy, connect the output sense pin (FBS) directly to the positive pin of the output capacitor. If not used, the FBS pin can be left floating or connected to ground. If the FBS pin is not used, the boost converter senses its output voltage using the OUTP pin.

8.3.2 Inverting Buck-Boost Converter

The inverting buck-boost converter uses a constant-off-time peak-current mode topology. The converter's default output voltage (V_NEG) is –4 V, but it can be programmed to any voltage in the range –1.4 V to –4.4  V (see Programming VNEG).

8.3.2.1 Programming V_NEG

The output voltage of the inverting buck-boost converter (V_NEG) can be programmed using the CTRL pin. If output voltage programming is not required, the CTRL pin can be used as a standard enable pin (see Enable (CTRL)).

Figure 6. Programming V_NEG Using the CTRL Pin

When the CTRL pin is pulled high, the inverting buck-boost converter starts up with its default voltage of –4V. The device now counts the rising edges applied to the CTRL pin and sets the output voltage (V_NEG) according to Table 1. For the timing diagram shown in Figure 6, V_NEG is programmed to –4.2 V, since three rising edges are detected.

The CTRL interface is designed to work with pulses whose duration is between 2 µs and 25 µs. Pulses shorter than 2 µs or longer than 25 µs are not ensured to be recognized.

Table 1. Programming Table for V_NEG

Number of Rising Edges	VNEG	Number of Rising Edges	VNEG
0 / no pulses	–4 V	16	–2.9 V
1	–4.4 V	17	–2.8 V
2	–4.3 V	18	–2.7 V
3	–4.2 V	19	–2.6 V
4	–4.1 V	20	–2.5 V
5	–4.0 V	21	–2.4 V
6	–3.9 V	22	–2.3 V
7	–3.8 V	23	–2.2 V
8	–3.7 V	24	–2.1 V
9	–3.6 V	25	–2.0 V
10	–3.5 V	26	–1.9 V
11	–3.4 V	27	–1.8 V
12	–3.3 V	28	–1.7 V
13	–3.2 V	29	–1.6 V
14	–3.1 V	30	–1.5 V
15	–3.0 V	31	–1.4 V

8.3.2.2 Controlling the V_NEG Transition Time

The transition time (t_set) is the time required to move V_NEG from one voltage level to the next. Users can control the transition time by connecting a capacitor between the CT pin and ground. When the CT pin is left open or is connected to ground, the transition time is as short as possible. When a capacitor is connected to the CT pin, the transition time is determined by the time constant (τ) of the external capacitor (C_(CT)) and the internal resistance of the CT pin (R_(CT)). The output voltage V_NEG reaches 70% of its programmed value after 1τ.

An example is given below for the case when using 100 nF for C_(CT).

Equation 1. τ ≈ t_set(70%) = R_(CT) × C_(CT) = 300 kΩ × 100 nF = 30 ms

The output voltage V_NEG reaches its programmed value after approximately 3τ.

The external capacitor connected to the CT pin has no effect on the first programming of V_NEG, when the inverting buck-boost converter ramps its output to the default voltage as fast as possible. Figure 7 shows the detail of programming of the V_NEG transition time with the CT pin during start-up.

Figure 7. Programming the Transition Time of V_NEG

8.3.3 Soft-Start and Start-Up Sequence

The TPS65631 features a soft-start function to limit inrush current. When the device is enabled by a high-level signal applied to the CTRL pin, the boost converter starts switching with a reduced switch current limit. Ten milliseconds after the CTRL pin goes high, the inverting buck-boost converter starts with a default value of –4 V. A typical start-up sequence is shown in Figure 8.

Figure 8. Typical Start-Up Sequence

8.3.4 Enable (CTRL)

The CTRL pin serves two functions. One is to enable and disable the device, and the other is to program the output voltage (V_NEG) of the inverting buck-boost converter (see Programming VNEG). If the digital interface is not required, the CTRL pin can be used as a standard enable pin for the device, which will come up with its default value on V_NEG of –4 V. When CTRL is pulled high, the device is enabled. The device is shut down with CTRL low.

8.3.5 Undervoltage Lockout

The TPS65631 features an undervoltage lockout function that disables the device when the input supply voltage is too low for normal operation.

8.3.6 Short Circuit Protection

The TPS65631 is protected against short-circuits of V_POS and V_NEG to ground and to each other.

8.3.7 Output Discharge During Shutdown

The TPS65631 actively discharges its outputs during shutdown. Figure 9 shows the output discharge control.

Figure 9. Active Discharge of V_POS and V_NEG During Shutdown

8.3.8 Thermal Shutdown

The TPS65631 enters thermal shutdown mode if its junction temperature exceeds 145°C (typical). During thermal shutdown mode none of the device functions are available. To resume normal operation, either cycle the input supply voltage or toggle the CTRL pin low and then high again.

8.4.1 Operation with V_I < 2.9 V

The recommended minimum input supply voltage for full performance is 2.9 V. The device continues to operate with input supply voltages below 2.9 V; however, full performance is not guaranteed. The device does not operate with input supply voltages below the UVLO threshold.

8.4.2 Operation with V_I ≈ V_POS (Diode Mode)

The TPS65631 features a "diode" mode that enables it to regulate its output voltage even when the input supply voltage is close to V_POS (that is, too high for normal boost operation). When operating in diode mode the converter's high-side switch stops switching and its body diode is used as the rectifier. Boost converter efficiency is reduced when operating in diode mode. At low output currents (≈2 mA and below), the boost converter automatically transitions from pulse-width modulation to pulse-skip mode. This ensures that V_POS stays in regulation but increases the output voltage ripple on V_POS.

8.4.3 Operation with CTRL

When a low-level signal is applied to the CTRL pin the device is disabled and switching is inhibited. When the input supply voltage is above the UVLO threshold and a high-level signal is applied to the CTRL pin the device is enabled and its start-up sequence begins.