8.3.1 Undervoltage Lockout

The undervoltage lockout circuit prevents the device from misoperation at low input voltages. It prevents the converter from turning on the switch or rectifier MOSFET under undefined conditions. The TPS6223x devices have a UVLO threshold set to 1.8 V (typical). Fully functional operation is permitted for input voltage down to the falling UVLO threshold level. The converter starts operation again once the input voltage trips the rising UVLO threshold level.

8.3.2 Enable and Shutdown

The device starts operation when EN is set high and starts up with the soft-start as previously described. For proper operation, the EN pin must be terminated and must not be left floating, except for the TPS622319, which has an integrated 1MΩ always active pull-down resistor.

Pulling the EN pin low forces the device into shutdown, with a shutdown quiescent current of typically 0.1 μA. In this mode, the P- and N-channel MOSFETs are turned off, the internal resistor feedback divider is disconnected, and the entire internal-control circuitry is switched off.

The EN input can be used to control power sequencing in a system with various DC – DC converters. The EN pin can be connected to the output of another converter, to drive the EN pin high and getting a sequencing of supply rails.

8.3.3 Thermal Shutdown

As soon as the junction temperature, T_J, exceeds 150°C (typical) the device goes into thermal shutdown. In this mode, the high-side and low-side MOSFETs are turned off. The device continues its operation when the junction temperature falls below the thermal shutdown hysteresis.

8.4.1 Soft-Start

The TPS6223x has an internal soft start circuit that controls the ramp up of the output voltage and limits the inrush current during start-up. This limits input voltage drops when a battery or a high-impedance power source is connected to the input of the converter.

The soft-start system generates a monotonic ramp up of the output voltage and reaches the nominal output voltage typically 100 μs after EN pin was pulled high.

If the output voltage does not reached its target value by this time, such as in the case of heavy load, the converter then operates in a current limit mode set by its switch current limits.

TPS6223x is able to start into a prebiased output capacitor. The converter starts with the applied bias voltage and ramps the output voltage to its nominal value.

8.4.2 Power-Save Mode

Connecting the MODE pin to GND enables the automatic PWM and power-save mode operation. The converter operates in quasi-fixed frequency PWM mode at moderate to heavy loads and in the pulse frequency modulation (PFM) mode during light loads, which maintains high efficiency over a wide load current range. In PFM mode, the device starts to skip switch pulses and generates only single pulses with an ON-time of t_ONmin. The PFM mode frequency depends on the load current and the external inductor and output capacitor values. The PFM mode of TPS6223x is optimized for low-output voltage ripple if small external components are used. Even at low output currents, the PFM frequency is above the audible noise spectrum and makes this operation mode suitable for audio applications.

The ON-time t_ONmin can be estimated to:

Equation 1.

Therefore, the peak inductor current in PFM mode is approximately:

Equation 2.

The transition from PFM into PWM mode and vice versa can be estimated to:

Equation 3.

where

• t_ON: High-side switch ON-time [ns]
• V_IN: Input voltage [V]
• V_OUT: Output voltage [V]
• L: Inductance [μH]
• I_LPFMpeak: PFM inductor peak current [mA]
• I_{OUT_PFM/PWM}: Output current for PFM to PWM mode transition and vice versa [mA]

8.4.3 Forced PWM Mode

Pulling the MODE pin high forces the converter to operate in a continuous conduction PWM mode even at light load currents. The advantage is that the converter operates with a quasi-fixed frequency that allows simple filtering of the switching frequency for noise-sensitive applications. In this mode, the efficiency is lower compared to the power save mode during light loads. This pin must be terminated except for the TPS622319 , which has an integrated 1MΩ always active pull-down resistor.

For additional flexibility, it is possible to switch from power-save mode to forced PWM mode during operation. This allows efficient power management by adjusting the operation of the converter to the specific system requirements.

8.4.4 100% Duty Cycle Low Dropout Operation

The device starts to enter 100% duty cycle mode once the input voltage comes close to the nominal output voltage. To maintain the output voltage, the high side switch is turned on 100% for one or more cycles.

With further decreasing V_IN the high-side MOSFET switch is turned on completely. In this case the converter offers a low input-to-output voltage difference. This is particularly useful in battery-powered applications to achieve longest operation time by taking full advantage of the whole battery voltage range.

The minimum input voltage to maintain regulation depends on the load current and output voltage, and can be calculated as:

Equation 4.

where

• I_OUTmax: maximum output current plus inductor ripple current
• R_DS(on)max: maximum P-channel switch RDSon
• R_L: DC resistance of the inductor
• V_OUTmax: nominal output voltage plus maximum output voltage tolerance

8.4.5 Short Circuit Protection

The TPS6223x integrates a high-side and low-side MOSFET current limit to protect the device against heavy load or short circuit. The current in the switches is monitored by current limit comparators. When the current in the P-channel MOSFET reaches its current limit, the P-channel MOSFET is turned off and the N-channel MOSFET is turned on to ramp down the current in the inductor. The high-side MOSFET switch can only turn on again, once the current in the low side MOSFET switch has decreased below the threshold of its current limit comparator.