7.3.1 Dynamic Voltage Positioning

This feature reduces the voltage undershoots and overshoots at load steps from light to heavy load and vice versa. It is active in power-save mode and regulates the output voltage 1% higher than the nominal value. This provides more headroom for both the voltage drop at a load step, and the voltage increase at a load throw-off.

Figure 5. Power-Save Mode Operation

7.3.2 Undervoltage Lockout

The undervoltage lockout circuit prevents the device from malfunctioning at low input voltages and from excessive discharge of the battery and disables the output stage of the converter. The undervoltage lockout threshold is typically 1.85 V with falling V_IN.

7.3.3 Mode Selection

The MODE pin allows mode selection between forced PWM mode and power-save mode.

Connecting this pin to GND enables the power-save mode with automatic transition between PWM and PFM mode. Pulling the MODE pin high forces the converter to operate in fixed frequency PWM mode even at light load currents. This 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.

The condition of the MODE pin can be changed during operation and allows efficient power management by adjusting the operation mode of the converter to the specific system requirements.

7.3.4 Enable

The device is enabled setting EN pin to high. During the start-up time t_{Start Up} the internal circuits are settled. Afterwards, the device activates the soft-start circuit. 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. With EN = GND, the device enters shutdown mode. In this mode, all circuits are disabled. In fixed output voltage versions, the internal resistor divider network is disconnected from FB pin.

7.3.5 Thermal Shutdown

As soon as the junction temperature T_J exceeds 140°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.

7.4.1 Soft-Start

The TPS62590 has an internal soft-start circuit that controls the ramp up of the output voltage. The output voltage ramps up from 5% to 95% of its nominal value within typical 250 μs. This limits the inrush current in the converter during ramp up and prevents possible input voltage drops when a battery or high impedance power source is used. The soft start circuit is enabled within the start up time t_{Start Up}.

7.4.2 Power-Save Mode

The power-save mode is enabled with MODE pin set to low level. If the load current decreases, the converter will enter power save mode operation automatically. During power save mode the converter skips switching and operates with reduced frequency in PFM mode with a minimum quiescent current to maintain high efficiency. The converter will position the output voltage +1% above the nominal output voltage typically. This voltage positioning feature minimizes voltage drops caused by a sudden load step.

The transition from PWM mode to PFM mode occurs once the inductor current in the low-side MOSFET switch becomes zero, which indicates discontinuous conduction mode.

During the power-save mode the output voltage is monitored with a PFM comparator. As the output voltage falls below the PFM comparator threshold of V_OUT nominal +1%, the device starts a PFM current pulse. For this the high-side MOSFET switch will turn on and the inductor current ramps up. After the ON-time expires, the switch is turned off and the low-side MOSFET switch is turned on until the inductor current becomes zero.

The converter effectively delivers a current to the output capacitor and the load. If the load is below the delivered current, the output voltage will rise. If the output voltage is equal to or higher than the PFM comparator threshold, the device stops switching and enters a sleep mode with typical 15-μA current consumption.

If the output voltage is still below the PFM comparator threshold, a sequence of further PFM current pulses are generated until the PFM comparator threshold is reached. The converter starts switching again once the output voltage drops below the PFM comparator threshold.

With a fast single threshold comparator, the output voltage ripple during PFM mode operation can be kept small. The PFM pulse is time controlled, which allows to modify the charge transferred to the output capacitor by the value of the inductor. The resulting PFM output voltage ripple and PFM frequency depend in first order on the size of the output capacitor and the inductor value. Increasing output capacitor values and inductor values will minimize the output ripple. The PFM frequency decreases with smaller inductor values and increases with larger values.

The PFM mode is left and PWM mode entered in case the output current can not longer be supported in PFM mode. The power save mode can be disabled through the MODE pin set to high. The converter will then operate in fixed frequency PWM mode.

7.4.3 100% Duty Cycle Low Dropout Operation

The device starts to enter 100% duty cycle mode once the input voltage comes close the nominal output voltage. To maintain the output voltage, the high-side MOSFET 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 by Equation 1:

7.4.4 Short-Circuit Protection

The high-side and low side MOSFET switches are short-circuit protected with maximum switch current equal to I_LIMF. The current in the switches is monitored by current limit comparators. Once the current in the high-side MOSFET switch exceeds the threshold of its current limit comparator, it turns off and the low-side MOSFET switch is activated to ramp down the current in the inductor and high-side MOSFET switch. 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.