The TPS62560/62 step-down converters operate with typically 2.25-MHz fixed-frequency pulse-width modulation (PWM) at moderate to heavy load currents. At light load currents, the converter can automatically enter power-save mode, and then operates in PFM mode. However, the TPS62561 operates with fixed-frequency PWM only, also at light load conditions.
During PWM operation, the converter uses a unique fast-response voltage-mode control scheme with input-voltage feed-forward to achieve good line and load regulation, allowing the use of small ceramic input and output capacitors. At the beginning of each clock cycle initiated by the clock signal, the high-side MOSFET switch is turned on. The current flows from the input capacitor via the high-side MOSFET switch through the inductor to the output capacitor and load. During this phase, the current ramps up until the PWM comparator trips and the control logic turns off the switch. The current-limit comparator also turns off the switch in case the current limit of the high-side MOSFET switch is exceeded. After a dead time, which prevents shoot-through current, the low-side MOSFET rectifier is turned on and the inductor current ramps down. The current flows from the inductor to the output capacitor and to the load. It returns back to the inductor through the low-side MOSFET rectifier.
The next cycle is initiated by the clock signal again turning off the low-side MOSFET rectifier and turning on the on the high-side MOSFET switch.
This feature reduces the voltage under/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.
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 VIN.
The MODE terminal allows mode selection between forced-PWM mode and power-save mode.
Connecting this terminal to GND enables the power-save mode with automatic transition between PWM and PFM modes. Pulling the MODE terminal 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 state of the MODE terminal can be changed during operation to allow efficient power management by adjusting the operation mode of the converter to the specific system requirements.
The device is enabled by setting the EN terminal to high. During the start-up time tStart Up, the internal circuits are settled and the soft-start circuit is activated. The EN input can be used to control power sequencing in a system with various dc/dc converters. The EN terminal can be connected to the output of another converter, to drive the EN terminal high to achieve a sequencing of the given supply rails. With EN = GND, the device enters shutdown mode, in which all internal circuits are disabled. In fixed-output-voltage versions, the internal resistor divider network is then disconnected from the FB terminal.
As soon as the junction temperature, TJ, 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.
The TPS62560 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 typically within 250 μs. This limits the inrush current into 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 tStart Up.
The power-save mode is enabled with the MODE terminal set to the low level. If the load current decreases, the converter enters the power-save mode of 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 positions the output voltage typically 1% above the nominal output voltage. 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 VOUT nominal + 1%, the device starts a PFM current pulse. The high-side MOSFET switch turns 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 rises. 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 is 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 modifying the charge transferred to the output capacitor by the value of the inductor. The resulting PFM output-voltage ripple and PFM frequency depend primarily on the size of the output capacitor and the inductor value. Increasing output capacitor values and inductor values minimizes 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 no longer be supported in PFM mode. The power-save mode can be disabled by setting the MODE terminal to high. The converter then operates in the fixed-frequency PWM mode.
The device starts to enter 100% duty-cycle mode once the input voltage comes close to the nominal output voltage. In order to maintain the output voltage, the high-side MOSFET switch is turned on 100% for one or more cycles.
With further decreasing VIN, 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:
The high-side and low-side MOSFET switches are short-circuit protected with maximum switch current = ILIMF. 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 after the current in the low-side MOSFET switch has decreased below the threshold of its current-limit comparator.