TPS63070RNMT

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1. Description

The TPS63070RNMT is a high efficiency, low quiescent current buck-boost converter suitable for applications where the input voltage can be higher or lower than the output voltage. Output currents can go as high as 2 A in boost mode and in buck mode. The buck-boost converter is based on a fixed frequency, pulse-widthmodulation (PWM) controller using synchronous rectification to obtain maximum efficiency. At low load currents, the converter enters Power Save Mode to maintain high efficiency over a wide load current range. The converter can be disabled to minimize battery drain. During shutdown, the load is disconnected from the battery. The device is available in a 2.5 mm x 3 mm QFN package.

2. Features

    1. Input Voltage Range: 2.0 V to 16 V

    2. Output Voltage Range: 2.5 V to 9 V

    3. Up to 95% Efficiency

    4. +/- 1% dc accuracy in PWM mode

    5. +3% / -1% dc accuracy in PFM mode

    6. 2 A Output Current in Buck Mode

    7. 2 A Output Current in Boost Mode (VIN = 4 V; Vout = 5 V)

    8. Precise ENABLE input allows

        – user defined undervoltage lockout

        – exact sequencing

    9. Automatic Transition Between Step Down and Boost Mode

  10. Typical Device Quiescent Current: 50 μA

  11. Fixed and Adjustable Output Voltage Options

  12. Output Discharge Option

  13. Power Save Mode for Improved Efficiency at Low Output Power

  14. Forced Fixed Frequency Operation at 2.4 MHz and Synchronization Option

  15. Power Good Output

  16. VSEL simply allows output voltage change

  17. Load Disconnect During Shutdown

  18. Overtemperature Protection

  19. Input / Output Overvoltage Protection

  20. Available in QFN Package

3. Applications

    1. Dual Li-Ion Applications

    2. Industrial Metering Equipment

    3. DSC's and Camcorders

    4. Notebook Computers

    5. Ultra Mobile PC's and Mobile Internet Devices

    6. Personal Medical Products

4. Pin Configuration 

5. Pin Functions

6. Control Loop Description

The controller circuit of the device is based on an average current mode topology. The average inductor current is regulated by a fast current regulator loop which is controlled by a voltage control loop. The non inverting input of the transconductance amplifier gmv can be assumed to be constant. The output of gmv defines the average inductor current. The inductor current is reconstructed by measuring the current through the high side buck MOSFET. This current corresponds exactly to the inductor current in boost mode. In buck mode, the current is measured during the on-time of the same MOSFET. During the off-time, the current is reconstructed internally starting from the peak value reached at the end of the on-time cycle. The average current is then compared to the desired value and the difference, or current error, is amplified and compared to the sawtooth ramp of either the Buck or the Boost. Depending on which of the two ramps is crossed by the signal, either the Buck MOSFETs or the Boost MOSFETs are activated. When the input voltage is close to the output voltage, one buck cycle is followed by a boost cycle. In this condition, not more than three cycle in a row of the same mode are allowed. This control method in the buck-boost region ensures a robust control and the highest efficiency. For an input voltage above 9 V, and Vout below 2.2 V, the switching frequency is reduced by a factor of 2 to keep the minimum on-time at a reasonable value. For short circuit protection, at an output voltage below 1.2V, the low side input FET and the high side output FET are not actively switched but their back-gate diode used for conduction. TPS6307x also contains a negative current limit. This allows the inductor current to reverse and flow from the output to the input. This is required for forced PWM operation at low output current but also for applications that require a fairly high current from the output to the input like TEC (thermo electric cooling) applications where the TEC cell is placed between input and output of the converter,

7. Power Good

The device has a built in power good output that indicates whether the output voltage has reached its nominal value. The PG signal is generated based on the status of the output voltage monitor. The power good circuit operates as long as the converter is enabled and VIN is above the undervoltage lockout threshold. If the output voltage has not reached the regulated condition, the PG pin is held low. When the regulated condition is reached, PG is high impedance. The PG output needs an external pull-up resistor. This resistor can be pulled to any voltage up to the maximum output voltage rating.

8. Soft Start

To minimize inrush current during start up, the device has a soft start. When the EN pin is set high, after a thermal shutdown or after the undervoltage lockout threshold is exceeded, a soft-start cycle is started and the input current is ramped until the output voltage reaches regulation. The device ramps up the output voltage in a controlled manner, even if a large capacitor is connected at the output. During soft-start, as long as the output voltage is below the power good threshold, the input current limit is reduced to typically 1A. The soft-start time is defined by the current limit during the soft-start phase along with the load current, output capacitance and the input to output voltage ratio.

9. Inductor Selection 

For high efficiencies, the inductor should have a low dc resistance to minimize conduction losses. Especially athigh switching frequencies, the core material has a higher impact on efficiency. When using small chip inductors,the efficiency is reduced mainly due to higher inductor core losses. This needs to be considered when selectingthe appropriate inductor. The inductor value determines the inductor ripple current. The larger the inductor value,the smaller the inductor ripple current and the lower the conduction losses of the converter. Conversely, largerinductor values cause a slower load transient response. To avoid saturation of the inductor, the peak current forthe inductor in steady state operation is calculated using Equation 8. Only the equation which defines the switchcurrent in boost mode is shown, because this provides the highest value of current and represents the criticalcurrent value for selecting the right inductor