SLVSBI5A May   2013  – October 2014 TPS54531

PRODUCTION DATA.  

  1. Features
  2. Applications
  3. Description
  4. Simplified Schematic
  5. Revision History
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Handling Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Fixed-Frequency PWM Control
      2. 8.3.2  Voltage Reference (Vref)
      3. 8.3.3  Bootstrap Voltage (BOOT)
      4. 8.3.4  Enable and Adjustable Input Undervoltage Lockout (VIN UVLO)
      5. 8.3.5  Programmable Slow Start Using SS Pin
      6. 8.3.6  Error Amplifier
      7. 8.3.7  Slope Compensation
      8. 8.3.8  Current-Mode Compensation Design
      9. 8.3.9  Overcurrent Protection and Frequency Shift
      10. 8.3.10 Overvoltage Transient Protection
      11. 8.3.11 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Eco-mode™
      2. 8.4.2 Operation With VIN < 3.5 V
      3. 8.4.3 Operation With EN Control
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1  Switching Frequency
        2. 9.2.2.2  Output Voltage Set Point
        3. 9.2.2.3  Undervoltage Lockout Set Point
        4. 9.2.2.4  Input Capacitors
        5. 9.2.2.5  Output Filter Components
          1. 9.2.2.5.1 Inductor Selection
          2. 9.2.2.5.2 Capacitor Selection
        6. 9.2.2.6  Compensation Components
        7. 9.2.2.7  Bootstrap Capacitor
        8. 9.2.2.8  Catch Diode
        9. 9.2.2.9  Slow-Start Capacitor
        10. 9.2.2.10 Output Voltage Limitations
        11. 9.2.2.11 Power Dissipation Estimate
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
    3. 11.3 Electromagnetic Interference (EMI) Considerations
  12. 12Device and Documentation Support
    1. 12.1 Device Support
      1. 12.1.1 Development Support
    2. 12.2 Trademarks
    3. 12.3 Electrostatic Discharge Caution
    4. 12.4 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8 Detailed Description

8.1 Overview

The TPS54531 device is a 28-V, 5-A, step-down (buck) converter with an integrated high-side n-channel MOSFET. To improve performance during line and load transients, the device implements a constant-frequency, current mode control which reduces output capacitance and simplifies external frequency compensation design. The TPS54531 device has a preset switching frequency of 570 kHz.

The TPS54531 device requires a minimum input voltage of 3.5 V for normal operation. The EN pin has an internal pullup current source that can be used to adjust the input-voltage undervoltage lockout (UVLO) with two external resistors. In addition, the pullup current provides a default condition when the EN pin is floating for the device to operate. The operating current is 110 μA (typical) when not switching and under no load. When the device is disabled, the supply current is 1 μA (typical).

The integrated 80-mΩ high-side MOSFET allows for high-efficiency power-supply designs with continuous output currents up to 5 A.

The TPS54531 device reduces the external component count by integrating the boot recharge diode. The bias voltage for the integrated high-side MOSFET is supplied by an external capacitor on the BOOT to PH pin. The boot capacitor voltage is monitored by an UVLO circuit and turns the high-side MOSFET off when the voltage falls below a preset threshold of 2.1 V (typical). The output voltage can be stepped down to as low as the reference voltage.

By adding an external capacitor, the slow-start time of the TPS54531 device can be adjustable which enables flexible output filter selection.

To improve the efficiency at light load conditions, the TPS54531 device enters a special pulse skipping Eco-mode when the peak inductor current drops below 160 mA (typical).

The frequency foldback reduces the switching frequency during startup and overcurrent conditions to help control the inductor current. The thermal shut down provides the additional protection under fault conditions.

8.2 Functional Block Diagram

fbd1a_lvsbi5.gif

8.3 Feature Description

8.3.1 Fixed-Frequency PWM Control

The TPS54531 device uses a fixed-frequency, peak-current mode control. The internal switching frequency of the TPS54531 device is fixed at 570 kHz.

8.3.2 Voltage Reference (Vref)

The voltage reference system produces a ±2% initial accuracy voltage reference (±3.5% over temperature) by scaling the output of a temperature stable bandgap circuit. The typical voltage reference is designed at 0.8 V.

8.3.3 Bootstrap Voltage (BOOT)

The TPS54531 device has an integrated boot regulator and requires a 0.1-μF ceramic capacitor between the BOOT and PH pins to provide the gate-drive voltage for the high-side MOSFET. A ceramic capacitor with an X7R- or X5R-grade dielectric is recommended because of the stable characteristics over temperature and voltage. To improve drop out, the TPS54531 device is designed to operate at 100% duty cycle as long as the BOOT-to-PH pin voltage is greater than 2.1 V (typical).

8.3.4 Enable and Adjustable Input Undervoltage Lockout (VIN UVLO)

The EN pin has an internal pullup current-source that provides the default condition of the TPS54531 device while operating when the EN pin floats.

The TPS54531 device is disabled when the VIN pin voltage falls below the internal VIN UVLO threshold. Using an external VIN UVLO to add at least 500-mV hysteresis is recommended unless the VIN voltage is greater than (VOUT + 2 V). To adjust the VIN UVLO with hysteresis, use the external circuitry connected to the EN pin as shown in Figure 9. When the EN pin voltage exceeds 1.25 V, an additional 3 μA of hysteresis is added. Use Equation 1 and Equation 2 to calculate the resistor values required for the desired VIN UVLO threshold voltages. The VSTOP should always be greater than 3.5 V.

UVlock_out_lvsbi5.gifFigure 9. Adjustable Input Undervoltage Lockout
Equation 1. q1_lvs839.gif

where

  • VSTART is the input start threshold voltage
  • VSTOP is the input stop threshold voltage
Equation 2. q2_lvs839.gif

where

  • VEN is the enable threshold voltage of 1.25 V

The external start and stop voltages are approximate. The actual start and stop voltages may vary.

8.3.5 Programmable Slow Start Using SS Pin

Programming the slow-start time externally is highly recommended because no slow-start time is implemented internally. The TPS54531 device effectively uses the lower voltage of the internal voltage reference or the SS pin voltage as the reference voltage of the power supply that is fed into the error amplifier and regulates the output accordingly. A capacitor (CSS) on the SS pin to ground implements a slow-start time. The TPS54531 device has an internal pullup current source of 2 μA that charges the external slow-start capacitor. Use Equation 3 to calculate the slow-start time (10% to 90%).

Equation 3. q3_lvs839.gif

where

  • Vref = 0.8 V
  • ISS = 2 μA

The slow-start time should be set between 1 ms to 10 ms to ensure good startup behavior. The value slow-start capacitor should not exceed 27 nF.

During normal operation, the TPS54531 device stops switching if the input voltage drops below the VIN UVLO threshold, the EN pin is pulled below 1.25 V, or a thermal shutdown event occurs.

8.3.6 Error Amplifier

The TPS54531 device has a transconductance amplifier for the error amplifier. The error amplifier compares the VSENSE voltage to the internal effective voltage reference presented at the input of the error amplifier. The transconductance of the error amplifier is 92 μA/V during normal operation. Frequency compensation components are connected between the COMP pin and ground.

8.3.7 Slope Compensation

In order to prevent the sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the TPS54531 device adds a built-in slope compensation which is a compensating ramp to the switch-current signal.

8.3.8 Current-Mode Compensation Design

The device is able to work with various types of output capacitors with appropriate compensation designs. For designs using ceramic output capacitors, proper derating of ceramic output capacitance is recommended when performing the stability analysis because the actual ceramic capacitance drops considerably from the nominal value when the applied voltage increases. For the detailed guidelines, see the Detailed Design Procedure section.

8.3.9 Overcurrent Protection and Frequency Shift

The TPS54531 device implements current mode control that uses the COMP pin voltage to turn off the high-side MOSFET on a cycle-by-cycle basis. During each cycle the switch current and the COMP pin voltage are compared. When the peak inductor current intersects the COMP pin voltage, the high-side switch is turned off. During overcurrent conditions that pull the output voltage low, the error amplifier responds by driving the COMP pin high, causing the switch current to increase. The COMP pin has a maximum clamp internally, which limits the output current.

The TPS54531 device provides robust protection during short circuits. Overcurrent runaway is possible in the output inductor during a short circuit at the output. The TPS54531 device solves this issue by increasing the off time during short-circuit conditions by lowering the switching frequency. The switching frequency is divided by 1, 2, 4, and 8 as the voltage ramps from 0 V to 0.8 V on VSENSE pin. The relationship between the switching frequency and the VSENSE pin voltage is listed in Table 1.

Table 1. Switching Frequency Conditions

SWITCHING FREQUENCY VSENSE PIN VOLTAGE
570 kHz VSENSE ≥ 0.6 V
570 kHz / 2 0.6 V > VSENSE ≥ 0.4 V
570 kHz / 4 0.4 V > VSENSE ≥ 0.2 V
570 kHz / 8 0.2 V > VSENSE

8.3.10 Overvoltage Transient Protection

The TPS54531 device incorporates an overvoltage transient-protection (OVTP) circuit to minimize output voltage overshoot when recovering from output fault conditions or strong unload transients. The OVTP circuit includes an overvoltage comparator to compare the VSENSE pin voltage and internal thresholds. When the VSENSE pin voltage goes above 109% × Vref, the high-side MOSFET is forced off. When the VSENSE pin voltage falls below 107% × Vref, the high-side MOSFET is enabled again.

8.3.11 Thermal Shutdown

The device implements an internal thermal shutdown to protect the device if the junction temperature exceeds 165°C. The thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal trip threshold. When the die temperature decreases below 165°C, the device reinitiates the power-up sequence.

8.4 Device Functional Modes

8.4.1 Eco-mode™

The TPS54531 is designed to operate in pulse skipping Eco-mode at light load currents to boost light load efficiency. When the peak inductor current is lower than 160 mA (typical), the COMP pin voltage falls to 0.5 V (typical) and the device enters Eco-mode . When the device is in Eco-mode, the COMP pin voltage is clamped at 0.5-V internally which prevents the high-side integrated MOSFET from switching. The peak inductor current must rise above 160 mA for the COMP pin voltage to rise above 0.5 V and exit Eco-mode. Because the integrated current comparator catches the peak inductor current only, the average load current entering Eco-mode varies with the applications and external output filters.

8.4.2 Operation With VIN < 3.5 V

The device is recommended to operate with input voltages above 3.5 V. The typical VIN UVLO threshold is not specified and the device can operate at input voltages down to the UVLO voltage. At input voltages below the actual UVLO voltage, the device does not switch. If the EN pin is externally pulled up or left floating, the device becomes active when the VIN pin passes the UVLO threshold. Switching begins when the slow-start sequence is initiated.

8.4.3 Operation With EN Control

The enable threshold voltage is 1.25 V (typical). With the EN pin is held below that voltage the device is disabled and switching is inhibited even if the VIN pin is above the UVLO threshold. The IC quiescent current is reduced in this state. If the EN voltage increases above the threshold while the VIN pin is above the UVLO threshold, the device becomes active. Switching is enabled, and the slow-start sequence is initiated.