Features

3.5 to 28 V input voltage range

Adjustable output voltage down to 0.8 V

Integrated 80-mΩ high-side MOSFET supports up to 3-A continuous output current

Light Load High Efficiency™ with Pulse Skip Eco Mode

Fixed 570 kHz switching frequency

Typical 1-µA Shutdown Quiescent Current

Adjustable slow start to limit inrush current

Programmable UVLO Threshold

Overvoltage transient protection

Cycle-by-Cycle Current Limit, Frequency Folding, and Thermal Shutdown Protection

Available in an easy-to-use SOIC8 package or

Thermally Enhanced SOIC8 Power Board™

pack

SUPPORTED BY WEBENCH® SOFTWARE TOOLS

2 apps

User applications such as set-top boxes, CPE devices, LCD monitors, peripherals and

battery charger

Industrial and Automotive Audio Power

5-V, 12-V, and 24-V Distributed Power Systems

4 sketches

illustrate

The TPS54331 device is a 28V, 3A non-synchronous buck converter that integrates a low RDS(on) high-side MOSFET. To improve efficiency at light loads, the pulse skip Eco mode function is automatically activated. Additionally, the 1µA shutdown supply current allows the device to be used in battery powered applications. Current-mode control with internal slope compensation simplifies external compensation calculations, reduces component count, and allows the use of ceramic output capacitors. The resistor divider programs the hysteresis of the input undervoltage lockout. Overvoltage transient protection circuitry limits voltage overshoot during startup and transient conditions. Cycle-by-cycle current limiting schemes, frequency folding, and thermal shutdown protect the device and load during overload conditions. Available in 8-pin SOIC and 8-pin SO PowerPAD packages, the TPS54331 device is internally optimized for improved thermal performance.

Overview

The TPS54331 device is a 28-V, 3-a, step-down (buck) converter with an integrated high-side n-channel MOSFET. To improve performance during line and load transients, the device implements constant frequency, current-mode control, reduces output capacitance, and simplifies external frequency compensation design. The TPS54331 device has a preset switching frequency of 570 kHz.

The TPS54331 device requires a minimum input voltage of 3.5 V for proper operation. The EN pin has an internal pull-up current source and the input voltage under-voltage lockout (UVLO) can be adjusted with two external resistors. Additionally, when the EN pin is floating, the pull-up current provides default conditions for device operation. Operating current is 110µA (typ) without switching and no load. When the device is disabled, the supply current is 1µA (typ).

The integrated 80-mΩ high-side MOSFET allows high-efficiency power supply designs with continuous output currents up to 3A.

The TPS54331 device reduces external component count by integrating a bootstrap charge diode. The bias voltage for the integrated high-side MOSFET is provided by an external capacitor on the PH pin. The boot capacitor voltage is monitored by a UVLO circuit, and when the voltage is below a preset threshold of 2.1v (typ), the high-side MOSFET is turned off. The output voltage can be stepped down as low as the reference voltage.

By adding external capacitors, the slow-start time of the TPS54331 device can be adjusted, allowing flexible output filter selection.

To improve efficiency at light load conditions, the TPS54331 device enters a special pulse-skipping Ecomode when the peak inductor current falls below 160 mA (typ).

Frequency folding reduces the switching frequency during startup and overcurrent conditions, helping to control the inductor current. Thermal shutdown provides additional protection under fault conditions.

Functional block diagram

Feature description

Fixed frequency PWM control

The TPS54331 device uses fixed frequency, peak current mode control. The internal switching frequency of the TPS54331 device is fixed at 570 kHz.

Reference voltage (Vref)

The voltage reference system stabilizes the output of the bandgap circuit by regulating temperature, resulting in an initial accuracy voltage reference of ±2% (±3.5% over temperature). A typical reference voltage design is 0.8V.

Boot voltage (boot)

The TPS54331 device has an integrated boot regulator that needs to be

Boot and PH pins that provide gate drive voltage for the high-side MOSFET. Ceramic capacitors with X7R or X5R grade dielectrics are recommended because of their stable characteristics over temperature and voltage. To improve power-down performance, the TPS54331 device is designed to operate at 100% duty cycle from power-on to PH pin voltage greater than 2.1V (typ).

Enable and Adjustable Input Undervoltage Lockout (VIN UVLO)

The EN pin has an internal pull-up current source that provides the default condition of the device when the EN pin is floating.

Characterization (continued)

The TPS54331 device is disabled when the VIN pin voltage is below the internal VIN UVLO threshold. Unless the VIN voltage is greater than (VOUT+2 V), it is recommended to use an external VIN UVLO to add hysteresis. To adjust the VIN UVLO with hysteresis, use an external circuit connected to the EN pin as shown in Figure 9. When EN-pin voltage exceeds 1.25v, add 3μA hysteresis. Use Equation 1 and Equation 2 to calculate the required resistor value for the desired VIN UVLO threshold voltage. The VSTOP threshold should always be greater than 3.5 V.

VEN is the enable threshold voltage of 1.25 V

Programmable slow start using SS pin

It is strongly recommended to program the slow-start time externally, as the slow-start time is not implemented internally. The TPS54331 device effectively uses the internal voltage reference or the lower of the SS pin voltage as the reference voltage for the power supply that feeds the error amplifier and regulates the output accordingly. A capacitor (CSS) from the SS pin to ground achieves a slow start-up time. The internal pull-up current source of the TPS54331 device is 2 μA to charge the external slow-start capacitor. Calculate the slow start time (10% to 90%) using Equation 3

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

During normal operation, if the input voltage falls below the VIN UVLO threshold, the EN pin is pulled below 1.25v, or a thermal shutdown event occurs, the TPS54331 device stops switching.

Characterization (continued)

Error amplifier

The TPS54331 device has a transconductance amplifier for the error amplifier. The error amplifier compares the VSENSE voltage to an internal effective voltage reference provided at the input of the error amplifier. In normal operation, the transconductance of the error amplifier is 92μA/V. The frequency compensation component is connected between the compressor pins and ground.

slope compensation

To prevent sub-harmonic oscillations when operating the device at duty cycles greater than 50%, the TPS54331 device adds a built-in slope compensation as a compensation ramp for the switch current signal.

Current Mode Compensation Design

To simplify the design effort using the TPS54331 device, Table 1 lists typical designs for common applications. For designs using ceramic output capacitors, it is recommended to appropriately de-rate the ceramic output capacitor when performing stability analysis, because the actual ceramic capacitor value is significantly lower than the nominal value as the applied voltage increases

Overcurrent protection and frequency shifting

The TPS54331 device implements current-mode control that uses the COMP pin voltage to cycle off the high-side MOSFET. In each cycle, the switch current and the compressor pin voltage are compared. When the peak inductor current crosses the COMP pin voltage, the high-side switch is turned off. During an overcurrent condition that pulls the output voltage low, the error amplifier responds by pulling the COMP pin high, causing the switch current to increase. There is a maximum clamp inside the COMP pin, limiting the output current.

The TPS54331 device provides reliable protection during short circuits. During a short circuit at the output, an overcurrent runaway can occur in the output inductor. The TPS54331 device solves this problem by reducing the switching frequency to increase the off-time under short-circuit conditions. The switching frequency is divided by 1, 2, 4 and 8 as the voltage ramps from 0 to 0.8V on the VSENSE pin

Voltage Transient Protection

The TPS54331 device contains an overvoltage transient protection (OVTP) circuit to minimize output voltage overshoot when recovering from output fault conditions or strong unloading transients. The OVTP circuit includes an overvoltage comparator that compares the VSENSE pin voltage with the internal threshold. When the VSENSE pin voltage is higher than 109%×Vref, the high-side MOSFET is forcibly turned off. When the VSENSE pin voltage is lower than 107% × Vref, the high-side MOSFET is enabled again.

thermal shutdown

The device implements an internal thermal shutdown to protect the device when the junction temperature exceeds 165°C. Thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal trip threshold. When the mold temperature drops below 165°C, the equipment restarts the power-up sequence.

Device functional mode

Eco-Mode™

The TPS54331 device is designed to operate in pulse skip Eco mode at light load currents to improve light load efficiency. When the inductor peak current is lower than 160ma (typ), the COMP pin voltage drops to 0.5v (typ), and the device enters Eco mode. When the device is in Eco mode, the internal voltage of the COMP pin is clamped to 0.5-V, preventing high-side integrated MOSFET switching. In order for the COMP pin voltage to rise above 0.5v and exit Eco mode, the inductor current peak must rise above 160ma. Since the integrated current comparator only captures the peak inductor current, the average load current entering Eco mode varies with the application and external output filter.

VIN <3.5 V operation

It is recommended that the device operate with an input voltage higher than 3.5 V. The typical vehicle identification number (VIN) UV lower threshold is not specified, and the device can operate with input voltages below the UV lower voltage. The device will not switch when the input voltage is lower than the actual UVLO voltage. If the EN pin is externally pulled up or left floating, the device will activate when the VIN pin exceeds the UVLO threshold. The switchover begins when the slow-start sequence is initiated.

EN control operation

The enable threshold voltage is 1.25 V (typ). When the EN pin is held below this voltage, the device will be disabled and switching will be disabled even if the VIN pin is above the UVLO threshold. In this state, the IC quiescent current decreases. If the EN voltage rises above the threshold while the VIN pin is above the UVLO threshold, the device will activate. Enable toggle to initiate a slow start sequence.

Typical application diagram

Detailed design procedure

The following design process can be used to select component values for the TPS54331 device. Alternatively, complete designs can be generated using WEBENCH software. WEBENCH software uses an iterative design process and accesses a comprehensive database of components as the design is generated. This section provides a simplified discussion of the design process.

On-off level

The switching frequency of the TPS54331 device is fixed at 570 kHz.

Output voltage setting value

The output voltage of the TPS54331 device can be adjusted externally using a resistor divider network. As shown in Figure 10, this segmentation network consists of R5 and R6. The relationship of the output voltage to the resistive divider is given by Equation 4 and Equation 5.

Choose an R5 value of approximately 10 kΩ. When using standard value resistors, slightly increasing or decreasing the value of R5 can result in a tighter output voltage match. In this design, R4 = 10.2 kΩ and R = 3.24 kΩ, resulting in a 3.31 V output voltage. A 0Ω resistor R4 is provided as a convenient place to disconnect the control loop for stability testing.

input capacitor

The TPS54331 device requires an input decoupling capacitor and, depending on the application, a bulk input capacitor. A typical recommended value for decoupling capacitors is 10µF. X5R or X7R type high quality ceramics are recommended. The rated voltage should be greater than the maximum input voltage. Smaller values can be used as long as all other requirements are met; however, a value of 10µF has been shown to work well in a variety of circuits. Also, some bulk capacitors may be required, especially if the TPS54331 circuit is not within about 2 inches of the input voltage source. The value of this capacitor is not critical, but it should be rated to handle the maximum input voltage including ripple voltage and should filter the output so that the input ripple voltage is acceptable. In this design, two 4.7μF capacitors are used for the input decoupling capacitors. The capacitors are X7R dielectric, rated at 50 V. Equivalent series resistance (ESR) is about 2 mΩ, and the current rating is 3 A. Additionally, a small 0.01-µF capacitor is included for high frequency filtering.

In this case, the input ripple voltage is 143mv and the RMS ripple current is 1.5a.

Notice

The actual input voltage ripple is greatly affected by parasitic circuits, which are related to the layout of the voltage source and the output impedance.

The actual input voltage ripple of this circuit, listed in Table 3, is greater than the calculated value. This measurement is still below the specified input limit of 300 mV. The maximum voltage of the input capacitor is VIN(MAX)+ΔVIN/2. The selected bulk capacitors and bypass capacitors are both rated at 50 V and have a ripple current capacity greater than 3 A, both of which provide sufficient margin. Under no circumstances should the maximum voltage and current ratings be exceeded.

Output Filter Components

Two components, L1 and C2, must be selected for the output filter. Since the TPS54331 device is an external compensation device, it can support a variety of filter component types and values.

Sensor selection

To calculate the minimum value of the output inductance, use Equation 8.

For this design example, using KIND=0.3, the minimum inductance value is calculated to be 5.7μH. For this design, a large value was chosen: 6.8 μH.

For output filter inductors, do not exceed the rms and saturation current ratings. Use Equation 9 to calculate the inductor ripple current (ILPP).

In this design, the rms inductor current is 3.01A and the peak inductor current is 3.47A. The selected inductor is Sumida CDRH103-6R8, 6.8μH. With a saturation current rating of 3.84A and an rms current rating of 3.6A, this inductor meets these requirements. Smaller or larger inductor values can be used depending on the amount of ripple current the designer wishes to allow, as long as other design requirements are met. A larger value inductor will have lower AC current and result in lower output voltage ripple, while a smaller value inductor will increase the AC current and output voltage ripple. Typically, the inductor value used with the TPS54331 device is in the range of 6.8 to 47 μH.

Capacitor selection

Important design factors for the output capacitor are the DC voltage rating, ripple current rating, and equivalent series resistance (ESR). The DC voltage and ripple current ratings cannot be exceeded. ESR is important because it, along with the inductor current, determines the magnitude of the output ripple voltage. The actual value of the output capacitor is not critical, but some practical limitations do exist. Consider the relationship between the closed-loop crossover frequency required for the design and the LC corner frequency of the output filter. In general, it is desirable that the closed-loop crossover frequency be less than 1/5 of the switching frequency. For high switching frequencies, such as 570 kHz for this design, internal circuit limitations of the TPS54331 device limit the practical maximum crossover frequency to about 25 kHz. In general, the closed-loop crossover frequency should be higher than the corner frequency determined by the load impedance and output capacitance. Use Equation 12 to calculate the limit for the minimum capacitance value

Power Recommendations

These devices are designed to operate over an input voltage range of 3.5 V to 28 V. This input voltage should be well regulated. If the input supply is more than a few inches away from the converter, additional bulk capacitance may be required in addition to ceramic bypass capacitors. Electrolytic capacitors with a value of 100µF are a typical choice.

layout

Layout Guidelines

The VIN pin should be bypassed to ground with a low ESR ceramic bypass capacitor. Care should be taken to minimize the loop area formed by the bypass capacitor connection, the VIN pin, and the capture diode anode. A typical recommended bypass capacitor is a 10µF ceramic with X5R or X7R dielectric, optimally positioned closest to the VIN pin and the source of the capture diode anode. Figure 25 shows an example PCB layout. The ground pins should be tied to the PCB ground plane of the device pins. The power supply for the low-side MOSFET should be connected directly to the top PCB ground area for connecting the ground sides of the input and output capacitors and the anode of the capture diode. The PH pin should be connected to the cathode of the catch diode and the output inductor. Since the PH connection is the switch node, the capture diode and output inductance are very close to the PH pin, and the area of the PCB conductors should be minimized to prevent excessive capacitive coupling. In order to operate at full rated load, the top floor area must provide sufficient heat dissipation area. The TPS54331 device uses a fuse leadframe, so the ground pin acts as a conductive path to dissipate heat from the die. Many applications have larger internal or back ground plane areas, and the top ground area can be connected to these areas using multiple vias under or near the device to aid heat dissipation. Other external components can be placed roughly as shown. Alternative layout schemes can be used to obtain acceptable performance, but this layout has been shown to produce good results and is used as a guideline.

layout example

enter

Electromagnetic Interference (EMI) Considerations

As EMI becomes a growing concern in more and more applications, the internal design of the TPS54331 device includes features to reduce EMI. The high-side MOSFET gate drive is designed to reduce PH pin voltage ringing. Inter-IC tracks are isolated to reduce noise sensitivity. In order to reduce parasitic effects, the wrapped junction scheme is adopted.