7.3.1 Fixed Frequency PWM Control

The device uses a adjustable fixed frequency, peak current mode control. The output voltage is compared through external resistors on the VSENSE pin to an internal voltage reference by an error amplifier which drives the COMP pin. An internal oscillator initiates the turnon of the high-side power switch. The error amplifier output is converted into a current reference which compares to the high-side power switch current. When the power switch current reaches current reference generated by the COMP voltage level the high-side power switch is turned off and the low-side power switch is turned on.

7.3.2 Continuous Current Mode Operation (CCM)

As a synchronous buck converter, the device normally works in CCM (Continuous Conduction Mode) under all load conditions.

7.3.3 VIN and Power VIN Pins (VIN and PVIN)

The device allows for a variety of applications by using the VIN and PVIN pins together or separately. The VIN pin voltage supplies the internal control circuits of the device. The PVIN pin voltage provides the input voltage to the power converter system.

If tied together, the input voltage for VIN and PVIN can range from 4.5 V to 17 V. If using the VIN separately from PVIN, the VIN pin must be between 4.5 V and 17 V, and the PVIN pin can range from as low as 1.6 V to 17 V. A voltage divider connected to the EN pin can adjust the either input voltage UVLO appropriately. Adjusting the input voltage UVLO on the PVIN pin helps to provide consistent power up behavior.

7.3.4 Voltage Reference

The voltage reference system produces a precise ±1% voltage reference over temperature by scaling the output of a temperature stable bandgap circuit.

7.3.5 Adjusting the Output Voltage

The output voltage is set with a resistor divider from the output (VOUT) to the VSENSE pin. TI recommends using 1% tolerance or better divider resistors. Referring to the application schematic of Figure 34, start with a 10 kΩ for R6 and use Equation 1 to calculate R5. To improve efficiency at light loads consider using larger value resistors. If the values are too high the regulator is more susceptible to noise and voltage errors from the VSENSE input current are noticeable.

Equation 1.

where

• Vref = 0.8V

The minimum output voltage and maximum output voltage can be limited by the minimum on time of the high-side MOSFET and bootstrap voltage (BOOT-PH voltage) respectively. More discussions are located in Minimum Output Voltage and Bootstrap Voltage (BOOT) and Low Dropout Operation.

7.3.6 Safe Start-Up into Prebiased Outputs

The device has been designed to prevent the low-side MOSFET from discharging a prebiased output. During monotonic prebiased start-up, the low-side MOSFET is not allowed to sink current until the SS/TR pin voltage is higher than 1.4 V.

7.3.7 Error Amplifier

The device uses a transconductance error amplifier. The error amplifier compares the VSENSE pin voltage to the lower of the SS/TR pin voltage or the internal 0.8-V voltage reference. The transconductance of the error amplifier is 1300 μA/V during normal operation. The frequency compensation network is connected between the COMP pin and ground.

7.3.8 Slope Compensation

The device adds a compensating ramp to the switch current signal. This slope compensation prevents sub-harmonic oscillations. The available peak inductor current remains constant over the full duty cycle range.

7.3.9 Enable and Adjusting Undervoltage Lockout

The EN pin provides electrical on/off control of the device. When the EN pin voltage exceeds the threshold voltage, the device starts operation. If the EN pin voltage is pulled below the threshold voltage, the regulator stops switching and enters low Iq state.

The EN pin has an internal pullup current source, allowing the user to float the EN pin for enabling the device. If an application requires controlling the EN pin, use either open-drain or open-collector output logic to interface with the pin.

The device implements internal UVLO circuitry on the VIN pin. The device is disabled when the VIN pin voltage falls below the internal VIN UVLO threshold. The internal VIN UVLO threshold has a hysteresis of 150 mV.

If an application requires either a higher UVLO threshold on the VIN pin or a secondary UVLO on the PVIN in split-rail applications, then the EN pin can be configured as shown in Figure 17, Figure 18, and Figure 19. When using the external UVLO function, TI recommends setting the hysteresis to be greater than 500 mV.

The EN pin has a small pullup current Ip which sets the default state of the pin to enable when no external components are connected. The pullup current is also used to control the voltage hysteresis for the UVLO function because it increases by I_h when the EN pin crosses the enable threshold. The UVLO thresholds can be calculated using Equation 2 and Equation 3.

Figure 17. Adjustable VIN Undervoltage Lockout

Figure 18. Adjustable PVIN Undervoltage Lockout, VIN ≥ 4.5 V

Figure 19. Adjustable VIN and PVIN Undervoltage Lockout

Equation 2.

Equation 3.

where

• I_h = 3.4 μA
• I_p = 1.15 μA
• V_ENRISING = 1.21 V
• V_ENFALLING = 1.17 V

7.3.10 Adjustable Switching Frequency and Synchronization (RT/CLK)

The RT/CLK pin can be used to set the switching frequency of the device in two modes.

In RT mode, a resistor (RT resistor) is connected between the RT/CLK pin and GND. The switching frequency of the device is adjustable from 200 kHz to 1600 kHz by placing a maximum of 240 kΩ and minimum of 29 kΩ, respectively. In CLK mode, an external clock is connected directly to the RT/CLK pin. The device is synchronized to the external clock frequency with PLL.

The CLK mode overrides the RT mode. The device is able to detect the proper mode automatically and switch from the RT mode to CLK mode.

7.3.11 Slow Start (SS/TR)

The device uses the lower voltage of the internal voltage reference or the SS/TR pin voltage as the reference voltage and regulates the output accordingly. A capacitor on the SS/TR pin to ground implements a slow-start time. The device has an internal pullup current source of 2.3 μA that charges the external slow-start capacitor. The calculations for the slow-start time (Tss, 10% to 90%) and slow-start capacitor (Css) are shown in Equation 4. The voltage reference (Vref) is 0.8 V and the slow-start charge current (Iss) is 2.3 μA.

Equation 4.

When the input UVLO is triggered, the EN pin is pulled below 1.21 V, or a thermal shutdown event occurs, the device stops switching and enters low current operation. At the subsequent power up when the shutdown condition is removed, the device does not start switching until it has discharged its SS/TR pin to ground ensuring proper soft-start behavior.

7.3.12 Power Good (PWRGD)

The PWRGD pin is an open-drain output. When the VSENSE pin is between 94% and 106% of the internal voltage reference the PWRGD pin pulldown is deasserted and the pin floats. TI recommends using a pullup resistor between the values of 10 kΩ and 100 kΩ to a voltage source that is 5.5 V or less. The PWRGD is in a defined state when the VIN input voltage is greater than 1 V but with reduced current sinking capability. The PWRGD achieves full current sinking capability when the VIN input voltage is above 4.5 V.

The PWRGD pin is pulled low when VSENSE is lower than 91% or greater than 109% of the nominal internal reference voltage. Also, if the PWRGD is pulled low and the input UVLO or thermal shutdown are asserted, the EN pin is pulled low or the SS/TR pin is set below 1.4 V.

7.3.13 Output Overvoltage Protection (OVP)

The device incorporates an output overvoltage protection (OVP) circuit to minimize output voltage overshoot. For example, when the power supply output is overloaded the error amplifier compares the actual output voltage to the internal reference voltage. If the VSENSE pin voltage is lower than the internal reference voltage for a considerable time, the output of the error amplifier demands maximum output current. When the condition is removed, the regulator output rises and the error amplifier output transitions to the steady-state voltage. In some applications with small output capacitance, the power supply output voltage can respond faster than the error amplifier. This leads to the possibility of an output overshoot. The OVP feature minimizes the overshoot by comparing the VSENSE pin voltage to the OVP threshold. If the VSENSE pin voltage is greater than the OVP threshold the high-side MOSFET is turned off preventing current from flowing to the output and minimizing output overshoot. When the VSENSE voltage drops lower than the OVP threshold, the high-side MOSFET is allowed to turn on at the next clock cycle.

7.3.14 Overcurrent Protection

The device is protected from overcurrent conditions by cycle-by-cycle current limiting on both the high-side MOSFET and the low-side MOSFET.

7.3.15 Thermal Shutdown

The internal thermal shutdown circuitry forces the device to stop switching if the junction temperature exceeds 175°C typically. The device reinitiates the power-up sequence when the junction temperature drops below 165°C typically.

7.3.16 Small Signal Model for Loop Response

Figure 20 shows an equivalent model for the device control loop which can be modeled in a circuit simulation program to check frequency response and transient responses. The error amplifier is a transconductance amplifier with a gm of 1300 μA/V. The error amplifier can be modeled using an ideal voltage controlled current source. The resistor Roea (2.38 MΩ) and capacitor Coea (20.7 pF) model the open-loop gain and frequency response of the error amplifier. The 1-mV AC voltage source between the nodes a and b effectively breaks the control loop for the frequency response measurements. Plotting a/c and c/b show the small signal responses of the power stage and frequency compensation respectively. Plotting a/b shows the small signal response of the overall loop. The dynamic loop response can be checked by replacing the R_L with a current source with the appropriate load step amplitude and step rate in a time domain analysis.

Figure 20. Small Signal Model for Loop Response

7.3.17 Simple Small Signal Model for Peak Current Mode Control

Figure 21 is a simple small signal model that can be used to understand how to design the frequency compensation. The device power stage can be approximated to a voltage-controlled current source (duty cycle modulator) supplying current to the output capacitor and load resistor. The control-to-output transfer function is shown in Equation 5 and consists of a DC gain, one dominant pole, and one ESR zero. The quotient of the change in switch current and the change in COMP pin voltage (node c in Figure 20) is the power stage transconductance (gm_ps), which is 16 A/V for the device. The DC gain of the power stage is the product of gm_ps, and the load resistance (R_L) as shown in Equation 6 with resistive loads. As the load current increases, the DC gain decreases. This variation with load may seem problematic at first glance, but fortunately the dominant pole moves with load current (see Equation 7). The combined effect is highlighted by the dashed line in Figure 22. As the load current decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover frequency the same for the varying load conditions, making it easier to design the frequency compensation.

Figure 21. Simplified Small Signal Model for Peak Current Mode Control

Figure 22. Simplified Frequency Response for Peak Current Mode Control

Equation 5.

Equation 6.

where

• gm_ps is the power stage gain (16 A/V).
• R_L is the load resistance

Equation 7.

where

• C_O is the output capacitance.
• R_L is the load resistance

Equation 8.

where

• C_O is the output capacitance.
• R_ESR is the equivalent series resistance of the output capacitor.

7.3.18 Small Signal Model for Frequency Compensation

The device uses a transconductance amplifier for the error amplifier and readily supports two of the commonly used Type II compensation circuits and a Type III frequency compensation circuit, as shown in Figure 23. In Type 2A, one additional high frequency pole, C6, is added to attenuate high-frequency noise. In Type III, one additional capacitor, C11, is added to provide a phase boost at the crossover frequency. See Designing Type III Compensation for Current Mode Step-Down Converters (SLVA352) for a complete explanation of Type III compensation.

The design guidelines below are provided for advanced users who prefer to compensate using the general method. The below equations only apply to designs whose ESR zero is above the bandwidth of the control loop. This is usually true with ceramic output capacitors. See the Application Information section for a step-by-step design procedure using higher ESR output capacitors with lower ESR zero frequencies.

Figure 23. Types of Frequency Compensation

The general design guidelines for device loop compensation are as follows:

1. Determine the crossover frequency, fc. A good starting point is 1/10^th of the switching frequency, fsw.
2. R4 can be determined by:
Equation 9.
where
• gm_ea is the GM amplifier gain (1300 μA/V)
• gm_ps is the power stage gain (12 A/V)
• Vref is the reference voltage (0.8 V)
3. Place a compensation zero at the dominant pole:
   C4 can be determined by:
Equation 10.
4. C6 is optional. It can be used to cancel the zero from the ESR (Equivalent Series Resistance) of the output capacitor Co.
Equation 11.
5. Type III compensation can be implemented with the addition of one capacitor, C11. This allows for slightly higher loop bandwidths and higher phase margins. If used, C11 is calculated from Equation 12.
Equation 12.

7.4.1 Adjustable Switching Frequency (RT Mode)

To determine the RT resistance for a given switching frequency, use Equation 13 or the curve in Figure 24. To reduce the solution size, one would set the switching frequency as high as possible, but tradeoffs of the supply efficiency and minimum controllable on-time must be considered.

Equation 13.

Figure 24. RT Set Resistor vs Switching Frequency

7.4.2 Synchronization (CLK Mode)

An internal Phase Locked Loop (PLL) has been implemented to allow synchronization between 200 kHz and 1600 kHz, and to easily switch from RT mode to CLK mode.

To implement the synchronization feature, connect a square wave clock signal to the RT/CLK pin with a duty cycle between 20% to 80%. The clock signal amplitude must transition lower than 0.8 V and higher than 2.0 V. The start of the switching cycle is synchronized to the falling edge of RT/CLK pin.

In applications where both RT mode and CLK mode are needed, the device can be configured as shown in Figure 25. Before the external clock is present, the device works in RT mode and the switching frequency is set by RT resistor. When the external clock is present, the CLK mode overrides the RT mode. The first time the SYNC pin is pulled above the RT/CLK high threshold (2.0 V), the device switches from the RT mode to the CLK mode and the RT/CLK pin becomes high impedance as the PLL starts to lock onto the frequency of the external clock. TI does not recommend switching from the CLK mode back to the RT mode because the internal switching frequency drops to 100 kHz first before returning to the switching frequency set by RT resistor.

Figure 25. Works With Both RT Mode and CLK Mode

7.4.3 Bootstrap Voltage (BOOT) and Low-Dropout Operation

The device has an integrated boot regulator, and requires a small ceramic capacitor between the BOOT and PH pins to provide the gate drive voltage for the high-side MOSFET. The boot capacitor is charged when the BOOT pin voltage is less than VIN and BOOT-PH voltage is below regulation. The value of this ceramic capacitor should be 0.1 μF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is recommended because of the stable characteristics over temperature and voltage.

To improve dropout, the device is designed to operate at 100% duty cycle as long as the BOOT to PH pin voltage is greater than the BOOT-PH UVLO threshold which is typically 2.1 V. When the voltage between BOOT and PH drops below the BOOT-PH UVLO threshold the high-side MOSFET is turned off and the low-side MOSFET is turned on allowing the boot capacitor to be recharged. In applications with split input voltage rails, 100% duty cycle operation can be achieved as long as (VIN – PVIN) > 4 V.

7.4.4 Sequencing (SS/TR)

Many of the common power supply sequencing methods can be implemented using the SS/TR, EN and PWRGD pins.

The sequential method is illustrated in Figure 26 using two TPS54620 devices. The power good of the first device is coupled to the EN pin of the second device which enables the second power supply when the primary supply reaches regulation. Figure 27 shows the results of Figure 26.

Figure 26. Sequential Start-Up Sequence

Figure 27. Sequential Start-Up Using EN and PWRGD

Figure 28 shows the method implementing ratiometric sequencing by connecting the SS/TR pins of two devices together. The regulator outputs ramp up and reach regulation at the same time. When calculating the slow start time the pullup current source must be doubled in Equation 4. Figure 29 shows the results of Figure 28.

Figure 28. Ratiometric Start-Up Sequence

Figure 29. Ratiometric Start-Up Using Coupled SS/TR Pins

Ratiometric and simultaneous power supply sequencing can be implemented by connecting the resistor network of R1 and R2 shown in Figure 30 to the output of the power supply that must be tracked or another voltage reference source. Using Equation 14 and Equation 15, the tracking resistors can be calculated to initiate the Vout2 slightly before, after or at the same time as Vout1. Equation 16 is the voltage difference between Vout1 and Vout2.

To design a ratiometric start-up in which the Vout2 voltage is slightly greater than the Vout1 voltage when Vout2 reaches regulation, use a negative number in Equation 14 and Equation 15 for ΔV. Equation 16 results in a positive number for applications where the Vout2 is slightly lower than Vout1 when Vout2 regulation is achieved. Figure 31 and Figure 32 show the results for positive ΔV and negative ΔV, respectively.

The ΔV variable is zero volt for simultaneous sequencing. To minimize the effect of the inherent SS/TR to VSENSE offset (Vssoffset, 29 mV) in the slow-start circuit and the offset created by the pullup current source (Iss, 2.3 μA) and tracking resistors, the Vssoffset and Iss are included as variables in the equations. Figure 33 shows the result when ΔV = 0 V.

To ensure proper operation of the device, the calculated R1 value from Equation 14 must be greater than the value calculated in Equation 17.

Equation 14.

Equation 15.

Equation 16.

Equation 17.

Figure 30. Ratiometric and Simultaneous Start-Up Sequence

Figure 31. Ratiometric Start-Up With Vout1 Leading Vout2

Figure 33. Simultaneous Start-Up

Figure 32. Ratiometric Start-Up With Vout2 Leading Vout1