8.2.2.1.1 Input Capacitors (C1, C11)

Input filter capacitor (C11) is used to filter out high frequency noise in the input line. Typical values of C11 are 0.1 µF to 0.01 µF. For higher frequency noise, low capacitor values are recommended.

To minimize the ripple voltage, input ceramic de-coupling capacitor (C1) of type X5R or X7R should be used. The DC voltage rating for the input decoupling capacitor must be greater than the maximum input voltage. This capacitor must have an input ripple current rating higher than the maximum input ripple current of the converter for the application; and is determined by Equation 25.

Equation 25.

The input capacitors for power regulators are chosen to have a reasonable capacitance-to-volume ratio and fairly stable over temperature. The value of the input capacitance also determines the input ripple voltage of the regulator, shown by Equation 26.

Equation 26.

Input ceramic filter capacitors should be located in close proximity to the VIN terminal. Surface mount capacitors are recommended to minimize lead length and reduce noise coupling.

8.2.2.1.2 Output Capacitor (C4, C12)

The selection of the output capacitor will determine several parameters in the operation of the converter (for example, voltage drop on the output capacitor and the output ripple). The capacitor value also determines the modulator pole and the roll-off frequency due to the LC output filter double pole. This is expressed in Equation 12.

The minimum capacitance needed to maintain desired output voltage during high to low load transition and prevent over shoot is given by Equation 27.

Equation 27.

where

• L = output inductor
• I_Load-Max = maximum load current
• I_Load-Min = minimum load current
• V_Reg-Max = maximum tolerance of regulated output voltage
• V_Reg-Min = minimum tolerance of regulated output voltage

During a load step from no load to full load or changes in the input voltage, the output capacitor must hold up the output voltage above a certain level for a specified time and not issue a reset, until the main regulator control loop responds to the change. The minimum output capacitance required to allow sufficient drop on the output voltage without issuing a reset is determined by Equation 28.

Equation 28.

where

• ΔV_Reg = transient response during load stepping

The minimum capacitance needed for output voltage ripple specification is given by Equation 29.

Equation 29.

Additional capacitance deratings for temperature, aging, and DC bias must be factored in, and so a value of 100 µF with ESR calculated using Equation 30 of less than 100 mΩ should be used on the output stage.

Maximum ESR of the output capacitor is based on output ripple voltage specification in Equation 30. The output ripple voltage is a product of the output capacitor ESR and ripple current.

Equation 30.

Output capacitor root mean square (RMS) ripple current is given by Equation 31. This is to prevent excess heating or failure due to high ripple currents. This parameter is sometimes specified by the manufacturers.

Equation 31.

Filter capacitor (C12) of value 0.1 µF (typical) is used to filter out the noise in the output line.

8.2.2.1.3 Soft-Start Capacitor (C6)

The soft-start capacitor determines the minimum time to reach the desired output voltage during a power-up cycle. This is useful when a load requires a controlled voltage slew rate, and helps to limit the current draw from the input voltage supply line. TI recommends a 100-nF capacitor for start-up loads of 1 A (maximum).

8.2.2.1.4 Bootstrap Capacitor (C3)

A 0.1-µF ceramic capacitor must be connected between the PH and BOOT terminals for the converter to operate and regulate to the desired output voltage. TI recommends using a capacitor with X5R or better grade dielectric material, and the voltage rating on this capacitor of at least 25 V to allow for derating.

8.2.2.1.5 Power-On Reset Delay (PORdly) Capacitor (C2)

The value of this capacitor can be calculated using Equation 6.

8.2.2.1.6 Output Inductor (L1)

Use a low EMI inductor with a ferrite type shielded core. Other types of inductors may be used; however, they must have low EMI characteristics and should be placed away from the low-power traces and components in the circuit.

To calculate the minimum value of the inductor, the ripple current should be first calculated using Equation 32.

The inductor ripple current is filtered by the output capacitor; therefore, K_IND is typically in the range of 0.2 to 0.3, depending on the ESR and the ripple current rating of the output capacitor (C4).

The minimum value of output inductor can be calculated using Equation 33.

Equation 33.

where

• VIN_Max = maximum input voltage
• V_Reg = regulated output voltage
• f_sw = switching frequency

The RMS and peak currents flowing in the inductor are given by Equation 34 and Equation 35.

Equation 34.

Equation 35.

8.2.2.1.7 Flyback Schottky Diode (D2)

The TPS54262-EP requires an external Schottky diode connected between the PH and power ground termination. The absolute voltage at PH pin should not go beyond the values in Absolute Maximum Ratings. The Schottky diode conducts the output current during the off state of the internal power switch. This Schottky diode must have a reverse breakdown voltage higher than the maximum input voltage of the application. A Schottky diode is selected for its lower forward voltage. The Schottky diode is selected based on the appropriate power rating, which factors in the DC conduction losses and the AC losses due to the high switching frequencies; this is determined by Equation 36.

Equation 36.

where

• P_diode = power rating
• V_fd = forward conducting voltage of Schottky diode
• C_J = junction capacitance of the Schottky diode

Recommended part numbers are PDS 360 and SBR8U60P5.

8.2.2.1.8 Resistor to Set Slew Rate (R7)

The slew rate setting is asymmetrical; that is, for a selected value of R7, the rise time and fall time are different. R7 can be approximately determined from Figure 11 and Figure 12. The minimum recommended value is 10 kΩ.

8.2.2.1.9 Resistor to Select Switching Frequency (R8)

See Selecting the Switching Frequency, Figure 10 and Equation 4.

8.2.2.1.10 Resistors to Select Output Voltage (R4, R5)

To minimize the effect of leakage current on the VSENSE terminal, the current flowing through the feedback network should be greater than 5 mA to maintain output accuracy. Higher resistor values help improve the converter efficiency at low-output currents, but may introduce noise immunity problems. See Equation 1. TI recommends fixing R4 to a standard value (for example, 187 kΩ) and calculate R5.

8.2.2.1.11 Resistors to Set Undervoltage, Overvoltage, and Reset Thresholds (R1, R2, R3)

8.2.2.1.12 Low-Power Mode (LPM) Threshold

An approximation of the output load current at which the converter is operating in discontinuous mode can be obtained from Equation 23 with ± 30% hysteresis. The values used in Equation 3 for minimum and maximum input voltage will affect the duty cycle and the overall discontinuous mode load current. These are the nominal values, and other factors are not taken into consideration like external component variations with temperature and aging.

8.2.2.1.13 Enable Pin Pull-Up Resistor (R11) and Voltage Divider Resistor (R10)

An external pull-up resistor, R11= 30.1 kΩ, is recommended to enable the device for operation and R10 can be left open.

Based on the application needs, if the device needs to be turned on at certain input voltage using EN pin threshold, R10 can be used as a voltage divider resistor along with pull-up resistor (R11=30.1 kΩ) and R10 can be calculated accordingly.

8.2.2.1.14 Pull-Up Resistor (R12) at RST Pin

A standard pull-up resistor, R12 = 2 K Ω can be used at this pin

8.2.2.1.15 Type 3 Compensation Components (R5, R6, R9, C5, C7, C8)

First, make the ZEROs close to double pole frequency, using Equation 12, Equation 13, and Equation 11.

fz1 = (50% to 70%) f_LC

fz2 = f_LC

Second, make the POLEs above the crossover frequency, using Equation 18 and Equation 19.

fp1 = f_ESR

fp2 = ½f_sw

8.2.2.1.15.1 Resistors

From Equation 1, knowing V_Reg and R4 (fix to a standard value), R5 can be calculated as shown in Equation 37:

Equation 37.

Using Equation 11 and Equation 15, R6 can be calculated as shown in Equation 38:

Equation 38.

R9 can be calculated as shown in Equation 39:

Equation 39.

8.2.2.1.15.2 Capacitors

Using Equation 20, C5 can be calculated as shown in Equation 40:

Equation 40.

C7 can be calculated as shown in Equation 41:

Equation 41.

C8 can be calculated as shown in Equation 42:

Equation 42.

8.2.2.1.16 Noise Filter on RST_TH and OV_TH Terminals (C9, C10)

These capacitors may be required in some applications to filter the noise on RST_TH and OV_TH pins. Typical capacitor values for RST_TH and OV_TH pins are from 10 pF to 100 pF for total resistance on RST_TH/OV_TH divider of less than 200 kΩ. See Noise Filter on RST_TH and OV_TH Terminals.

Table 7. Design Parameters – Example 1⁽¹⁾

8.2.2.2.1 Calculate the Switching Frequency (f_sw)

To reduce the size of output inductor and capacitor, higher switching frequency can be selected. It is important to understand that higher switching frequency results in higher switching losses, causing the device to heat up. This may result in degraded thermal performance. To prevent this, proper PCB layout guidelines must be followed (see Layout Guidelines).

Based upon the discussion in section Selecting the Switching Frequency, calculate the maximum and minimum duty cycle.

Knowing V_Reg and tolerance on V_Reg, the V_Reg-Max and V_Reg-Min are calculated to be:

V_Reg-Max = 102% of V_Reg = 5.1 V and V_Reg-Min = 98% of V_Reg = 4.9 V.

Using Equation 3, the minimum duty cycle is calculated to be, D_Min = 17.5%

Knowing: t_ON-Min = 150 ns from the device specifications, and using Equation 4, maximum switching frequency is calculated to be, f_sw-Max = 1166 kHz

Because the oscillator can also vary by ±10%, the switching frequency can be further reduced by 10% to add margin. Also, to improve efficiency and reduce power losses due to switching, the switching frequency can be further reduced by about 550 kHz. Therefore, f_sw = 500 kHz.

From Figure 10, R8 can be approximately determined to be, R8 = 205 kΩ.

8.2.2.2.2 Calculate the Ripple Current (I_Ripple)

Using Equation 32, for K_IND = 0.2 (typical), inductor ripple current is calculated to be: I_Ripple = 0.36 A.

The ripple current is chosen such that the converter enters discontinuous mode (DCM) at 20% of maximum load. The 20% is a typical value, it could go higher to a maximum of up to 40%.

8.2.2.2.3 Calculate the Inductor Value (L1)

Using Equation 33, the inductor value is calculated to be, L_Min = 22.8 µH. A closest standard inductor value can be used.

8.2.2.2.4 Calculate the Output Capacitor and ESR (C4)

8.2.2.2.5 Calculate the Feedback Resistors (R4, R5)

To keep the quiescent current low and avoid instability problems, TI recommends selecting R4 and R5 such that, R4 + R5 is approximately 250 kΩ.

Using Equation 1 and using a fixed standard value of R4 = 187 kΩ, R5 is calculated to be, R5 = 35.7 kΩ .

8.2.2.2.6 Calculate Type 3 Compensation Components

8.2.2.2.7 Calculate Soft-Start Capacitor (C6)

The recommended value of soft-start capacitor is 100 nF (typical).

8.2.2.2.8 Calculate Bootstrap Capacitor (C3)

The recommended value of bootstrap capacitor is 0.1 µF (typical).

8.2.2.2.9 Calculate Power-On Reset Delay Capacitor (C2)

To achieve 2.2-ms delay, the reset delay capacitor can be calculated using Equation 6 to be C2 = 2.2 nF.

8.2.2.2.10 Calculate Input Capacitor (C1, C11)

Typical values for C11 are 0.1 µF and 0.01 µF.

Input capacitor (C1) should be rated more than the maximum input voltage (VIN_Max). The input capacitor should be big enough to maintain supply in case of transients in the input line. Using Equation 26, C1 is calculated to be, C1 = 1.2 µF. For improved transient response, TI recommends a higher value of C1 such as 220 µF.

8.2.2.2.11 Calculate Resistors to Control Slew Rate (R7)

The value of slew rate resistor (R7) can be approximately determined from Figure 11 and Figure 12 at different typical input voltages. The minimum recommended value is 10 kΩ. To achieve rise time, t_r = 20 ns and fall time, t_f = 35 ns, the slew rate resistor is approximately of value 30 kΩ.

8.2.2.2.12 Resistors to Select Undervoltage, Overvoltage and Reset Threshold Values (R1, R2, R3)

The sum of these three resistors should be approximately equal to 100 kΩ. In this example,

• VReg_OV = 106% of V_Reg = 5.3 V
• VReg_RST = 92% of V_Reg = 4.6 V
• VReg_UV = 95% of V_Reg = 4.75 V

Using Equation 9, R3 = 15 kΩ.

Using Equation 8, R2 = 2.29 kΩ.

Using Equation 7, R1 = 82.6 kΩ

8.2.2.2.13 Diode D1 and D2 Selection

Diode D1 is used to protect the IC from the reverse input polarity connection. The diode should be rated at maximum load current. Only Schottky diode should be connected at the PH pin. The recommended part numbers are PDS360 and SBR8U60P5.

8.2.2.2.14 Noise Filter on RST_TH and OV_TH Terminals (C9 and C10)

Typical capacitor values for RST_TH and OV_TH pins are from 10 pF to 100 pF for total resistance on RST_TH/ OV_TH divider of less than 200 kΩ.

8.2.2.2.15 Power Budget and Temperature Estimation

Using Equation 43, conduction losses for typical input voltage are calculated to be, P_CON = 0.289 W.

Assuming slew resistance R7 = 30 kΩ, from Figure 11 and Figure 12, rise time, t_r = 20 ns and fall time, t_f = 35 ns. Using Equation 44, switching losses for typical input voltage are calculated to be, P_SW = 0.693 W.

Using Equation 45, gate drive losses are calculated to be, P_Gate = 3 mW.

Using Equation 46, power supply losses are calculated to be, P_IC = 1.8 mW.

Using Equation 47, the total power dissipated by the device is calculated to be, P_Total = 987 mW.

Using Equation 49, and knowing the thermal resistance of package = 35°C/W, the rise in junction temperature due to power dissipation is calculated to be, ?T = 34.5°C.

Using Equation 50, for a given maximum junction temperature 150°C, the maximum ambient temperature at which the device can be operated is calculated to be, T_A-Max = 115°C (approximately).

Table 8. Design Parameters – Example 2⁽¹⁾

8.2.2.3.1 Calculate the Switching Frequency (f_sw)

To reduce the size of output inductor and capacitor, higher switching frequency can be selected. It is important to understand that higher switching frequency results in higher switching losses, causing the device to heat up. This may result in degraded thermal performance. To prevent this, proper PCB layout guidelines must be followed (see Layout Guidelines).

Based upon the discussion in section Selecting the Switching Frequency, calculate the maximum and minimum duty cycle.

Knowing V_Reg and tolerance on V_Reg, the V_Reg-Max and V_Reg-Min are calculated to be:

V_Reg-Max = 102% of V_Reg = 3.366 V and V_Reg-Min = 98% of V_Reg = 3.234 V.

Using Equation 3, the minimum duty cycle is calculated to be, D_Min = 11.55%

Knowing t_ON-Min = 150 ns from the device specifications, and using Equation 4, maximum switching frequency is calculated to be, f_sw-Max = 770 kHz.

Because the oscillator can also vary by ±10%, the switching frequency can be further reduced by 10% to add margin. Also, to improve efficiency and reduce power losses due to switching, the switching frequency can be further reduced by about 100 kHz. Therefore f_sw = 593 kHz.

From Figure 10, R8 can be approximately determined to be, R8 = 170 kΩ.

8.2.2.3.2 Calculate the Ripple Current (I_Ripple)

Using Equation 32, for K_IND = 0.2 (typical), inductor ripple current is calculated to be: I_Ripple = 0.4 A.

The ripple current is chosen such that the converter enters discontinuous mode (DCM) at 20% of maximum load. The 20% is a typical value, although it could go higher to a maximum of up to 40%.

8.2.2.3.3 Calculate the Inductor Value (L1)

Using Equation 33, the inductor value is calculated to be, L_Min = 12.3 µH. A closest standard inductor value can be used.

8.2.2.3.4 Calculate the Output Capacitor and ESR (C4, C12)

8.2.2.3.5 Calculate the Feedback Resistors (R4, R5)

To keep the quiescent current low and avoid instability problems, TI recommends selecting R4 and R5 such that, R4 + R5 is approximately 250 kΩ.

Using Equation 1 and using a fixed standard value of R4 = 187 kΩ, R5 is calculated to be, R5 = 59.8 kΩ .

8.2.2.3.6 Calculate Type 3 Compensation Components

8.2.2.3.7 Calculate Soft-Start Capacitor (C6)

The recommended value of soft-start capacitor is 100 nF (typical).

8.2.2.3.8 Calculate Bootstrap Capacitor (C3)

The recommended value of bootstrap capacitor is 0.1 µF (typical).

8.2.2.3.9 Calculate Power-On Reset Delay Capacitor (C2)

To achieve 2.2-ms delay, the reset delay capacitor can be calculated using Equation 6 to be C2 = 2.2 nF.

8.2.2.3.10 Calculate Input Capacitor (C1, C11)

Typical values for C11 are 0.1 µF and 0.01 µF.

Input capacitor (C1) should be rated more than the maximum input voltage (VIN_Max). The input capacitor should be big enough to maintain supply in case of transients in the input line. Using Equation 26, C1 is calculated to be, C1 = 10.53 µF. For improved transient response, TI recommends a higher value of C1 such as 220 µF.

8.2.2.3.11 Calculate Resistors to Control Slew Rate (R7)

The value of slew rate resistor (R7) can be approximately determined from Figure 11 and Figure 12 at different typical input voltages. The minimum recommended value is 10 kΩ. To achieve rise time, t_r = 20 ns and fall time, t_f = 35 ns, the slew rate resistor is approximately of value 30 kΩ.

8.2.2.3.12 Resistors to Select Undervoltage, Overvoltage and Reset Threshold Values (R1, R2, R3)

The sum of these three resistors should be approximately equal to 100 kΩ. In this example,

VReg_OV = 106% of V_Reg = 3.498 V

VReg_RST = 92% of V_Reg = 3.036 V

VReg_UV = 95% of V_Reg = 3.135 V

Using Equation 9, R3 = 22.87 kΩ.

Using Equation 8, R2 = 3.48 kΩ.

Using Equation 7, R1 = 73.65 kΩ

8.2.2.3.13 Diode D1 and D2 Selection

Diode D1 is used to protect the IC from the reverse input polarity connection. The diode should be rated at maximum load current. Only Schottky diode should be connected at the PH pin. The recommended part numbers are PDS360 and SBR8U60P5.

8.2.2.3.14 Noise Filter on RST_TH and OV_TH Terminals (C9 and C10)

Typical capacitor values for RST_TH and OV_TH pins are from 10 pF to 100 pF for total resistance on RST_TH/ OV_TH divider of less than 200 kΩ.

8.2.2.3.15 Power Budget and Temperature Estimation

Using Equation 43, conduction losses for typical input voltage are calculated to be, P_CON = 0.235 W.

Assuming slew resistance R7 = 30 kΩ, from Figure 17 and Figure 18, rise time, t_r = 20 ns and fall time, t_f = 35 ns. Using Equation 19, switching losses for typical input voltage are calculated to be, P_SW = 0.913 W.

Using Equation 44, gate drive losses are calculated to be, P_Gate = 3.5 mW.

Using Equation 46, power supply losses are calculated to be, P_IC = 1.8 mW.

Using Equation 47, the total power dissipated by the device is calculated to be, P_Total = 1.15 W.

Using Equation 49, and knowing the thermal resistance of package = 35°C/W, the rise in junction temperature due to power dissipation is calculated to be, ?T = 40.4°C.

Using Equation 50, for a given maximum junction temperature 150°C, the maximum ambient temperature at which the device can be operated is calculated to be, T_A-Max approximately 105°C.