TPS54262-EP Datasheet, PDF (28/48 Pages) Texas Instruments – 2-A 60-V Step-Down DC-DC Converter With Low I(q) and Voltage Supervisor

TPS54262-EP
SLVSDP3 – DECEMBER 2016
www.ti.com
Typical Application (continued)
To calculate the minimum value of the inductor, the ripple current should be first calculated using Equation 32.
I_Ripple = K_IND × I_Load
where
• I_Load = maximum output load current
• I_Ripple = allowable peak to peak inductor ripple current, typically 20% of maximum I_Load
• K_IND = coefficient that represents the amount of inductor ripple current relative to the maximum output current.
(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.
(VIN_Max – V_Reg) × V_Reg
L_Min =
f_sw × I_Ripple × VIN_Max
where
• VIN_Max = maximum input voltage
• V_Reg = regulated output voltage
• f_sw = switching frequency
(33)
The RMS and peak currents flowing in the inductor are given by Equation 34 and Equation 35.
I = _L,RMS
I ₂
I + _{12 Load}
2
Ripple
(34)
I_Ripple
I = I + _{2 L,pk}
Load
(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.
P = _diode
(VIN_Max – V_Reg) × I_Load × V_fd
2
(VIN – V_fd) × f_sw × C_J
+
VIN_Max
2
where
• P_diode = power rating
• V_fd = forward conducting voltage of Schottky diode
• C_J = junction capacitance of the Schottky diode
(36)
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.
28
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