TPS54262-Q1 Datasheet, PDF (28/44 Pages) Texas Instruments – 2-A 60-V STEP-DOWN DC/DC CONVERTER WITH LOW QUIESCENT CURRENT

TPS54262-Q1
SLVS996C – SEPTEMBER 2009 – REVISED JUNE 2010
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5) Power-On Reset Delay (PORdly) Capacitor (C2)
The value of this capacitor can be calculated using Equation 9.
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 located 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 40.
I_Ripple = K_IND × I_Load
(40)
(40)
Where,
I_Load = maximum output load current
I_Ripple = allowable peak to peak inductor ripple current, typ. 20% of maximum I_Load
K_IND = coefficient that represents the amount of inductor ripple current relative to the maximum output
current. Since, 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 41.
(VIN_Max – V_Reg) × V_Reg
L_Min = _{fsw × IRipple × VINMax}
(41)
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 42 and Equation 43.
I = _L,RMS
I ₂
I + _{12 Load}
2
Ripple
I_Ripple
I = I + _{2 L,pk}
Load
(42)
(43)
7) Flyback Schottky Diode (D2)
The TPS54262 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 44.
(VIN_Max – V_Reg) × I_Load × V_fd (VIN – V_fd) × f_sw × C_J
P = _diode
+
VIN_Max
2
(44)
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) Resistor to Set Slew Rate (R7)
The slew rate setting is asymmetrical; i.e., for a selected value of R7, the rise time and fall time are different. R7
can be approximately determined from Figure 24 and Figure 25. The minimum recommended value is 10 kΩ.
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