Datasheet LT3570 (Analog Devices) - 10

LT3570
APPLICATIONS INFORMATION FB Resistor Network
The optimum inductor for a given application may differ from the one indicated by this simple design guide. A The output voltage is programmed with a resistor divider larger value inductor provides a slightly higher maximum (refer to the Block Diagram) between the output and the load current and will reduce the output voltage ripple. If FB pin. Choose the resistors according to: your load is lower than the maximum load current, then ⎛ V ⎞ you can relax the value of the inductor and operate with R1=R2 OUT – 1 ⎝⎜ ⎠⎟ higher ripple current. This allows you to use a physically 788mV smaller inductor or one with a lower DCR resulting in
Buck Inductor Selection and Maximum Output Current
higher effi ciency. Be aware that if the inductance differs from the simple rule above, then the maximum load current A good fi rst choice for the inductor value is will depend on input voltage. In addition, low inductance V may result in discontinuous mode operation, which further L = OUT2 + VF for SW2 reduces maximum load current. For details of maximum 0.75 • f output current and discontinuous mode operation, see where VF is the voltage drop of the catch diode (~0.4V) Linear Technology’s Application Note 44. Finally, for duty and f is the switching frequency. With this inductance cycles greater than 50% (VOUT2/VIN2 > 0.5) a minimum value or greater, the maximum load current will be 1A, inductance is required to avoid subharmonic oscillations, independent of input voltage. The inductor’s RMS current see Application Note 19. rating must be greater than the maximum load current and The current in the inductor is a triangle wave with an average its saturation current should be at least 30% higher. For value equal to the load current. The peak switch current highest effi ciency, the series resistance (DCR) should be is equal to the output current plus half the peak-to-peak less than 0.1Ω. Table 1 lists several vendors and types inductor ripple current. The LT3570 limits its switch cur- that are suitable. rent in order to protect itself and the system from overload
Table 1. Inductors
faults. Therefore, the maximum output current that the
VALUE ISAT DCR HEIGHT
LT3570 will deliver depends on the switch current limit,
PART NUMBER (μH) (A) (
Ω
) (mm)
the inductor value and the input and output voltages.
Sumida
CDRH4D28-3R3 3.3 1.57 0.049 3.0 When the switch is off, the potential across the inductor CDRH4D28-4R7 4.7 1.32 0.072 3.0 is the output voltage plus the catch diode drop. This gives CDC5D23-2R2 2.2 2.50 0.03 2.5 the peak-to-peak ripple current in the inductor: CR43-3R3 3.3 1.44 0.086 3.5 (1–DC2)(V ) OUT2 + VF CDRH5D28-100 10 1.3 0.048 3.0 ΔIL2 = L • f
Coilcraft
DO1608C-332 3.3 2.00 0.080 2.9 where DC2 is the duty cycle and is defi ned as: DO1608C-472 4.7 1.50 0.090 2.9 MOS6020-332 3.3 1.8 0.046 2.0 V DC2 = OUT2 D03314-103 10 0.8 0.520 1.4 VIN2 D03314-222 2.2 1.6 0.200 1.4
Toko
The peak inductor and switch current is: (D62F)847FY-2R4M 2.4 2.5 0.037 2.7 ΔI (D73LF)817FY-2R2M 2.2 2.7 0.03 3.0 I L2 SWPK2 = ILPK2 = IOUT2 +
Coiltronics
2 TP3-4R7 4.7 1.5 0.181 2.2 To maintain output regulation, this peak current must be TP1-2R2 2.2 1.3 0.188 1.8 less than the LT3570’s switch current limit ILIM2. ILIM2 is TP4-100 10 1.5 0.146 3.0 at least 1.5A at low duty cycles and decreases linearly 3570fb 10