Datasheet LT1941 (Analog Devices) - 10

LT1941
APPLICATIONS INFORMATION STEP-DOWN CONSIDERATIONS
where VF is the voltage drop of the catch diode (~0.4V) and L is in μH. With this value the maximum load current will
FB Resistor Network
be 2.1A for SW1 and 1.4A for SW2, independent of input The output voltage is programmed with a resistor divider voltage. The inductor’s RMS current rating must be greater (refer to the Block Diagram) between the output and the than the maximum load current and its saturation current FB pin. Choose the resistors according to should be at least 30% higher. For highest effi ciency, the series resistance (DCR) should be less than 0.1Ω. Table 1 R1 = R2(VOUT/628mV – 1) lists several vendors and types that are suitable. R2 should be 10k or less to avoid bias current errors.
Table 1. Inductors Input Voltage Range VALUE ISAT DCR HEIGHT PART NUMBER (μH) (A) (Ω) (mm)
The minimum operating voltage is determined either by the
Sumida
LT1941’s undervoltage lockout of ~3.3V or by its maximum CR43-1R4 1.4 2.52 0.056 3.5 duty cycle. The duty cycle is the fraction of time that the CR43-2R2 2.2 1.75 0.071 3.5 internal switch is on and is determined by the input and CDRH3D16-1R5 1.5 1.55 0.040 1.8 output voltages: CDRH4D28-3R3 3.3 1.57 0.049 3.0 DC = (V CDRH4D18-1R0 1.0 1.70 0.035 2.0 OUT + VF)/(VIN – VSW + VF) CDC5D23-2R2 2.2 2.50 0.03 2.5 where VF is the forward voltage drop of the catch diode CDRH5D28-2R6 2.6 2.60 0.013 3.0 (~0.4V) and VSW is the voltage drop of the internal switch
Coilcraft
(~0.3V at maximum load). This leads to a minimum input DO1606T-152 1.5 2.10 0.060 2.0 voltage of: DO1606T-222 2.2 1.70 0.070 2.0 VIN(MIN) = (VOUT + VF)/DCMAX – VF + VSW DO1608C-152 1.5 2.60 0.050 2.9 with DC DO1608C-222 2.2 2.30 0.070 2.9 MAX = 0.78. DO1608C-332 3.3 2.00 0.080 2.9 The maximum operating voltage is determined by the DO1608C-472 4.7 1.50 0.090 2.9 absolute maximum ratings of the VIN and BOOST pins MOS6020-222 2.2 2.15 0.035 2.0 and by the minimum duty cycle DCMIN = 0.15: MOS6020-332 3.3 1.8 0.046 2.0 VIN(MAX) = (VOUT + VF)/DCMIN – VF + VSW MOS6020-472 4.7 1.5 0.050 2.0 DO3314-222 2.2 1.6 0.200 1.4 This limits the maximum input voltage to ~14V with
Toko
VOUT = 1.8V and ~19V with VOUT = 2.5. Note that this is (D62F)847FY-2R4M 2.4 2.5 0.037 2.7 a restriction on the operating input voltage; the circuit will tolerate input voltage transients up to the Absolute (D73LF)817FY-2R2M 2.2 2.7 0.03 3.0 Maximum Rating. The optimum inductor for a given application may differ from the one indicated by this simple design guide. A
Inductor Selection and Maximum Output Current
larger value inductor provides a slightly higher maximum A good fi rst choice for the inductor value is load current and will reduce the output voltage ripple. If L = (VOUT + VF)/1.6 for SW1 L = (VOUT + VF)/1.1 for SW2 1941fb 10