Datasheet LT1936 (Analog Devices) - 8

LT1936
APPLICATIONS INFORMATION FB Resistor Network Inductor Selection and Maximum Output Current
The output voltage is programmed with a resistor divider A good fi rst choice for the inductor value is between the output and the FB pin. Choose the 1% resis- L = 2.2 (V tors according to: OUT + VD) where V D is the voltage drop of the catch diode (~0.4V) V R1=R2 OUT – 1 and L is in μH. With this value the maximum output cur- 1.200 rent will be above 1.2A at all duty cycles and greater than 1.4A for duty cycles less than 50% (VIN > 2 VOUT). The R2 should be 20k or less to avoid bias current errors. inductor’s RMS current rating must be greater than the Reference designators refer to the Block Diagram. maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions
Input Voltage Range
(start-up or short circuit) and high input voltage (>30V), The input voltage range for LT1936 applications depends the saturation current should be above 2.6A. To keep the on the output voltage and the Absolute Maximum Ratings effi ciency high, the series resistance (DCR) should be less of the VIN and BOOST pins. than 0.1Ω, and the core material should be intended for high frequency applications. Table 1 lists several vendors The minimum input voltage is determined by either the and suitable types. LT1936’s minimum operating voltage of ~3.45V or by its maximum duty cycle. The duty cycle is the fraction of
Table 1. Inductor Vendors
time that the internal switch is on and is determined by
VENDOR URL PART SERIES TYPE
the input and output voltages: Murata www.murata.com LQH55D Open V + V TDK www.component.tdk.com SLF7045 Shielded DC OUT D = SLF10145 Shielded V – V + V IN SW D Toko www.toko.com D62CB Shielded D63CB Shielded where VD is the forward voltage drop of the catch diode D75C Shielded (~0.5V) and V D75F Open SW is the voltage drop of the internal switch (~0.5V at maximum load). This leads to a minimum input Sumida www.sumida.com CR54 Open CDRH74 Shielded voltage of: CDRH6D38 Shielded CR75 Open V + V V OUT D = V + V IN MIN ( ) – DC D SW MAX Of course, such a simple design guide will not always result in the optimum inductor for your application. A with DCMAX = 0.87. larger value provides a slightly higher maximum load The maximum input voltage is determined by the absolute current and will reduce the output voltage ripple. If your maximum ratings of the VIN and BOOST pins and by the load is lower than 1.2A, then you can decrease the value minimum duty cycle DCMIN = 0.08: of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one V + V V OUT D = V + V with a lower DCR resulting in higher effi ciency. Be aware IN MA ( X) – DC D SW MIN that if the inductance differs from the simple rule above, then the maximum load current will depend on input volt- Note that this is a restriction on the operating input voltage; age. There are several graphs in the Typical Performance the circuit will tolerate transient inputs up to the absolute Characteristics section of this data sheet that show the maximum ratings of the VIN and BOOST pins. maximum load current as a function of input voltage and inductor value for several popular output voltages. Low 1936fd 8