Datasheet LT1934, LT1934 (Analog Devices) - 8

LT1934/LT1934-1
APPLICATIONS INFORMATION Which One to Use: LT1934 or LT1934-1?
The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output The only difference between the LT1934 and LT1934-1 voltages: is the peak current through the internal switch and the inductor. If your maximum load current is less than 60mA, DC = (VOUT + VD)/(VIN – VSW + VD) use the LT1934-1. If your maximum load is higher, use where V the LT1934; it can supply up to ~300mA. D is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch While the LT1934-1 can’t deliver as much output current, (~0.3V at maximum load for the LT1934, ~0.1V for the it has other advantages. The lower peak switch current LT1934-1). This leads to a minimum input voltage of: allows the use of smaller components (input capacitor, V inductor and output capacitor). The ripple current at the IN(MIN) = (VOUT + VD)/DCMAX – VD + VSW input of the LT1934-1 circuit will be smaller and may be with DCMAX = 0.85. an important consideration if the input supply is current limited or has high impedance. The LT1934-1’s current
Inductor Selection
draw during faults (output overload or short) and start- A good fi rst choice for the inductor value is: up is lower. L = 2.5 • (VOUT + VD) • 1.8μs/ILIM The maximum load current that the LT1934 or LT1934-1 where I can deliver depends on the value of the inductor used. LIM is the switch current limit (400mA for the LT1934 and 120mA for the LT1934-1). This choice provides Table 1 lists inductor value, minimum output capacitor a worst-case maximum load current of 250mA (60mA for and maximum load for 3.3V and 5V circuits. Increasing the LT1934-1). The inductor’s RMS current rating must the value of the capacitor will lower the output voltage be greater than the load current and its saturation current ripple. Component selection is covered in more detail in should be greater than I the following sections. LIM . To keep effi ciency high, the series resistance (DCR) should be less than 0.3Ω (1Ω
Minimum Input Voltage
for the LT1934-1). Table 2 lists several vendors and types that are suitable. The minimum input voltage required to generate a par- ticular output voltage is determined by either the LT1934’s This simple rule may not provide the optimum value for undervoltage lockout of ~3V or by its maximum duty cycle. your application. If the load current is less, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller
Table 1
inductor, or one with a lower DCR resulting in higher
MINIMUM MAXIMUM
effi ciency. The following provides more details to guide
PART VOUT L COUT LOAD
inductor selection. First, the value must be chosen so that LT1934 3.3V 100μH 100μH 300mA 47μH 47μH 250mA the LT1934 can supply the maximum load current drawn 33μH 33μH 200mA from the output. Second, the inductor must be rated ap- 5V 150μH 47μH 300mA propriately so that the LT1934 will function reliably and 68μH 33μH 250mA the inductor itself will not be overly stressed. 47μH 22μH 200mA LT1934-1 3.3V 150μH 15μH 60mA
Detailed Inductor Selection and
100μH 10μH 45mA 68μH 10μH 20mA
Maximum Load Current
5V 220μH 10μH 60mA The square wave that the LT1934 produces at its switch 150μH 4.7μH 45mA 100μH 4.7μH 20mA pin results in a triangle wave of current in the inductor. The LT1934 limits the peak inductor current to ILIM. Because 1934fe 8