Datasheet LT3505 (Analog Devices) - 9

LT3505 APPLICATIONS INFORMATION
FB Resistor Network
where fSW is the switching frequency in hertz and tON(MIN) is The output voltage is programmed with a resistor divider the worst-case minimum on-time in seconds. The minimum between the output and the FB pin. Choose the 1% resis- on-time of the LT3505 is a strong function of temperature. tors according to: The typical performance characteristics section of the datasheet contains a graph of minimum on-time versus  V temperature to help determine the worst-case minimum R = R OUT 1 2 – 1 on-time for the intended application.  0 78V  . If the input voltage is high enough that the duty-cycle R2 should be 20k or less to avoid bias current errors. requirement is lower than DC Reference designators refer to the Block Diagram. MIN, the part enters pulse- skipping mode. Specifi cally, the onset of pulse-skipping
Input Voltage Range
occurs at: The input voltage range for LT3505 applications depends VIN(PS) = (VOUT + VD) / DCMIN – VD + VSW on the output voltage, on the absolute maximum ratings Above VIN(PS) the part turns on for brief periods of time of the VIN and BOOST pins, and on the programmed to control the inductor current and regulate the output switching frequency. voltage, possibly producing a spectrum of frequencies The minimum input voltage is determined by either the below the programmed switching frequency. To remain LT3505’s minimum operating voltage of 3.6V, or by its in constant-frequency operation the input voltage should maximum duty cycle. The duty cycle is the fraction of remain below VIN(PS). See the “Minimum On Time” sec- time that the internal switch is on and is determined by tion of the data sheet for more information on operating the input and output voltages: above VIN(PS). Note that this is a restriction on the operating input voltage VOUT V DC D = + to remain in constant-frequency operation; the circuit will VIN – VSW + VD tolerate brief transient inputs up to the absolute maximum ratings of the V where V IN and BOOST pins when the output is in D is the forward voltage drop of the catch diode regulation. The input voltage should be limited to V (~0.4V) and V IN(PS) SW is the voltage drop of the internal switch during overload conditions (short-circuit or start-up). (~0.4V at maximum load). This leads to a minimum input voltage of:
Minimum On Time
VOUT V V D For switching frequencies less than 750kHz, the part IN M ( IN) = + – VD + VSW DCMAX will still regulate the output at input voltages that exceed V with DC IN(PS) (up to 40V), however, the output voltage ripple MAX = 1 – fSW/8.33, where fSW is in MHz. increases as the input voltage is increased. Figure 1 il- The maximum input voltage is determined by the abso- lustrates switching waveforms in continuous mode for a lute maximum ratings of the VIN and BOOST pins. For 3V output application near VIN(PS) = 33V. constant-frequency operation, the maximum input voltage As the input voltage is increased, the part is required to is determined by the minimum duty cycle requirement. switch for shorter periods of time. Delays associated with As the input voltage increases, the required duty cycle turning off the power switch determine the minimum on to regulate the output voltage decreases. The minimum time of the part. The worst-case typical minimum on-time duty-cycle is: is 130ns. Figure 2 illustrates the switching waveforms DCMIN = fSW • tON(MIN) when the input voltage is increased to VIN = 35V. 3505fc 9