Datasheet LT3508 (Analog Devices) - 9

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APPLICATIONS INFORMATION Setting the Output Voltage
where VF is the forward voltage drop of the catch diode The output voltage is programmed with a resistor divider (~0.4V) and VSW is the voltage drop of the internal switch between the output and the FB pin. Choose the 1% resis- (~0.4V at maximum load). tors according to: Example: ISW = 1.5A and IBOOST = 50mA, VOUT = 3.3V, ⎛⎛ V ⎞ βSW = 1.5A/50mA = 30, DCMAX = 1/(1+1/30) = 96%: R1=R2 OUT –1⎞ ⎝⎜⎝⎜ 0.8V ⎠⎟⎠⎟ 3.3V +0.4V VIN(MIN) = – 0.4V +0.4V = 3.8V 96% R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram.
Maximum Operating Voltage
The maximum operating voltage is determined by the
Minimum Operating Voltage
Absolute Maximum Ratings of the VIN and BOOST pins, The minimum operating voltage is determined either by and by the minimum duty cycle: the LT3508’s undervoltage lockout or by its maximum duty cycle. If V DCMIN = tON(MIN) • f IN1 and VIN2 are tied together, the under- voltage lockout is at 3.7V or below. If the two inputs are where tON(MIN) is equal to 130ns (for TJ > 125°C tON(MIN) used separately, then VIN1 has an undervoltage lockout is equal to 150ns) and f is the switching frequency. of 3.7V or below and VIN2 has an undervoltage lockout Running at a lower switching frequency allows a lower of 3V or below. Because the internal supply runs off VIN1 , minimum duty cycle. The maximum input voltage before channel 2 will not operate unless VIN1 > 3.7V. The duty pulse skipping occurs depends on the output voltage and cycle is the fraction of time that the internal switch is on the minimum duty cycle: and is determined by the input and output voltages: VOUT + VF V VIN(PS) = – VF + VSW DC = OUT + VF DCMIN VIN – VSW + VF Unlike many fixed frequency regulators, the LT3508 can Example: f = 790kHz, VOUT = 3.3V, DCMIN = 130ns • extend its duty cycle by turning on for multiple cycles. 790kHz = 0.103: The LT3508 will not switch off at the end of each clock 3.3V +0.4V cycle if there is sufficient voltage across the boost capac- VIN(PS) = – 0.4V +0.4V = 36V 0.103 itor (C3 in Figure 1). Eventually, the voltage on the boost capacitor falls and requires refreshing. Circuitry detects The LT3508 will regulate the output current at input volt- this condition and forces the switch to turn off, allowing ages greater than VIN(PS). For example, an application the inductor current to charge up the boost capacitor. This with an output voltage of 1.8V and switching frequency places a limitation on the maximum duty cycle as follows: of 1.5MHz has a VIN(PS) of 11.3V, as shown in Figure 2. 1 Figure 3 shows operation at 18V. Output ripple and peak DCMAX = inductor current have significantly increased. Exceeding 1+ 1 VIN(PS) is safe if the output is in regulation, if the external βSW components have adequate ratings to handle the peak where βSW is equal to the SW pin current divided by the conditions and if the peak inductor current does not BOOST pin current as shown in the Typical Performance exceed 3.2A. A saturating inductor may further reduce Characteristics section. This leads to a minimum input performance. Do not exceed VIN(PS) during start-up or voltage of: overload conditions (for outputs greater than 5V, use V VOUT = 5V to calculate VIN(PS)). For operation above 20V V OUT + VF IN(MIN) = – VF + VSW in pulse skipping mode, program the switching frequency DCMAX to 1.1MHz or less. Rev. E For more information www.analog.com 9 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts