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LT3508HUF-TRPBF 数据表(PDF) 9 Page - Linear Technology |
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LT3508HUF-TRPBF 数据表(HTML) 9 Page - Linear Technology |
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9 / 24 page  LT3508 9 3508fb APPLICATIONS INFORMATION Setting the Output Voltage The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the 1% resis- tors according to: RR V V OUT 12 08 1 = ⎛ ⎝⎜ ⎞ ⎠⎟ . – R2 should be 20k or less to avoid bias current errors. Reference designators refer to the Block Diagram. Minimum Operating Voltage The minimum operating voltage is determined either by the LT3508’s undervoltage lockout or by its maximum duty cycle. If VIN1 and VIN2 are tied together, the undervoltage lockout is at 3.7V or below. If the two inputs are used separately, then VIN1 has an undervoltage lockout of 3.7V or below and VIN2 has an undervoltage lockout of 3V or below. Because the internal supply runs off VIN1, chan- nel 2 will not operate unless VIN1 > 3.7V. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC VV VV V OUT F IN SW F = + + – Unlike many fixed frequency regulators, the LT3508 can extend its duty cycle by turning on for multiple cycles. The LT3508 will not switch off at the end of each clock cycle if there is sufficient voltage across the boost capacitor (C3 in Figure 1). Eventually, the voltage on the boost capacitor falls and requires refreshing. Circuitry detects this condi- tion and forces the switch to turn off, allowing the inductor current to charge up the boost capacitor. This places a limitation on the maximum duty cycle as follows: DCMAX SW = + 1 1 1 β where βSW is equal to the SW pin current divided by the BOOST pin current as shown in the Typical Performance Characteristics section. This leads to a minimum input voltage of: V VV DC VV IN MIN OUT F MAX FSW () – = + + where VF is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.4V at maximum load). Example: ISW = 1.5A and IBOOST = 50mA, VOUT = 3.3V, βSW = 1.5A/50mA = 30, DCMAX = 1/(1+1/30) = 96%: V VV VV V IN MIN () .. % –. . . = + += 33 04 96 04 04 38 Maximum Operating Voltage The maximum operating voltage is determined by the Absolute Maximum Ratings of the VIN and BOOST pins, and by the minimum duty cycle: DCMIN = tON(MIN) • f where tON(MIN) is equal to 130ns (for TJ > 125°C tON(MIN) is equal to 150ns) and f is the switching frequency. Running at a lower switching frequency allows a lower minimum duty cycle. The maximum input voltage before pulse skipping occurs depends on the output voltage and the minimum duty cycle: V VV DC VV IN PS OUT F MIN FSW () – = + + Example: f = 790kHz, VOUT = 3.3V, DCMIN = 130ns • 790kHz = 0.103: V VV VV V IN PS () .. . –. . = + += 33 04 0 103 04 04 36 The LT3508 will regulate the output current at input voltages greater than VIN(PS). For example, an application with an output voltage of 1.8V and switching frequency of 1.5MHz has a VIN(PS) of 11.3V, as shown in Figure 2. Figure 3 shows operation at 18V. Output ripple and peak inductor current have significantly increased. Exceeding VIN(PS) is safe if the output is in regulation, if the external components have adequate ratings to handle the peak conditions and if the peak inductor current does not exceed 3.2A. A saturating inductor may further reduce performance. Do not exceed VIN(PS) during start-up or overload conditions (for outputs greater than 5V, use VOUT = 5V to calculate VIN(PS)). For operation above 20V in pulse skipping mode, program the switching frequency to 1.1MHz or less. |
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类似说明 - LT3508HUF-TRPBF |
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