Datasheet LT3493 (Analog Devices) - 8
| Hersteller | Analog Devices |
| Beschreibung | 1.2A, 750kHz Step-Down Switching Regulator in 2mm × 3mm DFN |
| Seiten / Seite | 20 / 8 — APPLICATIONS INFORMATION. FB Resistor Network. Minimum On Time. Input … |
| Dateiformat / Größe | PDF / 607 Kb |
| Dokumentensprache | Englisch |
APPLICATIONS INFORMATION. FB Resistor Network. Minimum On Time. Input Voltage Range. Figure 1. Figure 2
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LT3493
APPLICATIONS INFORMATION FB Resistor Network
of the VIN and BOOST pins. The input voltage should be The output voltage is programmed with a resistor divider limited to the VIN operating range (36V) during overload between the output and the FB pin. Choose the 1% resis- conditions (short-circuit or start-up). tors according to:
Minimum On Time
R1= R2 VOUT – 1 The part will still regulate the output at input voltages that 0.78V exceed VIN(MAX) (up to 40V), however, the output voltage R2 should be 20k or less to avoid bias current errors. ripple increases as the input voltage is increased. Figure 1 Reference designators refer to the Block Diagram. illustrates switching waveforms in continuous mode for a 3V output application near V An optional phase lead capacitor of 22pF between V IN(MAX) = 33V. OUT and FB reduces light-load output ripple. As the input voltage is increased, the part is required to switch for shorter periods of time. Delays associated
Input Voltage Range
with turning off the power switch dictate the minimum The input voltage range for LT3493 applications depends on time of the part. The minimum on time for the LT3493 on the output voltage and on the absolute maximum rat- is ~120ns. Figure 2 illustrates the switching waveforms ings of the V when the input voltage is increased to V IN and BOOST pins. IN = 35V. The minimum input voltage is determined by either the LT3493’s minimum operating voltage of 3.6V, or by its maximum duty cycle. The duty cycle is the fraction of VSW 20V/DIV time that the internal switch is on and is determined by the input and output voltages: IL V 0.5A/DIV OUT + VD DC = V – V IN SW + VD VOUT 200mV/DIV where VD is the forward voltage drop of the catch diode AC COUPLED (~0.4V) and VSW is the voltage drop of the internal switch COUT = 10μF 2μs/DIV 3493 F01 (~0.4V at maximum load). This leads to a minimum input VOUT = 3V VIN = 30V voltage of: ILOAD = 0.75A L = 10μH V
Figure 1
V OUT + VD IN(MIN) = – VD + VSW DCMAX with DCMAX = 0.91 (0.88 over temperature). VSW 20V/DIV The maximum input voltage is determined by the absolute maximum ratings of the VIN and BOOST pins. For con- IL tinuous mode operation, the maximum input voltage is 0.5A/DIV determined by the minimum duty cycle DCMIN = 0.10: V VOUT 200mV/DIV V OUT + VD AC COUPLED IN(MAX) = – VD + VSW DCMIN COUT = 10μF 2μs/DIV 3493 F02 VOUT = 3V Note that this is a restriction on the operating input voltage VIN = 35V I for continuous mode operation; the circuit will tolerate LOAD = 0.75A L = 10μH transient inputs up to the absolute maximum ratings
Figure 2
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APPLICATIONS INFORMATION FB Resistor Network
of the VIN and BOOST pins. The input voltage should be The output voltage is programmed with a resistor divider limited to the VIN operating range (36V) during overload between the output and the FB pin. Choose the 1% resis- conditions (short-circuit or start-up). tors according to:
Minimum On Time
R1= R2 VOUT – 1 The part will still regulate the output at input voltages that 0.78V exceed VIN(MAX) (up to 40V), however, the output voltage R2 should be 20k or less to avoid bias current errors. ripple increases as the input voltage is increased. Figure 1 Reference designators refer to the Block Diagram. illustrates switching waveforms in continuous mode for a 3V output application near V An optional phase lead capacitor of 22pF between V IN(MAX) = 33V. OUT and FB reduces light-load output ripple. As the input voltage is increased, the part is required to switch for shorter periods of time. Delays associated
Input Voltage Range
with turning off the power switch dictate the minimum The input voltage range for LT3493 applications depends on time of the part. The minimum on time for the LT3493 on the output voltage and on the absolute maximum rat- is ~120ns. Figure 2 illustrates the switching waveforms ings of the V when the input voltage is increased to V IN and BOOST pins. IN = 35V. The minimum input voltage is determined by either the LT3493’s minimum operating voltage of 3.6V, or by its maximum duty cycle. The duty cycle is the fraction of VSW 20V/DIV time that the internal switch is on and is determined by the input and output voltages: IL V 0.5A/DIV OUT + VD DC = V – V IN SW + VD VOUT 200mV/DIV where VD is the forward voltage drop of the catch diode AC COUPLED (~0.4V) and VSW is the voltage drop of the internal switch COUT = 10μF 2μs/DIV 3493 F01 (~0.4V at maximum load). This leads to a minimum input VOUT = 3V VIN = 30V voltage of: ILOAD = 0.75A L = 10μH V
Figure 1
V OUT + VD IN(MIN) = – VD + VSW DCMAX with DCMAX = 0.91 (0.88 over temperature). VSW 20V/DIV The maximum input voltage is determined by the absolute maximum ratings of the VIN and BOOST pins. For con- IL tinuous mode operation, the maximum input voltage is 0.5A/DIV determined by the minimum duty cycle DCMIN = 0.10: V VOUT 200mV/DIV V OUT + VD AC COUPLED IN(MAX) = – VD + VSW DCMIN COUT = 10μF 2μs/DIV 3493 F02 VOUT = 3V Note that this is a restriction on the operating input voltage VIN = 35V I for continuous mode operation; the circuit will tolerate LOAD = 0.75A L = 10μH transient inputs up to the absolute maximum ratings
Figure 2
3493fb 8