Datasheet LT3663 (Analog Devices) - 8
| Fabricante | Analog Devices |
| Descripción | 1.2A Step-Down Switching Regulator with Output Current Limit |
| Páginas / Página | 16 / 8 — applicaTions inForMaTion. FB Resistor Network. Minimum Duty Cycle. Figure … |
| Formato / tamaño de archivo | PDF / 354 Kb |
| Idioma del documento | Inglés |
applicaTions inForMaTion. FB Resistor Network. Minimum Duty Cycle. Figure 2. Pulse-Skip Mode. Inductor Selection
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Versión de texto del documento
LT3663
applicaTions inForMaTion FB Resistor Network
Figure 2. il ustrates switching waveforms in pulse skip The output voltage of the LT3663 is programmed with a mode a 3.3V output application with VIN = 30V. resistor divider. Choose the 1% resistors according to: R1 = R2 (VOUT/0.8 – 1) Reference designators refer to the Block Diagram. VSW 20V/DIV The resistor divider is internal on the fixed output voltage versions, LT3663-X. IL 1A/DIV
Minimum Duty Cycle
VOUT As the input voltage increases the duty cycle decreases. At 200mV/DIV AC-COUPLED some point, the minimum duty cycle of the IC is reached 2.5µs/DIV 3663 F02 and pulse skipping begins. The output voltage continues to regulate but the output voltage ripple increases. The input
Figure 2. Pulse-Skip Mode
voltage at which this occurs is calculated as fol ows: (V +V ) V OUT F = – V + V IN F SW
Inductor Selection
DCMIN A good first choice for the inductor value is: Where VF is the forward voltage drop of the catch diode, L = V D1, and V OUT + VD (µH) SW is the voltage drop on the internal switch, Q1. LT3663 DC where V MIN is typical y 0.12. D is the voltage drop of the catch diode (~0.4V) and L is in µH. With this value there is no sub-harmonic For VF = 0.4V, VSW = 0.3V, and VOUT = 3.3V pulse skipping oscil ation for applications with 50% or greater duty occurs at about 24.5V. cycle. The inductor’s RMS current rating must be greater Figure 1 il ustrates switching waveforms in normal mode than the maximum load current. Size the inductor so the for a 3.3V output application with VIN = 20V. saturation current rating is about 30% higher than the maximum load current. The output current limit circuit tightly controls the maximum average inductor current, therefore the inductor RMS current rating does not have to be overrated to handle short circuit or overload condi- VSW 20V/DIV tions. For high efficiency, keep the series resistance (DCR) less than 0.1Ω. A higher value inductor is larger in size and slows transient response but reduces output voltage IL 1A/DIV ripple. A lower value inductor has higher ripple currents but is physical y smal er or, for the same size, it has lower VOUT 200mV/DIV DCR typical y resulting in higher efficiency. AC-COUPLED 2.5µs/DIV 3663 F01
Figure 1. Normal Operation
3663fc 8 For more information www.linear.com/LT3663 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Operation Applications Information Package Description
applicaTions inForMaTion FB Resistor Network
Figure 2. il ustrates switching waveforms in pulse skip The output voltage of the LT3663 is programmed with a mode a 3.3V output application with VIN = 30V. resistor divider. Choose the 1% resistors according to: R1 = R2 (VOUT/0.8 – 1) Reference designators refer to the Block Diagram. VSW 20V/DIV The resistor divider is internal on the fixed output voltage versions, LT3663-X. IL 1A/DIV
Minimum Duty Cycle
VOUT As the input voltage increases the duty cycle decreases. At 200mV/DIV AC-COUPLED some point, the minimum duty cycle of the IC is reached 2.5µs/DIV 3663 F02 and pulse skipping begins. The output voltage continues to regulate but the output voltage ripple increases. The input
Figure 2. Pulse-Skip Mode
voltage at which this occurs is calculated as fol ows: (V +V ) V OUT F = – V + V IN F SW
Inductor Selection
DCMIN A good first choice for the inductor value is: Where VF is the forward voltage drop of the catch diode, L = V D1, and V OUT + VD (µH) SW is the voltage drop on the internal switch, Q1. LT3663 DC where V MIN is typical y 0.12. D is the voltage drop of the catch diode (~0.4V) and L is in µH. With this value there is no sub-harmonic For VF = 0.4V, VSW = 0.3V, and VOUT = 3.3V pulse skipping oscil ation for applications with 50% or greater duty occurs at about 24.5V. cycle. The inductor’s RMS current rating must be greater Figure 1 il ustrates switching waveforms in normal mode than the maximum load current. Size the inductor so the for a 3.3V output application with VIN = 20V. saturation current rating is about 30% higher than the maximum load current. The output current limit circuit tightly controls the maximum average inductor current, therefore the inductor RMS current rating does not have to be overrated to handle short circuit or overload condi- VSW 20V/DIV tions. For high efficiency, keep the series resistance (DCR) less than 0.1Ω. A higher value inductor is larger in size and slows transient response but reduces output voltage IL 1A/DIV ripple. A lower value inductor has higher ripple currents but is physical y smal er or, for the same size, it has lower VOUT 200mV/DIV DCR typical y resulting in higher efficiency. AC-COUPLED 2.5µs/DIV 3663 F01
Figure 1. Normal Operation
3663fc 8 For more information www.linear.com/LT3663 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Operation Applications Information Package Description