Datasheet LT1765, LT1765-1.8, LT1765-2.5, LT1765-3.3, LT1765-5 (Analog Devices) - 9
| Fabricante | Analog Devices |
| Descripción | Monolithic 3A, 1.25MHz Step-Down Switching Regulator |
| Páginas / Página | 20 / 9 — APPLICATIONS INFORMATION. Table 2. Surface Mount Solid Tantalum Capacitor … |
| Formato / tamaño de archivo | PDF / 216 Kb |
| Idioma del documento | Inglés |
APPLICATIONS INFORMATION. Table 2. Surface Mount Solid Tantalum Capacitor ESR. and Ripple Current. E Case Size. ESR (Max, Ω)
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LT1765/LT1765-1.8/LT1765-2.5/ LT1765-3.3/LT1765-5
APPLICATIONS INFORMATION Table 2. Surface Mount Solid Tantalum Capacitor ESR
IOUT MAX ( ) = (V )(V – V OUT IN OUT)
and Ripple Current
IP – 2 L ( )(f)(V )
E Case Size ESR (Max, Ω) Ripple Current (A)
Continuous Mode IN AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 For V AVX TAJ 0.7 to 0.9 0.4 IN = 8V, VOUT = 5V and L = 3.3μH,
D Case Size
(5)(8−5) I = 3− AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 OUT MA ( X) 2 3.3 • − 10 6 1.25 • ( )( 106)(8)
C Case Size
AVX TPS 0.2 (typ) 0.5 (typ) = 3− 0 23 . = 2 7 . 7A Figure 3 shows a comparison of output ripple for a ceramic Note that the worst case (minimum output current avail- and tantalum capacitor at 200mA ripple current. able) condition is at the maximum input voltage. For the same circuit at 15V, maximum output current would be only 2.6A. VOUT USING 47μF, 0.1Ω TANTALUM CAPACITOR (10mV/DIV)
Inductor Selection
V The output inductor should have a saturation current rating OUT USING 2.2μF CERAMIC CAPACITOR greater than the peak inductor current set by the current (10mV/DIV) comparator of the LT1765. The peak inductor current will VSW depend on the output current, input and output voltages (5V/DIV) and the inductor value: 0.2μs/DIV 1765 F03 V V ( −V ) I =I OUT IN OUT PEAK OUT + 2 L f V
Figure 3. Output Ripple Voltage Waveform
( )( )( ) IN VIN = Maximum input voltage
INDUCTOR CHOICE AND MAXIMUM OUTPUT
f = Switching frequency, 1.25MHz
CURRENT
If an inductor with a peak current lower than the maximum Maximum output current for an LT1765 buck converter is switch current of the LT1765 is chosen a soft-start circuit equal to the maximum switch rating (IP) minus one half in Figure 10 should be used. Also, short-circuit conditions peak to peak inductor ripple current. The LT1765 main- should not be allowed because the inductor may saturate tains a constant switch current rating at all duty cycles. resulting in excessive power dissipation. (Patent Pending) Also, consideration should be given to the resistance For most applications, the output inductor will be in the of the inductor. Inductor conduction loses are directly 1μH to 10μH range. Lower values are chosen to reduce proportional to the DC resistance of inductor. Sometime, the physical size of the inductor, higher values allow higher the manufacturers will also provide maximum current output currents due to reduced peak to peak ripple current. rating based on the allowable losses in the inductor. Care The following formula gives maximum output current for should be taken, however. At high input voltages and low continuous mode operation, implying that the peak to peak DCR, excessive switch current could fl ow during shorted ripple (2x the term on the right) is less than the maximum output condition. switch current. Suitable inductors are available from Coilcraft, Coiltronics, Dale, Sumida, Toko, Murata, Panasonic and other manufacturers. 1765fd 9
APPLICATIONS INFORMATION Table 2. Surface Mount Solid Tantalum Capacitor ESR
IOUT MAX ( ) = (V )(V – V OUT IN OUT)
and Ripple Current
IP – 2 L ( )(f)(V )
E Case Size ESR (Max, Ω) Ripple Current (A)
Continuous Mode IN AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 For V AVX TAJ 0.7 to 0.9 0.4 IN = 8V, VOUT = 5V and L = 3.3μH,
D Case Size
(5)(8−5) I = 3− AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1 OUT MA ( X) 2 3.3 • − 10 6 1.25 • ( )( 106)(8)
C Case Size
AVX TPS 0.2 (typ) 0.5 (typ) = 3− 0 23 . = 2 7 . 7A Figure 3 shows a comparison of output ripple for a ceramic Note that the worst case (minimum output current avail- and tantalum capacitor at 200mA ripple current. able) condition is at the maximum input voltage. For the same circuit at 15V, maximum output current would be only 2.6A. VOUT USING 47μF, 0.1Ω TANTALUM CAPACITOR (10mV/DIV)
Inductor Selection
V The output inductor should have a saturation current rating OUT USING 2.2μF CERAMIC CAPACITOR greater than the peak inductor current set by the current (10mV/DIV) comparator of the LT1765. The peak inductor current will VSW depend on the output current, input and output voltages (5V/DIV) and the inductor value: 0.2μs/DIV 1765 F03 V V ( −V ) I =I OUT IN OUT PEAK OUT + 2 L f V
Figure 3. Output Ripple Voltage Waveform
( )( )( ) IN VIN = Maximum input voltage
INDUCTOR CHOICE AND MAXIMUM OUTPUT
f = Switching frequency, 1.25MHz
CURRENT
If an inductor with a peak current lower than the maximum Maximum output current for an LT1765 buck converter is switch current of the LT1765 is chosen a soft-start circuit equal to the maximum switch rating (IP) minus one half in Figure 10 should be used. Also, short-circuit conditions peak to peak inductor ripple current. The LT1765 main- should not be allowed because the inductor may saturate tains a constant switch current rating at all duty cycles. resulting in excessive power dissipation. (Patent Pending) Also, consideration should be given to the resistance For most applications, the output inductor will be in the of the inductor. Inductor conduction loses are directly 1μH to 10μH range. Lower values are chosen to reduce proportional to the DC resistance of inductor. Sometime, the physical size of the inductor, higher values allow higher the manufacturers will also provide maximum current output currents due to reduced peak to peak ripple current. rating based on the allowable losses in the inductor. Care The following formula gives maximum output current for should be taken, however. At high input voltages and low continuous mode operation, implying that the peak to peak DCR, excessive switch current could fl ow during shorted ripple (2x the term on the right) is less than the maximum output condition. switch current. Suitable inductors are available from Coilcraft, Coiltronics, Dale, Sumida, Toko, Murata, Panasonic and other manufacturers. 1765fd 9