Datasheet LT3508 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Dual Monolithic 1.4A Step-Down Switching Regulator |
| Pages / Page | 26 / 10 — APPLICATIONS INFORMATION. Table 1. Programming the Switching Frequency. … |
| Revision | E |
| File Format / Size | PDF / 1.2 Mb |
| Document Language | English |
APPLICATIONS INFORMATION. Table 1. Programming the Switching Frequency. SWITCHING FREQUENCY (MHz). RT (k
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APPLICATIONS INFORMATION Table 1. Programming the Switching Frequency
VOUT 100mV/DIV
SWITCHING FREQUENCY (MHz) RT (k
Ω
)
(AC) 2.5 7.50 IL 2.2 9.76 500mA/DIV 2 11.5 1.8 14 2µs/DIV 3508 F02 1.6 16.9 1.4 20.5
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and
1.2 26.1
fSW = 1.5MHz
1 33.2 0.9 38.3 V 0.8 44.2 OUT 100mV/DIV 0.7 52.3 (AC) 0.6 61.9 0.5 76.8 IL 500mA/DIV 0.45 88.7 0.4 100 2µs/DIV 3508 F03 0.35 115 0.3 140
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
0.25 169
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current Increase Inductor Selection and Maximum Output Current Setting the Switching Frequency
A good first choice for the inductor value is: The switching frequency is programmed either by driving 1.2µH L = (V )• the RT/SYNC pin with a logic level SYNC signal or by tying OUT + VF f a resistor from the RT/SYNC pin to ground. A graph for where VF is the voltage drop of the catch diode (~0.4V) selecting the value of RT for a given operating frequency and f is in MHz. The inductor’s RMS current rating must be is shown in the Typical Applications section. Suggested greater than the maximum load current and its saturation programming resistors for various switching frequencies current should be at least 30% higher. For highest efficiency, are shown in Table 1. the series resistance (DCR) should be less than 0.1Ω. Table 2 lists several vendors and types that are suitable. Choosing a high switching frequency will allow the small- est overall solution size. However, at high input voltages
Table 2. Inductor Vendors
the efficiency can drop significantly with increasing
VENDOR URL PART SERIES TYPE
switching frequency. The choice of switching frequency Coilcraft www.coilcraft MSS7341 Shielded will also impact the input voltage range, inductor and Murata www.murata.com LQH55D Open capacitor selection, and compensation. See the related TDK www.component.tdk.com SLF7045 Shielded SLF10145 Shielded sections for details. Toko www.toko.com DC62CB Shielded D63CB Shielded D75C Shielded D75F Open Sumida www.sumida.com CR54 Open CDRH74 Shielded CDRH6D38 Shielded CR75 Open Rev. E 10 For more information www.analog.com 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
APPLICATIONS INFORMATION Table 1. Programming the Switching Frequency
VOUT 100mV/DIV
SWITCHING FREQUENCY (MHz) RT (k
Ω
)
(AC) 2.5 7.50 IL 2.2 9.76 500mA/DIV 2 11.5 1.8 14 2µs/DIV 3508 F02 1.6 16.9 1.4 20.5
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and
1.2 26.1
fSW = 1.5MHz
1 33.2 0.9 38.3 V 0.8 44.2 OUT 100mV/DIV 0.7 52.3 (AC) 0.6 61.9 0.5 76.8 IL 500mA/DIV 0.45 88.7 0.4 100 2µs/DIV 3508 F03 0.35 115 0.3 140
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
0.25 169
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current Increase Inductor Selection and Maximum Output Current Setting the Switching Frequency
A good first choice for the inductor value is: The switching frequency is programmed either by driving 1.2µH L = (V )• the RT/SYNC pin with a logic level SYNC signal or by tying OUT + VF f a resistor from the RT/SYNC pin to ground. A graph for where VF is the voltage drop of the catch diode (~0.4V) selecting the value of RT for a given operating frequency and f is in MHz. The inductor’s RMS current rating must be is shown in the Typical Applications section. Suggested greater than the maximum load current and its saturation programming resistors for various switching frequencies current should be at least 30% higher. For highest efficiency, are shown in Table 1. the series resistance (DCR) should be less than 0.1Ω. Table 2 lists several vendors and types that are suitable. Choosing a high switching frequency will allow the small- est overall solution size. However, at high input voltages
Table 2. Inductor Vendors
the efficiency can drop significantly with increasing
VENDOR URL PART SERIES TYPE
switching frequency. The choice of switching frequency Coilcraft www.coilcraft MSS7341 Shielded will also impact the input voltage range, inductor and Murata www.murata.com LQH55D Open capacitor selection, and compensation. See the related TDK www.component.tdk.com SLF7045 Shielded SLF10145 Shielded sections for details. Toko www.toko.com DC62CB Shielded D63CB Shielded D75C Shielded D75F Open Sumida www.sumida.com CR54 Open CDRH74 Shielded CDRH6D38 Shielded CR75 Open Rev. E 10 For more information www.analog.com 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
