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. … |
| File Format / Size | PDF / 322 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Table 1. Programming the Switching Frequency. SWITCHING FREQUENCY (MHz). RT (k
Text Version of Document
LT3508
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 3508 F02 2μs/DIV 1.6 16.9 1.4 20.5
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and f
1.2 26.1
SW = 1.5MHz
1 33.2 0.9 38.3 VOUT 0.8 44.2 100mV/DIV (AC) 0.7 52.3 0.6 61.9 IL 0.5 76.8 500mA/DIV 0.45 88.7 3508 F03 0.4 100 2μs/DIV 0.35 115
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
0.3 140
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current
0.25 169
Increase Inductor Selection and Maximum Output Current Setting the Switching Frequency
A good fi rst choice for the inductor value is: The switching frequency is programmed either by driving 1.2µH the RT/SYNC pin with a logic level SYNC signal or by tying L = V ( + ) 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 Application 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 effi ciency, are shown in Table 1. the series resistance (DCR) should be less than 0.1Ω. Choosing a high switching frequency will allow the smallest Table 2 lists several vendors and types that are suitable. overall solution size. However, at high input voltages the
Table 2. Inductor Vendors
effi ciency can drop signifi cantly with increasing switching
VENDOR URL PART SERIES TYPE
frequency. The choice of switching frequency will also Coilcraft www.coilcraft MSS7341 Shielded impact the input voltage range, inductor and capacitor Murata www.murata.com LQH55D Open selection, and compensation. See the related sections TDK www.component.tdk.com SLF7045 Shielded for details. SLF10145 Shielded Toko www.toko.com DC62CB Shielded D63CB Shielded D75C Shielded D75F Open Sumida www.sumida.com CR54 Open CDRH74 Shielded CDRH6D38 Shielded CR75 Open 3508fd 10 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION 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 TYPICAL APPLICATION 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 3508 F02 2μs/DIV 1.6 16.9 1.4 20.5
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and f
1.2 26.1
SW = 1.5MHz
1 33.2 0.9 38.3 VOUT 0.8 44.2 100mV/DIV (AC) 0.7 52.3 0.6 61.9 IL 0.5 76.8 500mA/DIV 0.45 88.7 3508 F03 0.4 100 2μs/DIV 0.35 115
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
0.3 140
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current
0.25 169
Increase Inductor Selection and Maximum Output Current Setting the Switching Frequency
A good fi rst choice for the inductor value is: The switching frequency is programmed either by driving 1.2µH the RT/SYNC pin with a logic level SYNC signal or by tying L = V ( + ) 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 Application 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 effi ciency, are shown in Table 1. the series resistance (DCR) should be less than 0.1Ω. Choosing a high switching frequency will allow the smallest Table 2 lists several vendors and types that are suitable. overall solution size. However, at high input voltages the
Table 2. Inductor Vendors
effi ciency can drop signifi cantly with increasing switching
VENDOR URL PART SERIES TYPE
frequency. The choice of switching frequency will also Coilcraft www.coilcraft MSS7341 Shielded impact the input voltage range, inductor and capacitor Murata www.murata.com LQH55D Open selection, and compensation. See the related sections TDK www.component.tdk.com SLF7045 Shielded for details. SLF10145 Shielded Toko www.toko.com DC62CB Shielded D63CB Shielded D75C Shielded D75F Open Sumida www.sumida.com CR54 Open CDRH74 Shielded CDRH6D38 Shielded CR75 Open 3508fd 10 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION 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 TYPICAL APPLICATION RELATED PARTS
