Datasheet LT3988 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Dual 60V Monolithic 1A Step-Down Switching Regulator |
| Pages / Page | 24 / 10 — applicaTions inForMaTion. Switching Frequency. Inductor Selection and … |
| File Format / Size | PDF / 287 Kb |
| Document Language | English |
applicaTions inForMaTion. Switching Frequency. Inductor Selection and Maximum Output Current. Table 1. Inductors. MFG. URL
Model Line for this Datasheet
Text Version of Document
LT3988
applicaTions inForMaTion
at frequencies above fMAX1. It will continue to regulate but 1.31 46.56 with increased inductor current and increased output ripple. RT = + – 7.322 f2 f fMAX2 is the frequency at which the maximum duty cycle is exceeded. If there is sufficient charge on the BOOST 250kHz ≤ f ≤ 2.5MHz capacitor, the regulator will skip OFF periods to increase where f is in MHz and RT is in kΩ. the overall duty cycle at frequencies above fMAX2. Note that the restriction on the operating input voltage refers The frequency sync signal will support VIH logic levels from to steady-state limits to keep the output in regulation; 1.5V to 5V CMOS or TTL. The duty cycle is not important, the circuit will tolerate input voltage transients up to the but it needs a minimum on time of 100ns and a minimum absolute maximum rating. off time of 100ns. RT should be set to provide a frequency within ±25% of the final sync frequency.
Switching Frequency
The slope recovery circuit sets the slope compensation Once the upper and lower bounds for the switching to the appropriate value for the synchronized frequency. frequency are found from the duty cycle requirements, Choose the inductor value based on the lowest potential the frequency may be set within those bounds. Lower switching frequency. frequencies result in lower switching losses, but require larger inductors and capacitors. The user must decide
Inductor Selection and Maximum Output Current
the best trade-off. A good first choice for the inductor value is: The switching frequency is set by a resistor connected from VOUT + VF the RT pin to ground, or by forcing a clock signal into the L = 0.6A • f SYNC pin. The LT3988 applies a voltage across this resistor and uses the current to set the oscillator speed. The R where V T F is the voltage drop of the catch diode (~0.4V) and f resistor value for a given switching frequency is given by: is in MHz. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current
Table 1. Inductors MFG URL PART SERIES INDUCTANCE RANGE (µH) SIZE (mm) (L
×
W
×
H)
Coilcraft http://www.coilcraft.com XPL7030 0.13 to 22 7 × 7 × 3 XFL4020 1 to 4.7 4 × 4 × 2.15 XAL50XX 0.16 to 22 5.28 × 5.48 × 5.1 Cooper http://www.cooperbussmann.com DRA74 0.33 to 1000 7.6 × 7.6 × 4.35 DR1040 1.5 to 330 10.5 × 10.3 × 4 CWS http://www.coilws.com SP-0703 3 to 100 7 × 7 × 3 SP-0704 2.2 to 100 7 × 7 × 4 SB-1004 10 to 1500 10.1 × 10.1 × 4.5 Murata http://www.murata.com LQH55D 0.12 to 10000 5 × 5.7 × 4.7 LQH6PP 1 to 100 6 × 6 × 4.3 LQH88P 1 to 100 8 × 8 × 3.8 Sumida http://www.sumida.com CDMC6D28 0.2 to 4.7 7.25 × 6.7 × 3 CDEIR8D38F 4 to 22 8.5 × 8.3 × 4 Toko http://www.toko.co.jp DS84LCB 1 to 100 8.4 × 8.3 × 4 FDV0620 0.2 to 4.7 6.7 × 7.4 × 2 Vishay http://www.vishay.com IHLP-2020AB-11 0.1 to 4.7 5.49 × 5.18 × 1.2 IHLP-2020BZ-11 0.1 to 10 5.49 × 5.18 × 2 IHLP-2525CZ-11 1 to 22 6.86 × 6.47 × 3 Würth http://www.we-online.de WE-PD2-S 1 to 68 4 × 4.5 × 3.2 WE-PD-M 1 to 1000 7.3 × 7.3 × 4.5 WE-PD2-XL 10 to 820 9 × 10 × 5.4 3988f 10 Document Outline Features Applications Description 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 Typical Application Related Parts
applicaTions inForMaTion
at frequencies above fMAX1. It will continue to regulate but 1.31 46.56 with increased inductor current and increased output ripple. RT = + – 7.322 f2 f fMAX2 is the frequency at which the maximum duty cycle is exceeded. If there is sufficient charge on the BOOST 250kHz ≤ f ≤ 2.5MHz capacitor, the regulator will skip OFF periods to increase where f is in MHz and RT is in kΩ. the overall duty cycle at frequencies above fMAX2. Note that the restriction on the operating input voltage refers The frequency sync signal will support VIH logic levels from to steady-state limits to keep the output in regulation; 1.5V to 5V CMOS or TTL. The duty cycle is not important, the circuit will tolerate input voltage transients up to the but it needs a minimum on time of 100ns and a minimum absolute maximum rating. off time of 100ns. RT should be set to provide a frequency within ±25% of the final sync frequency.
Switching Frequency
The slope recovery circuit sets the slope compensation Once the upper and lower bounds for the switching to the appropriate value for the synchronized frequency. frequency are found from the duty cycle requirements, Choose the inductor value based on the lowest potential the frequency may be set within those bounds. Lower switching frequency. frequencies result in lower switching losses, but require larger inductors and capacitors. The user must decide
Inductor Selection and Maximum Output Current
the best trade-off. A good first choice for the inductor value is: The switching frequency is set by a resistor connected from VOUT + VF the RT pin to ground, or by forcing a clock signal into the L = 0.6A • f SYNC pin. The LT3988 applies a voltage across this resistor and uses the current to set the oscillator speed. The R where V T F is the voltage drop of the catch diode (~0.4V) and f resistor value for a given switching frequency is given by: is in MHz. The inductor’s RMS current rating must be greater than the maximum load current and its saturation current
Table 1. Inductors MFG URL PART SERIES INDUCTANCE RANGE (µH) SIZE (mm) (L
×
W
×
H)
Coilcraft http://www.coilcraft.com XPL7030 0.13 to 22 7 × 7 × 3 XFL4020 1 to 4.7 4 × 4 × 2.15 XAL50XX 0.16 to 22 5.28 × 5.48 × 5.1 Cooper http://www.cooperbussmann.com DRA74 0.33 to 1000 7.6 × 7.6 × 4.35 DR1040 1.5 to 330 10.5 × 10.3 × 4 CWS http://www.coilws.com SP-0703 3 to 100 7 × 7 × 3 SP-0704 2.2 to 100 7 × 7 × 4 SB-1004 10 to 1500 10.1 × 10.1 × 4.5 Murata http://www.murata.com LQH55D 0.12 to 10000 5 × 5.7 × 4.7 LQH6PP 1 to 100 6 × 6 × 4.3 LQH88P 1 to 100 8 × 8 × 3.8 Sumida http://www.sumida.com CDMC6D28 0.2 to 4.7 7.25 × 6.7 × 3 CDEIR8D38F 4 to 22 8.5 × 8.3 × 4 Toko http://www.toko.co.jp DS84LCB 1 to 100 8.4 × 8.3 × 4 FDV0620 0.2 to 4.7 6.7 × 7.4 × 2 Vishay http://www.vishay.com IHLP-2020AB-11 0.1 to 4.7 5.49 × 5.18 × 1.2 IHLP-2020BZ-11 0.1 to 10 5.49 × 5.18 × 2 IHLP-2525CZ-11 1 to 22 6.86 × 6.47 × 3 Würth http://www.we-online.de WE-PD2-S 1 to 68 4 × 4.5 × 3.2 WE-PD-M 1 to 1000 7.3 × 7.3 × 4.5 WE-PD2-XL 10 to 820 9 × 10 × 5.4 3988f 10 Document Outline Features Applications Description 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 Typical Application Related Parts
