Datasheet LT3471 (Analog Devices) - 6
| Manufacturer | Analog Devices |
| Description | Dual 1.3A, 1.2MHz Boost/Inverter in 3mm × 3mm DFN |
| Pages / Page | 16 / 6 — OPERATION. APPLICATIONS INFORMATION. Duty Cycle. Setting Output Voltage. … |
| File Format / Size | PDF / 307 Kb |
| Document Language | English |
OPERATION. APPLICATIONS INFORMATION. Duty Cycle. Setting Output Voltage. Switching Frequency and Inductor Selection
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LT3471
OPERATION
current reaches this value, the SR latch is reset regardless both latches are reset regardless of the state of compara- of the state of the comparator A2 (A4). Also not shown tors A2 and A4. The current limit and thermal shutdown in the Block Diagram is the thermal shutdown circuit. If circuits protect the power switch as well as the external the temperature of the part exceeds approximately 160°C, components connected to the LT3471.
APPLICATIONS INFORMATION Duty Cycle
For inverting topologies, VFBN is tied to ground and VFBP is connected between R1 and R2. R2 is between V The typical maximum duty cycle of the LT3471 is 94%. FBP and V The duty cycle for a given application is given by: REF and R1 is between VFBP and VOUT (see the Ap- plications section for examples). In this case: | V DC = OUT |+| VD | – | VIN | R1 | V V OUT |+| VD | – | VCESAT | OUT = VREF R2 Where VD is the diode forward voltage drop and VCESAT Select values of R1 and R2 according to the following is in the worst case 330mV (at 1.3A) equation: The LT3471 can be used at higher duty cycles, but it must V be operated in the discontinuous conduction mode so that R1=R2 OUT the actual duty cycle is reduced. V REF
Setting Output Voltage
A good value for R2 is 15k, which sets the current in the resistor divider chain to 1.00V/15k = 67μA. Setting the output voltage depends on the topology used. For normal noninverting boost regulator topologies:
Switching Frequency and Inductor Selection
R1 V The LT3471 switches at 1.2 MHz, allowing for small valued OUT = VFBP 1 + R2 inductors to be used. 4.7μH or 10μH will usually suffi ce. Choose an inductor that can handle at least 1.4A without where VFBN is connected between R1 and R2 (see the saturating, and ensure that the inductor has a low DCR Typical Applications section for examples). (copper-wire resistance) to minimize I2R power losses. Select values of R1 and R2 according to the following Note that in some applications, the current handling equation: requirements of the inductor can be lower, such as in the SEPIC topology where each inductor only carries one half V R1= R2 OUT – 1 of the total switch current. For better effi ciency, use similar V REF valued inductors with a larger volume. Many different sizes and shapes are available from various manufacturers. A good value for R2 is 15k which sets the current in the Choose a core material that has low losses at 1.2 MHz, resistor divider chain to 1.00V/15k = 67μA. such as ferrite core. VFBP is usually just tied to VREF = 1.00V, but VFBP can also
Table 1. Inductor Manufacturers
be tied to a divided down version of VREF or some other Sumida (847) 956-0666 www.sumida.com voltage as long as the absolute maximum ratings for the TDK (847) 803-6100 www.tdk.com feedback pins are not exceeded (see Absolute Maximum Murata (714) 852-2001 www.murata.com Ratings). 3471fb 6
OPERATION
current reaches this value, the SR latch is reset regardless both latches are reset regardless of the state of compara- of the state of the comparator A2 (A4). Also not shown tors A2 and A4. The current limit and thermal shutdown in the Block Diagram is the thermal shutdown circuit. If circuits protect the power switch as well as the external the temperature of the part exceeds approximately 160°C, components connected to the LT3471.
APPLICATIONS INFORMATION Duty Cycle
For inverting topologies, VFBN is tied to ground and VFBP is connected between R1 and R2. R2 is between V The typical maximum duty cycle of the LT3471 is 94%. FBP and V The duty cycle for a given application is given by: REF and R1 is between VFBP and VOUT (see the Ap- plications section for examples). In this case: | V DC = OUT |+| VD | – | VIN | R1 | V V OUT |+| VD | – | VCESAT | OUT = VREF R2 Where VD is the diode forward voltage drop and VCESAT Select values of R1 and R2 according to the following is in the worst case 330mV (at 1.3A) equation: The LT3471 can be used at higher duty cycles, but it must V be operated in the discontinuous conduction mode so that R1=R2 OUT the actual duty cycle is reduced. V REF
Setting Output Voltage
A good value for R2 is 15k, which sets the current in the resistor divider chain to 1.00V/15k = 67μA. Setting the output voltage depends on the topology used. For normal noninverting boost regulator topologies:
Switching Frequency and Inductor Selection
R1 V The LT3471 switches at 1.2 MHz, allowing for small valued OUT = VFBP 1 + R2 inductors to be used. 4.7μH or 10μH will usually suffi ce. Choose an inductor that can handle at least 1.4A without where VFBN is connected between R1 and R2 (see the saturating, and ensure that the inductor has a low DCR Typical Applications section for examples). (copper-wire resistance) to minimize I2R power losses. Select values of R1 and R2 according to the following Note that in some applications, the current handling equation: requirements of the inductor can be lower, such as in the SEPIC topology where each inductor only carries one half V R1= R2 OUT – 1 of the total switch current. For better effi ciency, use similar V REF valued inductors with a larger volume. Many different sizes and shapes are available from various manufacturers. A good value for R2 is 15k which sets the current in the Choose a core material that has low losses at 1.2 MHz, resistor divider chain to 1.00V/15k = 67μA. such as ferrite core. VFBP is usually just tied to VREF = 1.00V, but VFBP can also
Table 1. Inductor Manufacturers
be tied to a divided down version of VREF or some other Sumida (847) 956-0666 www.sumida.com voltage as long as the absolute maximum ratings for the TDK (847) 803-6100 www.tdk.com feedback pins are not exceeded (see Absolute Maximum Murata (714) 852-2001 www.murata.com Ratings). 3471fb 6
