Datasheet LT1946A (Linear Technology) - 6
| Manufacturer | Linear Technology |
| Description | 2.7MHz Boost DC/DC Converter with 1.5A Switch and Soft-Start |
| Pages / Page | 12 / 6 — OPERATIO. APPLICATIO S I FOR ATIO. Table 1. Recommended Inductors - … |
| Revision | S |
| File Format / Size | PDF / 326 Kb |
| Document Language | English |
OPERATIO. APPLICATIO S I FOR ATIO. Table 1. Recommended Inductors - LT1946A. Inductor Selection. MAX. Size. DCR. LxWxH. PART. (mm). VENDOR
Model Line for this Datasheet
Text Version of Document
LT1946A
U OPERATIO
The LT1946A uses a constant frequency, current mode a nominal value of 0.5V. This is accomplished via com- control scheme to provide excellent line and load regula- parator A3. This feature reduces the minimum duty cycle tion. Please refer to Figure 2 for the following description that the part can achieve thus allowing better control of the of the part’s operation. At the start of the oscillator cycle, switch current during start-up. When the FB pin voltage the SR latch is set, turning on the power switch Q1. The goes above 0.5V, the oscillator returns to the normal switch current flows through the internal current sense frequency of 2.7MHz. A soft-start function is also provided resistor generating a voltage. This voltage is added to a by the LT1946A. When the part is brought out of shut- stabilizing ramp and the resulting sum is fed into the down, 4µA of current is sourced out of the SS pin. By positive terminal of the PWM comparator A2. When this connecting an external capacitor to the SS pin, the rate of voltage exceeds the level at the negative input of A2, the SR voltage rise on the pin can be set. Typical values for the latch is reset, turning off the power switch. The level at the soft-start capacitor range from 10nF to 200nF. The SS pin negative input of A2 (VC pin) is set by the error amplifier directly limits the rate of rise on the VC pin, which in turn (A1) and is simply an amplified version of the difference limits the peak switch current. Current limit is not shown between the feedback voltage and the reference voltage of in Figure 2. The switch current is constantly monitored and 1.250V. In this manner, the error amplifier sets the correct not allowed to exceed the nominal value of 2.1A. If the peak current level to keep the output in regulation. switch current reaches 2.1A, the SR latch is reset regard- less of the output of comparator A2. This current limit Two functions are provided to enable a very clean start-up protects the power switch as well as various external for the LT1946A. Frequency foldback is used to reduce the components connected to the LT1946A. oscillator frequency by one-third when the FB pin is below
U U W U APPLICATIO S I FOR ATIO Table 1. Recommended Inductors - LT1946A Inductor Selection MAX Size
Several inductors that work well with the LT1946A are
L DCR LxWxH
listed in Table 1. This table is not complete, and there are
PART (
µ
H) (m
Ω
) (mm) VENDOR
many other manufacturers and devices that can be used. RLF5018-1R5M2R1 1.5 25 5.2x5.6x1.8 TDK Consult each manufacturer for more detailed information RLF5018-2R7M1R8 2.7 33 (847) 803-6100 RLF5018-4R7M1R4 4.7 45 www.tdk.com and for their entire selection of related parts, as many RLF5018-100MR94 10.0 67 different sizes and shapes are available. Ferrite core induc- LPO1704-122MC 1.2 80 5.5x6.6x1.0 Coilcraft tors should be used to obtain the best efficiency, as core LPO1704-222MC 2.2 120 (800) 322-2645 losses at 2.7MHz are much lower for ferrite cores than for www.coilcraft.com the cheaper powdered-iron ones. Choose an inductor that CR43-2R2 2.2 71 4.5x4.0x3.2 Sumida can handle at least 1.5A without saturating, and ensure CR43-3R3 3.3 86 (847) 956-0666 www.sumida.com that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. A 1.5µH to 4.7µH inductor will be the best choice for most LT1946A designs. Note
Capacitor Selection
that in some applications, the current handling require- Low ESR (equivalent series resistance) capacitors should ments of the inductor can be lower, such as in the SEPIC be used at the output to minimize the output ripple voltage. topology where each inductor only carries one-half of the Multilayer ceramic capacitors are an excellent choice, as total switch current. they have an extremely low ESR and are available in very The inductors shown in Table 1 were chosen for small size. small packages. X5R dielectrics are preferred, followed by For better efficiency, use similar valued inductors with a X7R, as these materials retain the capacitance over wide larger volume. voltage and temperature ranges. A 2.2µF to 20µF output sn1946a 1946afs 6
U OPERATIO
The LT1946A uses a constant frequency, current mode a nominal value of 0.5V. This is accomplished via com- control scheme to provide excellent line and load regula- parator A3. This feature reduces the minimum duty cycle tion. Please refer to Figure 2 for the following description that the part can achieve thus allowing better control of the of the part’s operation. At the start of the oscillator cycle, switch current during start-up. When the FB pin voltage the SR latch is set, turning on the power switch Q1. The goes above 0.5V, the oscillator returns to the normal switch current flows through the internal current sense frequency of 2.7MHz. A soft-start function is also provided resistor generating a voltage. This voltage is added to a by the LT1946A. When the part is brought out of shut- stabilizing ramp and the resulting sum is fed into the down, 4µA of current is sourced out of the SS pin. By positive terminal of the PWM comparator A2. When this connecting an external capacitor to the SS pin, the rate of voltage exceeds the level at the negative input of A2, the SR voltage rise on the pin can be set. Typical values for the latch is reset, turning off the power switch. The level at the soft-start capacitor range from 10nF to 200nF. The SS pin negative input of A2 (VC pin) is set by the error amplifier directly limits the rate of rise on the VC pin, which in turn (A1) and is simply an amplified version of the difference limits the peak switch current. Current limit is not shown between the feedback voltage and the reference voltage of in Figure 2. The switch current is constantly monitored and 1.250V. In this manner, the error amplifier sets the correct not allowed to exceed the nominal value of 2.1A. If the peak current level to keep the output in regulation. switch current reaches 2.1A, the SR latch is reset regard- less of the output of comparator A2. This current limit Two functions are provided to enable a very clean start-up protects the power switch as well as various external for the LT1946A. Frequency foldback is used to reduce the components connected to the LT1946A. oscillator frequency by one-third when the FB pin is below
U U W U APPLICATIO S I FOR ATIO Table 1. Recommended Inductors - LT1946A Inductor Selection MAX Size
Several inductors that work well with the LT1946A are
L DCR LxWxH
listed in Table 1. This table is not complete, and there are
PART (
µ
H) (m
Ω
) (mm) VENDOR
many other manufacturers and devices that can be used. RLF5018-1R5M2R1 1.5 25 5.2x5.6x1.8 TDK Consult each manufacturer for more detailed information RLF5018-2R7M1R8 2.7 33 (847) 803-6100 RLF5018-4R7M1R4 4.7 45 www.tdk.com and for their entire selection of related parts, as many RLF5018-100MR94 10.0 67 different sizes and shapes are available. Ferrite core induc- LPO1704-122MC 1.2 80 5.5x6.6x1.0 Coilcraft tors should be used to obtain the best efficiency, as core LPO1704-222MC 2.2 120 (800) 322-2645 losses at 2.7MHz are much lower for ferrite cores than for www.coilcraft.com the cheaper powdered-iron ones. Choose an inductor that CR43-2R2 2.2 71 4.5x4.0x3.2 Sumida can handle at least 1.5A without saturating, and ensure CR43-3R3 3.3 86 (847) 956-0666 www.sumida.com that the inductor has a low DCR (copper-wire resistance) to minimize I2R power losses. A 1.5µH to 4.7µH inductor will be the best choice for most LT1946A designs. Note
Capacitor Selection
that in some applications, the current handling require- Low ESR (equivalent series resistance) capacitors should ments of the inductor can be lower, such as in the SEPIC be used at the output to minimize the output ripple voltage. topology where each inductor only carries one-half of the Multilayer ceramic capacitors are an excellent choice, as total switch current. they have an extremely low ESR and are available in very The inductors shown in Table 1 were chosen for small size. small packages. X5R dielectrics are preferred, followed by For better efficiency, use similar valued inductors with a X7R, as these materials retain the capacitance over wide larger volume. voltage and temperature ranges. A 2.2µF to 20µF output sn1946a 1946afs 6
