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ADP3000AR-5 Datasheet(PDF) 10 Page - Analog Devices |
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ADP3000AR-5 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADP3000 Rev. A | Page 10 of 16 APPLICATIONS INFORMATION COMPONENT SELECTION Inductor Selection For most applications, the inductor used with the ADP3000 falls in the range of 4.7 µH to 33 µH. Table 4 shows recommended inductors and their vendors. When selecting an inductor for the ADP3000, it is very important to make sure the inductor is able to handle a current higher than the ADP3000’s current limit, without becoming saturated. As a general rule, powdered iron cores saturate softly, whereas Ferrite cores saturate abruptly. Rod and open drum core geometry inductors saturate gradually. Inductors that saturate gradually are easier to use. Even though rod and drum core inductors are attractive in both price and physical size, they must be used with care because they have high magnetic radiation. When minimizing EMI is critical, toroid and closed drum core geometry inductors should be used. In addition, inductor dc resistance causes power loss. To minimize power loss, it is best to use an inductor with a dc resistance lower than 0.2 Ω. Table 4. Recommended Inductors Vendor Series Core Type Phone Number Coiltronics OCTAPAC Toroid (561) 752-5000 Coiltronics UNIPAC Open (561) 752-5000 Sumida CR43, CR54 Open (847) 545-6700 Sumida CDRH6D28, CDRH73, CDRH64 Semi-Closed Geometry (847) 545-6700 Capacitor Selection For most applications, the capacitor used with the ADP3000 falls in the range of 33 µF to 220 µF. Table 5 shows recommended capacitors and their vendors. For input and output capacitors, use low ESR type capacitors for best efficiency and lowest ripple. Recommended capacitors include the AVX TPS series, the Sprague 595D series, the Panasonic HFQ series, and the Sanyo OS-CON series. When selecting a capacitor, it is important to make sure the maximum capacitor ripple current rms rating is higher than the ADP3000’s rms switching current. It is best to protect the input capacitor from high turn-on current charging surges by derating the capacitor voltage by 2:1. For very low input or output voltage ripple requirements, use capacitors with very low ESR, such as the Sanyo OS-CON series. Alternatively, two or more tantalum capacitors can be used in parallel. Table 5. Recommended Capacitors Vendor Series Type Phone Number AVX TPS Surface Mount (843) 448-9411 Sanyo OS-CON Through Hole (619) 661-6835 Sprague 595D Surface Mount (603) 224-1961 Panasonic HFQ Through Hole (800) 344-2112 Diode Selection The ADP3000’s high switching speed demands the use of Schottky diodes. Suitable choices include the 1N5817, the 1N5818, the 1N5819, the MBRS120LT3, and the MBR0520LT1. Fast recovery diodes are not recommended because their high forward drop lowers efficiency. General-purpose and small- signal diodes should be avoided as well. PROGRAMMING THE SWITCHING CURRENT LIMIT The ADP3000’s RLIM pin permits the cycle-by-cycle switch current limit to be programmed with a single external resistor. This feature offers major advantages that ultimately decrease the component’s cost and the PCB’s real estate. First, the RLIM pin allows the ADP3000 to use low value, low saturation current and physically small inductors. Additionally, it allows for a physically small surface-mount tantalum capacitor with a typical ESR of 0.1 Ω. With this capacitor, it achieves an output ripple as low as 40 mV to 80 mV, as well as a low input ripple. The current limit is usually set to approximately 3 to 5 times the full load current for boost applications, and about 1.5 to 3 times the full load current in buck applications. The internal structure of the ILIM circuit is shown in Figure 24. Q1, the ADP3000’s internal power switch, is paralleled by sense transistor Q2. The relative sizes of Q1 and Q2 are scaled so that IQ2 is 0.5% of IQ1. Current flows to Q2 through both the RLIM resistor and an internal 80 Ω resistor. The voltage on these two resistors biases the base-emitter junction of the oscillator-disable transistor, Q3. When the voltage across R1 and RLIM exceeds 0.6 V, Q3 turns on and terminates the output pulse. If only the 80 Ω internal resistor is used (when the ILIM pin is connected directly to VIN), the maximum switch current is 1.5 A. Figure 13, Figure 14, and Figure 15 give values for lower current limit levels. VIN SW2 SW1 RLIM (EXTERNAL) DRIVER 80 Ω (INTERNAL) ILIM IQ1 200 VIN Q2 ADP3000 Q1 400kHz OSCILLATOR Q3 R1 POWER SWITCH Figure 24. ADP3000 Current Limit Operation |
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