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MAX1612EEE Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX1612EEE Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page where IPEAK is the peak current, IOUT is the load cur- rent, VBBATT is the bridge-battery voltage, VD is the for- ward drop across D1, VOUT is the output voltage, IIN is average current provided by the bridge battery, and VRDS(ON) is the voltage drop across the internal N- channel power transistor at LX (typically 0.5V). A larger number of cells reduces the IPEAK and, in effect, reduces the discharge current, thereby extending the discharge time. The same is true for decreasing the output voltage or output current. For example, choose the following values: IOUT = 100mA, VOUT = 5V, and VBBATT = 2V (two cells). Using the minimum voltage of 1V for each cell, Table 2 summarizes some common values. Step 2: To avoid saturation, choose an inductor (L) with a peak current rating above the IPEAK calculated in Step 1. Use low series resistance ( ≤ 200mΩ), to opti- mize efficiency. In this example, a 15µH inductor is used. See Table 4 for a list of component suppliers. The “edge-of-continuous” DC-DC algorithm causes the inductor value to fall out of the peak current equation. Therefore, the exact inductor value chosen is not criti- cal to the design. However, the switching frequency is inversely proportional to inductance, so trade-offs of switching losses versus physical inductor size can be made by adjusting the inductor value. where f is the switching frequency, VOUT is the output voltage, VRDSON is the voltage across the internal MOS- FET switch, VD is the forward voltage of D1, IPEAK is the peak current, and VBBATT is the bridge battery voltage. The maximum practical switching frequency is 400kHz. Step 3: Choose the charging (CCC) and discharging (CCD) timing capacitors. These capacitors set the fre- quency that the counter increments/decrements. CCC (nF) = 4.3 · expected charge time (in hours) CCD (nF) = 4.3 · expected discharge time (in hours) For instance, using a charge time of 16 hours and a dis- charge time of one hour, CCC = 68nF and CCD = 4.3nF. (Consult battery manufacturers’ specifications for stan- dard charging information, which generally compen- sates for battery inefficiencies.) Step 4: Using the peak current calculated in Step 1, calculate the series resistor (RBBON) as follows: R BBON = (V BBON · 42,000) / IPEAK where V BBON = 2V (internally regulated). Step 5: Resistors R1, R2, and R3 set the DC-DC con- verter’s output voltage and the low-battery comparator trip value. The sum of R1, R2, and R3 must be less than 2M Ω, to minimize leakage errors. Choose resistor R1 = 750k Ω for the example. Calculate R2 and R3 as follows: R2 = [ VOUT (R3) - 2 (R1) - 2 (R3) ] / (2 - VOUT ) R3 = (R1 + R2) / [ (VTRIP / 1.8) - 1] Bridge-Battery Backup Controllers for Notebooks 10 ______________________________________________________________________________________ VOUT (V) VBBATT (V) AVERAGE IPEAK (mA) IIN (mA) MINIMUM DISCHARGE TIME (MINUTES) 6 4 300 150 20 6 2 600 300 10 5 2 500 250 12 4.5 2 450 225 13.2 6 3 400 200 15 5 3 333 167 18 4.5 3 300 150 20 5 4 250 125 24 Table 2. Summary of Common Values for Designing with the MAX1612/MAX1613 Note: In this table, IOUT = 100mA and battery capacity = 50mAh. Table 3. Component List INDUCTORS CAPACITORS RECTIFIERS BATTERY Sumida CD43 or CD54 series Sprague 595D series, AVX TPS series Motorola MBR0530, NIEC EC10QS03L Sanyo N-50AAA SUPPLIER PHONE FAX AVX USA: 207-287-5111 USA: 207-283-1941 Motorola USA: 408-749-0510 800-521-6274 — NIEC USA: 805-867-2555 Japan: 81-3-3494-7411 USA: 805-867-2556 Japan: 81-3-3494-7414 Sumida USA: 708-956-0666 Japan: 81-3-3607-5111 USA: 708-956-0702 Japan: 81-3-3607-5144 Table 4. Component Suppliers Sanyo USA: 619-661-6835 Japan: 81-7-2070-6306 USA: 619-661-1055 Japan: 81-7-2070-1174 f 1 L(I ) (V V ) (V V V ) (V V V ) PEAK BBATT RDSON OUT BBATT D OUT RDSON D = − − − − − |
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