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LTC1559-3.3 Datasheet(PDF) 10 Page - Linear Technology |
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LTC1559-3.3 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  10 LTC1559-3.3/LTC1559-5 If VCC is still less than VCC (rated value) – 7% (LTC1559- 3.3) after the first boost cycle, the LTC1559 immediately reconnects SW to ground, repeating the boost cycle. If after two consecutive pulses, VCC is still not above the boost threshold VCC (rated value) – 7% (LTC1559-3.3), the LTC1559 decides that the load is not so light after all, and doubles the internal inductor charging current limit to 330mA for subsequent cycles. This is high current mode. By doubling the peak inductor current, each boost cycle effectively carries four times more energy compared to low current mode (E = 1/2 • LI2), doubling the available output power. When VCC exceeds the VCC (rated value) – 7% (LTC1559-3.3) boost threshold, the LTC1559 stops the boost converter and resets the internal 2-pulse counter. The next time VCC falls below VCC (rated value) – 7% (LTC1559-3.3), the boost converter restarts in low current mode for at least two boost cycles. Moderate or changing loads cause the LTC1559 to shift between the two peak inductor current limits, keeping the output in tight regula- tion. Near its maximum load capability, the LTC1559 will stay in 330mA high current mode and the output voltage VBAK will hover around VCC (rated value) – 7% (LTC1559-3.3). VCC Capacitor ESR The type of output capacitor and the rated VCC value will affect the LTC1559’s output ripple and efficiency. In most applications, the VCC capacitor is primarily determined by the requirements of the main power supply. Such a capacitor will generally meet the requirements of the LTC1559. In unusual circumstances or circuits where the main system VCC capacitor is located some distance away from the LTC1559, a local output capacitor may be necessary. The ripple on the VCC pin is equal to the capacitor ESR voltage drop due to the boost converter’s output current pulses. The ripple frequency and output duty cycle is proportional to the inductor discharge time. Given a fixed inductor value (22 µH) and a known peak current limit, the booster’s discharge time in each boost cycle is propor- tional to the difference between VBAK (93% of the rated VCC for the LTC1559-3.3 and 92.5% of the rated VCC for the LTC1559-5) and the battery cell voltage, VBAT (1.2V). APPLICATIONS INFORMATION Assuming ESR = 0.2 Ω, IIND(PEAK) = 330mA, VCC = 5V, VRIPPLE(P-P) = (IIND(PEAK))(RESR(CAP)) = (330mA)(0.2 Ω) = 66mV Since VCC must be scaled down internally, the external resistor ratio: = 5V/1.272V = 3.931 Therefore the ripple seen by the VCC comparators is: = 66mV/3.931 = 16.79mV The discharge time period, tDISCH = (L • IIND(PEAK))/(VBAK – VBAT) = (22 µH • 330mA)/(4.625 – 1.2V) = 2.12 µs For VCC = 3.3V and IIND(PEAK) = 330mA, VRIPPLE(P-P) = 66mV RB resistor ratio = 3.3/1.272 = 2.594 Ripple voltage = 25.4mV tDISCH = 3.9µs The internal VCC comparators are designed to have a slow response time to filter away this ripple. The VCC (rated value) – 5.5% (LTC1559-3.3) and VCC(rated value) – 9% comparators have a 6 µs rising edge delay and 2µs falling edge delay. The VCC (rated value) – 7% (LTC1559-3.3) comparator has a similar 6 µs rising time delay but a much longer falling time delay of 20 µs. This enables the com- parator to control the boost converter properly, and avoids turning off the boost converter prematurely due to false triggering by the ESR ripple. Exit from Backup When a main battery is inserted into the system, the LTC1559 follows a specific sequence to exit backup mode and return control to the main supply. The sequence depends on the type of main power supply used. In systems where the main supply’s output impedance is |
Similar Part No. - LTC1559-3.3 |
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Similar Description - LTC1559-3.3 |
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