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LTC1263 Datasheet(PDF) 5 Page - Linear Technology |
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LTC1263 Datasheet(HTML) 5 Page - Linear Technology |
5 / 8 page 5 LTC1263 The LTC1263 uses a charge pump tripler to generate 12V from a VCC of 5V. The charge pump is clocked by an internal oscillator. The oscillator frequency is not critical and may vary from the typical value of 300kHz. When the oscillator output is low, C1 and C2 are each connected between VCC and GND, charging them to VCC (see Figure 3). When the oscillator output goes high, C1 and C2 are stacked in series with the bottom plate of C1 pulled to VCC (see Figure 4). The top plate of C2 is switched to charge COUT, which enables VOUT to rise. VOUT is regulated to within 5% of 12V by an oscillator pulse gating scheme that turns the charge pump on and off based on the comparator results of VOUT and a reference voltage. First, a resistor divider senses VOUT; if the output of the divider (VDIV) is less than the output of a bandgap (VBGAP) by the hysteresis voltage (VHYST) of the compara- tor, then oscillator pulses are applied to the charge pump to raise VOUT. When VDIV is above VBGAP by VHYST, the OPERATION oscillator pulses are prevented from clocking the charge pump. As a result, VOUT drops until VDIV is below VBGAP by VHYST again. To ensure proper start-up when VOUT is lower than VCC and maintain proper operation when VOUT is higher than VCC, the gates of all internal switches are driven between GND and the higher of either VOUT or VCC. To reduce supply current, the LTC1263 may be put into shutdown mode by “floating” the SHDN pin or connecting it to VCC. In this mode, the bandgap, comparator, oscilla- tor and resistor divider are switched off to reduce the supply current to typically 0.5 µA. At the same time an internal switch shorts VOUT to VCC; VOUT takes 10ms (typ) to reach 5.1V (see t OFF in Figure 1). When the SHDN pin is low, the LTC1263 exits shutdown and the charge pump operates to raise VOUT to 12V. VOUT takes 600µs (typ) to reach the lower regulation limit of 11.4V (see t ON in Figure 1). + C1 + C2 VCC LTC1263 • F03 Figure 3. C1 and C2 Charge to VCC Figure 4. C1 and C2 Stacked in Series with C1– Tied to VCC + C1 + C2 VCC LTC1263 • F04 VOUT COUT APPLICATIONS INFORMATION Choice of Capacitors The LTC1263 is tested with the capacitors shown in Figure 2. C1 and C2 are 0.47 µF ceramic capacitors and CIN and COUT are 10µF tantalum capacitors. Refer to Table 1 if other choices are desired. Table 1. Recommended Capacitor Types and Values CAPACITOR CERAMIC TANTALUM ALUMINUM C1, C2 0.47 µF to 1µF Not Recommended Not Recommended COUT 10 µF (Min) 10 µF (Min) 10 µF (Min) CIN 10 µF (Min) 10 µF (Min) 10 µF (Min) C1 and C2 should be ceramic capacitors with values in the range of 0.47 µF to 1µF. Higher values provide better load regulation. Tantalum capacitors are not recommended as the higher ESR of these capacitors degrades performance at high load currents and VCC = 4.75V. CIN and COUT can be ceramic, tantalum or electrolytic capacitors. The ESR of COUT introduces steps in the VOUT waveform whenever the charge pump charges COUT. This tends to increase VOUT ripple. Ceramic or tantalum capaci- tors are recommended for COUT if minimum ripple is |
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Similar Description - LTC1263_15 |
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