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LTC1911EMS8-1.5 Datasheet(PDF) 8 Page - Linear Technology |
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LTC1911EMS8-1.5 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page 8 LTC1911-1.5/LTC1911-1.8 sn1911 1911is 4.7nF capacitor on SS/SHDN results in a 3ms ramp time from 0.6V to 1.9V on the pin. If COUT is 10µF, the 3ms VREF ramp time results in an average COUT charge current of only 6mA (see Figure 2). Low Current Burst Mode Operation To improve efficiency at low output currents, a Burst Mode function was included in the design of the LTC1911. An output current sense circuit is used to detect when the required output current drops below 30mA typ. When this occurs, the oscillator shuts down and the part goes into a low current operating state. The LTC1911 will remain in the low current operating state until VOUT has dropped enough to require another burst of current. Unlike tradi- tional charge pumps who’s burst current is dependant on many factors (i.e., supply, switch strength, capacitor selection, etc.), the LTC1911 burst current is set by the burst threshold. This means that the output ripple voltage during Burst Mode operaton will be fixed and is typically 5mV. Short-Circuit/Thermal Protection The LTC1911 has built-in short-circuit current limiting as well as overtemperature protection. During short-circuit conditions it will automatically limit its output current to approximately 600mA. The LTC1911 will shut down if the junction temperature exceeds approximately 160 °C. Un- der normal operating conditions, the LTC1911 should not go into thermal shutdown but it is included to protect the IC in cases of excessively high ambient temperatures, or in cases of excessive power dissipation inside the IC (i.e., overcurrent or short circuit). The charge transfer will reactivate once the junction temperature drops back to approximately 150 °C. The LTC1911 can cycle in and out of thermal shutdown indefinitely without latch-up or damage until the fault condition is removed. VOUT Ripple and Capacitor Selection The type and value of capacitors used with the LTC1911 determine several important parameters such as regulator control loop stability, output ripple and charge pump strength. The value of COUT directly controls the amount of output ripple for a given load current. Increasing the size of COUT will reduce the output ripple. APPLICATIO S I FOR ATIO Figure 2. Shutdown/Soft-Start Operation SS/SHDN CSS ON OFF VCTRL 6 8 VOUT RLOAD LTC1911 (2a) VCTRL 2V/DIV VOUT 1V/DIV CSS = 0nF 2ms/DIV 1911 F02b COUT = 10µF RLOAD = 10Ω (2b) VCTRL 2V/DIV VOUT 1V/DIV CSS = 4.7nF 2ms/DIV 1911 F02c COUT = 10µF ROUT = 10Ω (2c) |
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