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LTC1144 Datasheet(PDF) 7 Page - Linear Technology |
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LTC1144 Datasheet(HTML) 7 Page - Linear Technology |
7 / 12 page LTC3261 7 3261f OPERATION (Refer to the Block Diagram) The LTC3261 is a high voltage inverting charge pump. It supports a wide input power supply range from 4.5V to 32V. Shutdown Mode In shutdown mode, all circuitry except the internal bias is turned off. The LTC3261 is in shutdown when a logic low is applied to the enable input (EN). The LTC3261 only draws 2μA (typical) from the VIN supply in shutdown. Constant Frequency Operation The LTC3261 provides low noise constant frequency opera- tion when a logic low is applied to the MODE pin. The charge pump and oscillator circuit are enabled using the EN pin. At the beginning of a clock cycle, switches S1 and S2 are closed. The external flying capacitor across the C+ and C– pins is charged to the VIN supply. In the second phase of the clock cycle, switches S1 and S2 are opened, while switches S3 and S4 are closed. In this configuration the C+ side of the flying capacitor is grounded and charge is delivered through the C– pin to VOUT. In steady state the VOUT pin regulates at –VIN less any voltage drop due to the load current on VOUT. The charge transfer frequency can be adjusted between 50kHz and 500kHz using an external resistor on the RT pin. At slower frequencies the effective open-loop output resistance (ROL) of the charge pump is larger and it is able to provide smaller average output current. Figure 1 can be used to determine a suitable value of RT to achieve a required oscillator frequency. If the RT pin is grounded, the part operates at a constant frequency of 500kHz. Burst Mode Operation The LTC3261 provides low power Burst Mode operation when a logic high is applied to the MODE pin. In Burst Mode operation, the charge pump charges the VOUT pin to –0.94 • VIN (typical). The part then shuts down the internal oscillator to reduce switching losses and goes into a low current state. This state is referred to as the sleep state in which the IC consumes only about 60μA. When the output voltage droops enough to overcome the burst comparator hysteresis, the part wakes up and commences charge pump cycles until output voltage exceeds –0.94 • VIN (typical). This mode provides lower operating current at the cost of higher output ripple and is ideal for light load operation. Thefrequencyofchargingcyclesissetbytheexternalresistor on the RT pin. The charge pump has a lower ROL at higher frequencies.ForBurstModeoperationitisrecommendedthat the RT pin be tied to GND. This minimizes the charge pump ROL, quickly charges the output up to the burst threshold and optimizes the duration of the low current sleep state. Figure 1. Oscillator Frequency vs RT Soft-Start The LTC3261 has built in soft-start circuitry to prevent excessive current flow during start-up. The soft-start is achieved by internal circuitry that slowly ramps the amount of current available at the output storage capacitor. The soft-start circuitry is reset in the event of a commanded shutdown or thermal shutdown. Short-Circuit/Thermal Protection The LTC3261 has built-in short-circuit current limit as well as overtemperature protection. During a short-circuit condition, the part automatically limits its output current to approximately 160mA. If the junction temperature ex- ceeds approximately 175°C the thermal shutdown circuitry disables current delivery to the output. Once the junction temperature drops back to approximately 165°C current delivery to the output is resumed. When thermal protection is active the junction temperature is beyond the specified operating range. Thermal protection is intended for mo- mentary overload conditions outside normal operation. Continuous operation above the specified maximum op- erating junction temperature may impair device reliability. RT (kΩ) 1 0 500 600 400 300 200 100 100 1000 10 10000 3261 F01 |
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Similar Description - LTC1144 |
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