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LTC1682CMS8-5 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1682CMS8-5 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 9 LTC1682/LTC1682-3.3/LTC1682-5 The following formula can be used to find the maximum output voltage that may be programmed for a given minimum input voltage and output current load: VOUT(MAX) = (2)(VIN(MIN)) – (IOUT)(RCPO) – VDROPOUT with the condition that (IOUT)(RCPO) < 0.55VIN. Example: VIN(MIN) = 3V IOUT = 10mA RCPO(MAX) = 20Ω Max unloaded CPO voltage = 6V Loaded CPO voltage = 6V – (10mA)(20 Ω) = 5.8V VDROPOUT(MAX) = 0.08V VOUT(MAX) = (6V) – (0.2V) – (0.08V) = 5.72V VOUT < 5.5V and (IOUT)(RCPO) < 0.55VIN, 0.2V < 1.65V. For minimum noise applications, the LDO must be kept out of dropout to prevent CPO noise from coupling into VOUT. External CPO Loading The CPO output can drive an external load (an LDO, for example). The current required by this additional load will reduce the available current from VOUT. If the external load requires 5mA, then the maximum available current at VOUT will be reduced by 5mA. Short-Circuit and Thermal Protection VOUT can be shorted to ground indefinitely. Internal cir- cuitry will limit the output current. If the junction tempera- ture exceeds 150 °C, the part will shut down. Excessive power dissipation due to heavy loads will also cause the part to shut down when the junction temperature exceeds 150 °C. The part will become enabled when the junction temperature drops below 140 °C. If the fault condition remains in place, the part will cycle between the shutdown and enabled states. Capacitor Selection For best performance it is recommended that low ESR capacitors be used for C2, C3 and C4 in Figure 1 to reduce noise and ripple. C2 must be ≥ 2µF and C3 must be equal to or greater than C2. C4 is dependent on the source impedance. The charge pump demands large APPLICATIONS INFORMATION instantaneous currents which may induce ripple onto a common voltage rail. C4 should be ≥ 2µF and a spike reducing resistor of 2.2 Ω may be required between VIN and the supply. A low ESR ceramic capacitor is recommended for the flying capacitor C1 with a value of 0.22 µF. At low load or high VIN a smaller capacitor could be used to reduce ripple on CPO which would reflect as lower ripple on VOUT. If a minimum enable time is required, the CPO output filter capacitor should be at least 2 × the VOUT filter capacitor. When the LDO is first enabled, the CPO capacitor will dump a large amount of charge into the VOUT capacitor. If the drop in the CPO voltage falls below 1.45(VIN), the LDO will be disabled and the CPO voltage will have to charge up to 1.75(VIN) to enable the LDO. The resulting cycling extends the enable time. A 1nF filter capacitor for the LTC1682-3.3/LTC1682-5 should be connected between the FILT pin and ground for optimum noise performance. Output Ripple The output noise and ripple on CPO includes a spike component from the charge pump switches and a droop component which is dependent on the load current and the value of C3. The charge pump has been carefully designed to minimize the spike component; however, low ESR capacitors are essential to reduce the remaining spike energy effect on the CPO voltage. C3 should be increased for high load currents to minimize the droop component. Ripple components on CPO are greatly reduced at VOUT by the LDO; however, C2 should also be a low ESR capacitor to improve filtering of the CPO noise. Shutdown When SHDN pin is pulled low (< 0.4V), the part will be in shutdown, the supply current will be < 5 µA and VOUT will be connected to ground through a 100 Ω switch. In addi- tion, CPO will be high impedance and disconnected from VIN. If shutdown is not required, connect SHDN to VIN which will continuously enable the part. |
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Similar Description - LTC1682CMS8-5 |
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