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LTC3202 Datasheet(PDF) 6 Page - Linear Technology |
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LTC3202 Datasheet(HTML) 6 Page - Linear Technology |
6 / 12 page 6 LTC3202 3202fa Figure 1. Current Control Mode 3202 F01 LTC3202 VOUT FB GND 1 µF • • • ILED = VFB RX 5, 11 3 2 RX RX OPERATIO The LTC3202 uses a fractional conversion switched ca- pacitor charge pump to boost VOUT to as much as 1.5 times the input voltage. A two-phase nonoverlapping clock acti- vates the charge pump switches. On the first phase of the clock the flying capacitors are charged in series from VIN. On the second phase of the clock they are connected in parallel and stacked on top of VIN. This sequence of charging and discharging the flying capacitors continues at a free running frequency of 1.5MHz (typ). Regulation is achieved by sensing the voltage at the FB pin and modulating the charge pump strength based on the error signal. The control pins, D0 and D1, program the set point of the internal digital-to-analog converter. The regu- lation loop will increase VOUT until FB comes to balance at the set-point voltage. Table 1 shows the regulation voltage as a function of D0 and D1. Table 1. Feedback Control Voltage Settings D1 D0 Feedback Set Point Voltage 0 0 Shutdown 0 1 0.2V 1 0 0.4V 1 1 0.6V In shutdown mode all circuitry is turned off and the LTC3202 draws only leakage current from the VIN supply. Furthermore, VOUT is disconnected from VIN. The D0 and D1 pins are CMOS inputs with a threshold voltage of approximately 0.8V. The LTC3202 is in shutdown when a logic low is applied to both D0 and D1. Since the D0 and D1 pins are high impedance CMOS inputs they should never be allowed to float. To ensure that their states are defined they must always be driven with valid logic levels. Shutdown Current Output voltage detection circuitry will draw a current of 5 µA when the LTC3202 is in shutdown. This current will be eliminated when the output voltage (VOUT) is at 0V. To ensure that VOUT is at 0V in shutdown a bleed resistor can be used from VOUT to GND. 10k to 100k is acceptable. Short-Circuit/Thermal Protection The LTC3202 has built-in short-circuit current limiting as well as over temperature protection. During short-circuit conditions it will automatically limit its output current to approximately 250mA. At higher temperatures, or if the input voltage is high enough to cause excessive self heating on-chip, thermal shutdown circuitry will shut down the charge pump when the junction tempera- ture exceeds approximately 160 °C. It will reenable the charge pump once the junction temperature drops back to approximately 155 °C. The LTC3202 will cycle in and out of thermal shutdown indefinitely without latchup or damage until the short-circuit on VOUT is removed. Soft-Start To prevent excessive current flow at VIN during start-up, the LTC3202 has built-in soft-start circuitry. Soft-start is achieved by increasing the amount of current available to the output charge storage capacitor linearly over a period of approximately 500 µs. The soft-start feature activates any time an input, D0 or D1, changes state. This will prevent large inrush current during initial start-up as well as when the feedback setting is changed from one value to the next. Note that the set point voltage will drop to zero during the soft-start period. Under heavy load conditions there may be observable droop at VOUT until the soft-start circuit catches up. Programming the LTC3202 for Voltage or Current The LTC3202 can be configured to control either a voltage or a current. In white LED applications the LED current is programmed by the ratio of the feedback set point voltage and a sense resistor as shown in Figure 1. The current of the remaining LEDs is controlled by virtue of their similar- ity to the reference LED and the ballast voltage across the sense resistor. (Refer to Simplified Block Diagram) |
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