Datasheet LTC3805-5 (Analog Devices) - 9

LTC3805-5
OPERATION Start-Up/Shutdown Setting the Oscillator Frequency
The LTC3805-5 has two shutdown mechanisms to disable Connect a frequency set resistor RFS from the FS pin to and enable operation: an undervoltage lockout on the VCC ground to set the oscillator frequency over a range from supply pin voltage, and a precision-threshold RUN pin. The 70kHz to 700kHz. The oscillator frequency is calculated voltage on both pins must exceed the appropriate threshold from: before operation is enabled. The LTC3805-5 transitions into and out of shutdown according to the state diagram 24 • 109 f shown in Figure 1. Operation in fault timeout is discussed OSC = RFS −1500 in a subsequent section. During shutdown the LTC3805-5 draws only a small 40µA current. The oscillator may be synchronized to an external clock using the SYNC input. The rising edge of the external clock The undervoltage lockout (UVLO) mechanism prevents on the SYNC pin triggers the beginning of a switching the LTC3805-5 from trying to drive the external MOSFET period, i.e., the GATE pin going high. The pulse width of gate with insufficient voltage on the VCC pin. The voltage the external clock is quite flexible. at the VCC pin must initially exceed VTURNON = 4.5V to en- able LTC3805-5 operation. After operation is enabled, the
Overcurrent Protection
voltage on the VCC pin may fall as low as VTURNOFF = 4V With the OC pin connected to the external MOSFET’s current before undervoltage lockout disables the LTC3805-5. See sense resistor, the converter is protected in the event of the Applications Information section for more detail. an overload or short-circuit on the output. During normal The RUN pin is connected to the input voltage using a operation the peak value of current in the external MOSFET, voltage divider. Converter operation is enabled when the as measured by the current sense resistor (plus any adjust- voltage on the RUN pin exceeds V ment for slope compensation), is set by the voltage on the RUNON = 1.207V and disabled when the voltage falls below V I RUNOFF = 1.170V. TH pin operating through the current comparator. As the Additional hysteresis is added by a 5µA current source output current increases, so does the voltage on the ITH acting on the voltage divider’s Thevenin resistance. Setting pin and so does the peak MOSFET current. the input voltage range and hysteresis is further discussed in the Applications Information section. VRUN > VRUNON AND VCC > VTURNON LTC3805-5 VRUN < VRUNOFF LTC3805-5 SHUTDOWN ENABLED VCC < VTURNOFF LTC3805-5 FAULT VSSFLT < 0.7V V TIMEOUT OC > 100mV 38055 F01
Figure 1. Start-Up/Shutdown State Diagram
38055fe 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration order information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts