Datasheet LTC3805 (Analog Devices) - 9

LTC3805
operation Start-Up/Shutdown Setting the Oscillator Frequency
The LTC3805 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 thresh- from: old before operation is enabled. The LTC3805 transitions 24 • 109 into and out of shutdown according to the state diagram fOSC = R shown in Figure 1. Operation in fault timeout is discussed FS − 1500 in a subsequent section. During shutdown the LTC3805 The oscillator may be synchronized to an external clock draws only a small 40µA current. 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 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 = 8.4V to enable LTC3805 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. This sense resistor, the converter is protected in the event of wide UVLO hysteresis range supports the use of trickle an overload or short-circuit on the output. During normal charger powering schemes. See the Applications Informa- operation the peak value of current in the external MOS- tion section for more detail. FET, as measured by the current sense resistor (plus any The RUN pin is connected to the input voltage using a adjustment for slope compensation), is set by the voltage voltage divider. Converter operation is enabled when the on the ITH pin operating through the current comparator. voltage on the RUN pin exceeds VRUNON = 1.207V and As the output current increases, so does the voltage on disabled when the voltage falls below VRUNOFF = 1.170V. the ITH pin and so does the peak MOSFET current. Additional hysteresis is added by a 5µA current source acting on the voltage divider’s Thevenin resistance. Setting the input voltage range and hysteresis is further discussed in the Applications Information section. VRUN > VRUNON AND VCC > VTURNON LTC3805 VRUN < VRUNOFF LTC3805 SHUTDOWN ENABLED VCC < VTURNOFF LTC3805 FAULT VSSFLT < 0.7V V TIMEOUT OC > 100mV 3805 F01
Figure 1. Start-Up/Shutdown State Diagram
3805fg 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration 0rder Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Application Package Description Revision History Typical Application Related Parts