Datasheet LT3999 (Analog Devices) - 5

LT3999
TYPICAL PERFORMANCE CHARACTERISTICS Soft-Start (ILIM/SS) Current Switch Duty Cycle
14 40 13 12 35 11 30 10 9 25 8 DUTY CYCLE (%) 20 7 SOFT-START CURRENT (µA) 6 15 5 4 10 –50 –25 0 25 50 75 100 125 150 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) TEMPERATURE (°C) 3999 G09 3999 G10
PIN FUNCTIONS SWA, SWB (Pin 1, Pin 10):
SWA and SWB pins are the
RDC (Pin 6):
The RDC pin is the duty cycle control pin. A open-collector nodes of the power switches. These pins resistor to ground sets the duty cycle. If unused leave the drive the transformer and are connected to the outer ter- pin floating or connect to the OVLO/DC pin. minals of the center tapped transformer. Large currents
RT (Pin 7):
The RT pin sets the switching frequency of flow through these pins so keep PCB traces short and wide. the power switches.
RBIAS (Pin 2):
The RBIAS pin sets the bias current of
SYNC (Pin 8):
The SYNC pin synchronizes the part to an the power switches (SWA and SWB). Connect the pin to external clock. Set the internal oscillator frequency below a 49.9k resistor to GND. the external clock frequency. Synchronizing the clock to
VIN (Pin 3):
The VIN pin is the main supply pin for the an external reference is useful for creating more stable switch driver and internal regulator. Short duration, high positioning of the switcher voltage or current harmonics. current pulses are produced during the turn on and turn Connect the SYNC pin to ground if not used. off of the power switches. Connect a low ESR capacitor
ILIM/SS (Pin 9):
The ILIM/SS pin sets a threshold level of 4.7µF or greater. for the cycle by cycle maximum switch current. Imple-
UVLO (Pin 4):
The UVLO pin has a precision threshold ment soft-start with a capacitor, CSS, placed on this pin to with hysteresis to implement an accurate VIN undervolt- ground. An internal current source charges the capacitor. age lockout. The UVLO function disables switching and The RILIM, CSS time constant sets the soft-start time and sets the part into a low current shutdown mode. Connect ramps the maximum switch current threshold at start-up. the UVLO pin directly to VIN or to a resistor divider string. If the ILIM/SS function is not used, float this pin and the
OVLO/DC (Pin 5):
The OVLO/DC pin has a precision thresh- current limit will default to the internal limit. old with hysteresis to implement an accurate VIN overvolt-
GND (Pin 11):
The ground pin is the exposed pad of the age lockout. The OVLO function disables the switching. package. Solder the exposed pad directly to the ground Connect OVLO/DC pin to ground to disable the function plane. or to a resistor divider string to program the duty cycle. 3999fa For more information www.linear.com/LT3999 5 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 Package Description Typical Application Related Parts