Datasheet LT3781 (Analog Devices) - 6

LT3781
W U TYPICAL PERFOR A CE CHARACTERISTICS Soft-Start Output Current Current Sense Amplifier vs Soft-Start Pin Voltage Bandwidth vs Temperature
60 8 TA = 25°C µA) 7 40 6 5 20 4 3 SOFT-START OUTPUT CURRENT ( CURRENT SENSE AMP BANDWIDTH (MHz) 0 2 0 1 2 3 4 5 –55 –35 –15 5 25 45 65 85 105 125 SOFT-START PIN VOLTAGE (V) TEMPERATURE (°C) 3781 G13 3781 G14
U U U PI FU CTIO S SHDN (Pin 1):
Shutdown Pin. Pin voltages exceeding
THERM (Pin 3):
System Thermal Shutdown. Auxiliary positive going threshold of 1.25V enables the LT3781. shutdown pin that is typically used for system thermal 150mV of input hysteresis resists mode switching insta- protection. If pin voltage exceeds 1.25V, LT3781 bility. switching function is disabled. 40mV of input hysteresis resists mode switching instability. Exceeding the THERM The SHDN pin can be controlled by either a logic level input threshold also triggers soft-start reset, resulting in a or with an analog signal. This shutdown feature is typically graceful recovery. used for input supply undervoltage protection. A resistor divider from the converter input supply to the SHDN pin
SGND (Pin 4):
Signal Ground Reference. Careful board monitors that supply for control of system power-up layout techniques must be used to prevent corruption of sequencing, etc. signal ground reference. High-current switching paths must be oriented on the converter ground plane such that An 18V clamp on the VCC pin is enabled during shutdown currents to/from the switches do not affect the integrity of mode, preventing a trickle start circuit from pulling that pin the LT3781 signal ground reference. above maximum operational levels. All other internal functions are disabled during shutdown.
5VREF (Pin 5):
5V Local Reference. Allows connection of external loads up to 20mA DC. Typically bypassed with
OVLO (Pin 2):
Overvoltage Shutdown Sense. Typically 1µF ceramic capacitor to SGND. Reference output is connected to input supply through a resistor divider. If pin current limit protected to a typical value of 45mA. If the voltage exceeds 1.25V, LT3781 switching function is load on the 5V reference exceeds the current limit value, disabled to protect boosted circuitry from exceeding ab- LT3781 switching function is disabled and the soft-start solute maximum voltage. 40mV of input hysteresis resists function is reset. mode switching instability. Exceeding the OVLO threshold also triggers soft-start reset, resulting in a graceful recov-
FSET (Pin 6):
Oscillator Timing Pin. Connect a resistor ery from an input transient event. (RFSET) from the 5VREF pin to this pin and a capacitor (CFSET) from this pin to ground. 3781f 6