Datasheet LT1681 (Analog Devices) - 6

LT1681
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) 1681 G13 1681 G14
U U U PI FU CTIO S SHDN (Pin 1):
Shutdown Pin. Pin voltages exceeding resists mode switching instability. Exceeding the THERM positive-going threshold of 1.25V enables the LT1681. threshold also triggers soft-start reset, resulting in a 150mV of input hysteresis resists mode switching insta- graceful recovery. bility.
SGND (Pin 4):
Signal Ground Reference. Careful board The SHDN pin can be controlled by either a logic-level layout techniques must be used to prevent corruption of input or with an analog signal. This shutdown feature is the signal ground reference. High current switching paths typically used for input supply undervoltage protection. A must be oriented on the converter ground plane such that resistor divider from the converter input supply to the currents to/from the switches do not affect the integrity of SHDN pin monitors that supply for control of system the LT1681 signal ground reference. power-up sequencing, etc. All internal functions are dis-
5V
abled during shutdown.
REF (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, the LT1681 switching function is load on the 5V reference exceeds the current limit value, disabled to protect boosted circuitry from exceeding ab- LT1681 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
FSET (Pin 6):
Oscillator Timing Pin. Connect a resistor also triggers soft-start reset, resulting in a graceful recov- (R ery from an input transient event. FSET) from the 5VREF pin to this pin and a capacitor (CFSET) from this pin to ground.
THERM (Pin 3):
System Thermal Shutdown. Auxiliary The LT1681 oscillator operates by monitoring the voltage shutdown pin that is typically used for system thermal on C protection. If pin voltage exceeds 1.25V, the LT1681 FSET as it is charged via RFSET. When the voltage on the FSET pin reaches 2.5V, the oscillator rapidly discharges switching function is disabled. 40mV of input hysteresis the capacitor with an average current of 0.8mA. Once the 1681f 6