LT3580
OPERATION
Start-Up Operation
Current Limit and Thermal Shutdown Operation
The LT3580 has a current limit circuit not shown in the
Block Diagram. The switch current is consistently moni-
tored and not allowed to exceed the maximum switch
current at a given duty cycle (see the Electrical Charac-
teristics table). If the switch current reaches this value,
the SR latch (SR1) is reset regardless of the state of the
comparator (A1/A2). Also not shown in the Block Diagram
is the thermal shutdown circuit. If the temperature of the
part exceeds approximately 165°C, the SR2 latch is set
regardless of the state of the comparator (A1/A2). A full
soft-start cycle will then be initiated. The current limit and
thermalshutdowncircuitsprotectthepowerswitchaswell
as the external components connected to the LT3580.
Several functions are provided to enable a very clean
start-up for the LT3580.
• First, the SHDN pin voltage is monitored by an internal
voltagereferencetogiveapreciseturn-onvoltagelevel.
Anexternalresistor(orresistordivider)canbeconnected
from the input power supply to the SHDN pin to provide
a user-programmable undervoltage lockout function.
• Second, the soft-start circuitry provides for a gradual
ramp-upoftheswitchcurrent. Whenthepartisbrought
out of shutdown, the external SS capacitor is first
discharged (providing protection against SHDN pin
glitches and slow ramping), then an integrated 275k
resistor pulls the SS pin up to ~2.2V. By connecting an
external capacitor to the SS pin, the voltage ramp rate
on the pin can be set. Typical values for the soft-start
capacitor range from 100nF to 1μF.
• Finally,thefrequencyfoldbackcircuitreducestheswitch-
ing frequency when the FB pin is in a nominal range of
350mV to 900mV. This feature reduces the minimum
duty cycle that the part can achieve thus allowing better
control of the switch current during start-up. When the
FB voltage is pulled outside of this range, the switching
frequency returns to normal.
3580fc
8
Linear [ Linear ]
