LT3580
OPERATION
Start-Up Operation
Several functions are provided to enable a very clean
start-up for the LT3580.
• First, the
SHDN
pin voltage is monitored by an internal
voltage reference to give a precise turn-on voltage level.
An external resistor (or resistor divider) can be connected
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-up of the switch current. When the part is brought
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, the frequency foldback circuit reduces the switch-
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.
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
thermal shutdown circuits protect the power switch as well
as the external components connected to the LT3580.
3580fc
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