lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 11

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltage ramp minimizes output voltage overshoot, re-
duces inrush current from the V
supply sequencing. A capacitor, C
V
slew rate. The capacitor provides a current to the C
which is proportional to the dV/dt of the output voltage.
The soft-start circuit overrides the control loop and ad-
justs the inductor current until the output voltage slew rate
yields a 2μA current through the soft-start capacitor. If the
current is greater than 2μA, then the current threshold set
by the DC control voltage, V
inductor current is lowered. This in turn lowers the output
current and the output voltage slew rate is decreased. If the
current is less than 2μA, then the current threshold set by
the DC control voltage, V
current is raised. This in turn increases the output current
and the output voltage slew rate is increased. Once the
output voltage is within 5% of its regulation voltage, the
soft-start circuit is disabled and the main control regulates
the output. The soft-start circuit is reactivated when the
output voltage drops below 70% of its regulation voltage.
Slope/Antislope Compensation
The IC incorporates slope compensation to eliminate
potential subharmonic oscillations in the current control
loop. The IC’s slope compensation circuit imposes an
OUT
of the supply and the C
U
(Refer to Functional Diagram)
C
, is increased and the inductor
SS
pin of the IC, programs the
C
IN
, is decreased and the
SS
supply, and facilitates
, connected between
SS
pin
artificial ramp on the sensed current to increase the rising
slope as duty cycle increases.
Unfortunately, this additional ramp typically affects the
sensed current value, thereby reducing the achievable
current limit value by the same amount as the added ramp
represents. As such, the current limit is typically reduced
as the duty cycle increases. The LT3724, however, con-
tains antislope compensation circuitry to eliminate the
current limit reduction associated with slope compensa-
tion. As the slope compensation ramp is added to the
sensed current, a similar ramp is added to the current limit
threshold. The end result is that the current limit is not
compromised so the LT3724 can provide full power re-
gardless of required duty cycle.
Shutdown
The LT3724 includes a shutdown mode where all the
internal IC functions are disabled and the V
reduced to less than 10μA. The shutdown pin can be used
for undervoltage lockout with hysteresis, micropower
shutdown or as a general purpose on/off control of the
converter output. The shutdown function has two thresh-
olds. The first threshold, a precision 1.23V threshold with
120mV of hysteresis, disables the converter from switch-
ing. The second threshold, approximately a 0.7V refer-
enced to SGND, completely disables all internal circuitry
and reduces the V
Application Information section for more information.
IN
current to less than 10μA. See the
LT3724
IN
current is
11
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