LTC4380HMS-1TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC4380HMS-1TRPBF Datasheet PDF : 20 Pages
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LTC4380
Applications Information
effective means of limiting the VCC voltage. Using a 68V
Zener assures that D1 will not override the internal GATE
pin clamp in the LTC4380-1 and LTC4380-2 devices. For
the LTC4380-3 and LTC4380-4, the VCC operating range
extends from 4V to 72V. Since the GATE pin is regulated to
VOUT + 11.5V, D1 is chosen to achieve the desired output
clamping effect while at the same time keeping the VCC
pin within its 4V to 72V range.
Fault Timer Overview
Overvoltage and overcurrent conditions, and high VDS con-
ditions in Q1 are limited in duration by an adjustable fault
timer. A capacitor at the TMR pin CTMR sets the delay time
before a fault condition is reported at the FLT pin and Q1 is
turned off. CTMR also sets the cool down time before Q1 is
permitted to turn back on for the LTC4380-2 and LTC4380-4
auto retry versions. The LTC4380-1 and LTC4380-3 versions
simply latch off at the end of the timer delay.
Fault timing starts as soon as the input power is applied
with the part in the on condition, or when the part is turn
on after application of power. A 1.5µA current is gener-
ated to pull up the TMR pin when the voltage across the
MOSFET is higher than 0.7V. The timer speeds up with
an additional current that varies with the power dissipated
in the MOSFET, Q1. The power dissipation is the product
of the voltage across the MOSFET (VDS) and the current
flowing through it (ID). VDS is inferred from the voltage
drop across the drain pin resistor, RDRN, while ΔVSNS
represents ID.
while the LTC4380-2/LTC4380-4 automatically restart.
Fault Timer Operation in Overvoltage
During an overvoltage condition, where the MOSFET’s VDS
exceeds 0.7V, the TMR pin charges from 0V to 1.215V
with a current that varies principally as a function of VDS
and ID. VDS is inferred from the current flowing in the DRN
pin resistor, RDRN, while the voltage difference between
the SNS and OUT pins (∆VSNS) represents the MOSFET
current, ID.
The TMR pin current is given by
ITMR
=
1.5
•
10–6A
+
⎡
0.0917⎢
⎣
A
V
⎤
⎥
⎦
•
ΔVSNS
•
IDRN
Where 1.5 • 10–6A is the minimum TMR current and
0.0917√A/V is the gain term of the multiplier.
Substituting for ∆VSNS and IDRN
ITMR
=
1.5
•
10–6A
+
⎡
0.0917⎢
⎣
A
V
⎤
⎥
⎦
•
ID
•
VDS
RSNS
RDRN
When TMR reaches 1.215V, the FLT pin pulls low and the
MOSFET is turned off and allowed to cool for an extended
period. The total elapsed time between the onset of output
clamping and turning off is given by:
tTMR = VTMR(F) •
CTMR
ITMR
At initial power-up, the 1.5µA pilot current charges the
TMR pin capacitor because the input supply is, at least
for a short time, more than 0.7V above the output voltage.
When the output rises to within 0.7V of the input supply
voltage, the pull-up current disappears and an internal
2µA current source discharges the TMR pin capacitor. The
capacitor must be sized to ride through the initial start-up
interval for successful power-up.
In the presence of a sustained fault, the timer current charges
the TMR pin to 1.215V. At this point the FLT pin pulls low
to indicate a fault condition and the GATE pin pulls low,
shutting off the MOSFET. After faulting off, the timer enters
the cool down phase. At the end of the cool down period
the LTC4380-1/LTC4380-3 remain off until manually reset,
Because ITMR is a function of VDS and ID, the exact time
spent in overvoltage before turning off depends upon the
input waveform and the load current.
Fault Timer Operation in Overcurrent
TMR pin behavior in overcurrent is substantially the same as
in overvoltage. In the presence of an overcurrent condition
when the LTC4380 regulates the output current, the TMR
pin charges from 0V to 1.215V with a current that varies
principally as a function of the power dissipated on the
MOSFET. In addition to the variable current, an additional
27µA hastens timeout in a low impedance short where the
output is less than 1.5V. This additional current is reduced
to 7µA when VOUT is above 3V.
4380f
For more information www.linear.com/LTC4380
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