LTC4217 Linear Technology, LTC4217 Datasheet - Page 11

• Manufacturer Part Number: LTC4217
• Description: 2A Integrated Hot Swap Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
will turn off the switch. These include an input overvoltage
(OV pin), overcurrent circuit breaker (SENSE pin) or over
temperature. Normally the switch is turned off with a 250μA
current pulling down the GATE pin to ground. With the
switch turned off, the OUT voltage drops which pulls the
FB pin below its threshold. PG then pulls low to indicate
output power is no longer good.
If V
drops below 2.5V for greater than 1μs, a fast shutdown
of the switch is initiated. The GATE is pulled down with a
170mA current to the OUT pin.
Overcurrent Fault
The LTC4217 features an adjustable current limit with
foldback that protects against short circuits or excessive
load current. To prevent excessive power dissipation in the
switch during active current limit, the available current is
reduced as a function of the output voltage sensed by the
FB pin. A graph in the Typical Performance curves shows
the current limit versus FB voltage.
An overcurrent fault occurs when the current limit circuitry
has been engaged for longer than the time-out delay set
by the TIMER. Current limiting begins when the MOSFET
current reaches 0.5A to 2A (depending on the foldback).
The GATE pin is then brought down with a 170mA GATE-
to-OUT current. The voltage on the GATE is regulated in
order to limit the current to less than 2A. At this point, a
circuit breaker time delay starts by charging the external
timing capacitor from the TIMER pin with a 100μA pull-up
current. If the TIMER pin reaches its 1.2V threshold, the
internal switch turns off (with a 250μA current from GATE
to ground). Included in the Typical Performance Curves is
a graph of the Safe Operating Area for the MOSFET. From
this graph one can determine the MOSFET’s maximum
time in current limit for a given output power.
DD
drops below 2.65V for greater than 5μs or INTV
Figure 3. Compensation for Small C
LTC4217
GATE
4217 F03
C
2.2nF
P
*OPTIONAL
RC TO LOWER
INRUSH CURRENT
LOAD
CC
Tying the TIMER pin to INTV
the internally generated (circuit breaker) delay of 2ms.
In either case the FLT pin is pulled low to indicate an
overcurrent fault has turned off the pass MOSFET. For a
given the circuit breaker time delay, the equation for setting
the timing capacitor’s value is as follows:
After the switch is turned off, the TIMER pin begins dis-
charging the timing capacitor with a 2μA pull-down current.
When the TIMER pin reaches its 0.2V threshold, an internal
100ms timer is started. After the 100ms delay, the switch
is allowed to turn on again if the overcurrent fault has been
cleared. Bringing the UV pin below 0.6V and then high will
clear the fault. If the TIMER pin is tied to INTV
switch is allowed to turn on again (after an internal 100ms
delay) if the overcurrent fault is cleared.
Tying the FLT pin to the UV pin allows the part to self-
clear the fault and turn the MOSFET on as soon as TIMER
pin has ramped below 0.2V. In this auto-retry mode the
LTC4217 repeatedly tries to turn on after an overcurrent
at a period determined by the capacitor on the TIMER pin.
The auto-retry mode also functions when the TIMER pin
is tied to INTV
The waveform in Figure 4 shows how the output latches
off following a short circuit. The current in the MOSFET
is 0.5A as the timer ramps up.
Current Limit Adjustment
The default value of the active current limit is 2A. The
current limit threshold can be adjusted lower by placing
C
T
10V/DIV
10V/DIV
= t
ΔV
1A/DIV
2V/DIV
TIMER
V
GATE
I
CB
OUT
OUT
• 0.083(μF/ms)
CC
Figure 4. Short-Circuit Waveform
.
1ms/DIV
CC
will force the part to use
LTC4217
4217 F04
CC
then the
11
4217fb