Datasheet

LTC4217
11
4217fe
applicaTions inForMaTion
Tying the TIMER pin to INTV
CC
will force the part to use
the internally generated (circuit breaker) delay of 2ms.
In either case the F LT 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 set-
ting the timing capacitor’s value is as follows:
C
T
= t
CB
• 0.083(µF/ms)
After the switch is turned off, the TIMER pin begins
discharging 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
CC
then the switch is allowed to turn on again (after
an internal 100ms delay) if the overcurrent fault is cleared.
Tying the F LT 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
CC
.
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
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
DD
drops below 2.65V for greater than 5µs or INTV
CC
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 Characteristics
curves shows the current limit versus FB voltage.
An overcurrent fault occurs when the current limit circuitry
has been engaged for longer than the timeout 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 140mA 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
Characteristics 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.
Figure 3. Compensation for Small C
LOAD
4217 F03
LTC4217
*OPTIONAL
RC TO LOWER
INRUSH CURRENT
GATE
C
P
2.2nF
Figure 4. Short-Circuit Waveform
I
OUT
1A/DIV
V
OUT
10V/DIV
∆V
GATE
10V/DIV
TIMER
2V/DIV
1ms/DIV
4217 F04