LTC4216
12
4216fa
For more information www.linear.com/LTC4216
Analog Current Limiting
In addition to an electronic circuit breaker, the LTC4216
has included a novel analog current limit (ACL) amplifier
that does not require an external compensation capacitor
at the GATE pin. The amplifiers stability is compensated
by the large gate input capacitance (C
ISS
) of the external
MOSFET used. These MOSFETs usually have C
ISS
e 1nF.
However, if the MOSFETs gate input capacitance (C
ISS
)
is too small for loop stability, then connect an external
capacitor between the GATE pin and ground to increase
the total gate capacitance to e 1nF. As given by Equation
(4), the MOSFET current, I
ACL
, is limited to the analog
current limit voltage, 擵
ACL(TH)
, 40mV typical, across
the sense resistor, R
SENSE
, connected between SENSEP
and SENSEN pins.
I
V
R
ACL
ACL TH
EN E
=
( )
(4)
The 擵
ACL(TH)
threshold is 1.6 times higher than the
擵
CB(TH)
threshold (25mV typical) to provide dual level cur-
rent sensing. When the ACL amplifier servos the MOSFET
current at 擵
ACL(TH)
across the sense resistor, it exceeds
擵
CB(TH)
threshold causing the FILTER pin to charge C3
with 60礎 pull-up. If the condition persists long enough
for C3 to reach the V
FILT(TH)
threshold (1.253V), GATE is
pulled low and FAULT latched low.
If the voltage across the sense resistor is greater than
擵
ACL(TH)
during an overload condition, the ACL amplifier
will servo GATE downwards in an attempt to control the
MOSFET current. Since the GATE pin voltage overdrives
the MOSFET in normal operation, the ACL amplifier needs
time to discharge the GATE to the threshold of the MOSFET
for gate regulation. For mild overload, the ACL amplifier
can control the MOSFET current, but in the event of a
severe overload, the MOSFET current may overshoot as
the MOSFET has large GATE overdrive initially. The GATE
is quickly discharged to ground followed by the ACL ampli-
fier taking control. For applications that require very fast
analog current limit recovery from the GATE undershoot as
it discharges, connect a series resistor, R
Z
, with an exter-
nal capacitor, C
Z
, at the GATE pin as shown in Figure 17.
The value of R
Z
should be between 10?and 100?for
optimum performance.
Soft-Start
The LTC4216 features a soft-start function that controls
the di/dt of the inrush current during power-up. As large
output load capacitors are commonly used in low voltage
applications, the normal inrush can be large enough to
glitch the load supply. With the soft-start function, the
gate of the external MOSFET is allowed to turn on very
gradually to control the inrush current flowing into the
load capacitor without causing a supply glitch.
With an external capacitor, C2, connected between the SS
pin and ground, the GATE is servoed by the ACL amplifier
to track the rate of SS ramp-up during power-up. There
are two slopes in the SS ramp-up profile: 10礎 current
source pull-up for a normal ramp rate; and 1礎 current
source pull-up for a slower ramp rate. Both the SS ramp
rates are given as follows:
Normal SS Ramp Rate:
dV
A
SS NOM
(
)
=
?/DIV>
10
2
(5)
Slower SS Ramp Rate:
dV
A
SS SLOW
(
)
=
?/DIV>
1
2
(6)
Figure 6. Circuit Breaker Filter
Response for Intermittent Overload
OVERLOAD DUTY CYCLE, D (%)
0
20
40
60
80
100
1
0.1
0.01
4216 F06
t/C3(s/礔) = 1.253/[(60 " D) 2.4]
applicaTions inForMaTion
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