LTC4216 datasheet(11/24 Pages) LINER

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LTC4216

4216f

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 ampliﬁer’s stability is compensated

by the large gate input capacitance (CISS) of the external

MOSFET used. These MOSFETs usually have CISS ≥ 1nF.

However, if the MOSFET’s gate input capacitance (CISS)

is too small for loop stability, then connect an external

capacitor between the GATE pin and ground to increase

the total gate capacitance to ≥ 1nF. As given by Equation

(4), the MOSFET current, IACL, is limited to the analog

current limit voltage, ΔVACL(TH), 40mV typical, across

the sense resistor, RSENSE, connected between SENSEP

and SENSEN pins.

ACL

ACL TH

SENSE

∆

()

(4)

The ΔVACL(TH) threshold is 1.6 times higher than the

ΔVCB(TH)threshold(25mVtypical)toprovideduallevelcur-

rent sensing. When the ACL ampliﬁer servos the MOSFET

current at ΔVACL(TH) across the sense resistor, it exceeds

ΔVCB(TH) threshold causing the FILTER pin to charge C3

with 60µA pull-up. If the condition persists long enough

for C3 to reach the VFILT(TH) threshold (1.253V), GATE is

pulled low and ⎯F⎯A⎯U⎯L⎯T latched low.

If the voltage across the sense resistor is greater than

ΔVACL(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, RZ, with an external

capacitor, CZ, at the GATE pin as shown in Figure 17.

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: 10µA current

source pull-up for a normal ramp rate; and 1µA current

source pull-up for a slower ramp rate. Both the SS ramp

rates are given as follows:

Normal SS Ramp Rate:

SS NOM

() = µ

(5)

Slower SS Ramp Rate:

SS SLOW

() = µ

(6)

Figure 6. Circuit Breaker Filter

Response for Intermittent Overload

APPLICATIO S I FOR ATIO

OVERLOAD DUTY CYCLE, D (%)

20406080

100

0.1

0.01

4216 F06

t/C3(s/

µF) = 1.253/[(60 • D) – 2.4]

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