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LTC4223-1/LTC4223-2

422312f

APPLICATIONS INFORMATION

As the TIMER capacitor is used for fault ﬁltering during

power-up for both the auxiliary and 12V supplies, only

one supply can be started up at any one time. The other

supply waits until the power-good signal is generated by

the powering-up supply and the TIMER pin voltage falls

below 0.2V. By default, the 3.3V auxiliary supply starts up

ﬁrst if both AUXON and 12ON are high at the end of the

debounce cycle.

Whenever both AUXON and 12ON are pulled low, the de-

vice is in reset mode and TIMER capacitor is discharged

to ground by an 8mA current sink.

Thermal Shutdown

The internal 3.3V auxiliary supply switch is protected by

thermal shutdown. If the switch’s temperature reaches

150°C, the aux switch will shut off immediately and ⎯F⎯A⎯U⎯L⎯T

will pull low. The external 12V supply switch also turns

off. The switches are allowed to turn on again by cycling

both the AUXON and 12ON pins low then high after the

internal switch’s temperature falls below 120°C.

Overcurrent Fault

The LTC4223 features an adjustable current limit with circuit

breaker function that protects the external MOSFET against

short circuits or excessive load current on 12V supply. The

voltage across the external sense resistor is monitored by

the analog current limit (ACL) ampliﬁer and the electronic

circuit breaker (ECB) comparator. If an overcurrent fault

occurs that causes the sense voltage to reach the ACL

threshold (60mV), the ACL ampliﬁer regulates the MOSFET

to prevent any further increase in current. This overcurrent

condition results in a sense voltage that exceeds the ECB

threshold. As a result, the TIMER capacitor is charged

by a 200μA current. If the condition persists, the TIMER

pin voltage will reach its threshold (1.235V). When this

occurs, the ⎯F⎯A⎯U⎯L⎯T pin pulls low and a 1mA current pulls

the GATE pin to ground causing the MOSFET to turn off.

The circuit breaker time delay, the time required for the

TIMER pin capacitor to charge from ground to the TIMER

pin threshold, is given by CT • 6[ms/μF].

After the MOSFET turns off, the TIMER pin capacitor dis-

charges with a 2μA pull-down current. For the auto-retry

version (LTC4223-2), if the TIMER discharges to below

0.2V, a new start-up cycle will begin. The TIMER starts

ramping up and clears faults when it exceeds 1.235V;

thereafter it ramps down (see the section on Auto-Retry

for details). Figure 5 shows an overcurrent fault on the

12V output.

In the event of a severe short-circuit fault on 12V output

as shown in Figure 6, the output current can surge to

tens of amperes. The LTC4223 responds within a very

short time to bring the current under control by pulling

the MOSFET’s GATE-to-SOURCE pin voltage down to zero

volts. Thereafter, the GATE of the MOSFET recovers rapidly

due to the RG/CG compensation network and enters into

active current limiting until the TIMER times out. Due to

parasitic supply lead inductance, an input supply without

any bypass capacitor will collapse during the high cur-

rent surge and then spike upwards when the current is

interrupted. An input supply transient protection network

comprising of Z1, R1 and C1 shown in Figure 13 is recom-

mended if there is no input capacitance.

Figure 5. Overcurrent Fault on 12V Output

0.1ms/DIV

12VGATE

5V/DIV

FAULT

5V/DIV

12VOUT

5V/DIV

422312 F05

ILOAD

5A/DIV

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