LTC4210 LINER [Linear Technology], LTC4210 Datasheet - Page 13

• Manufacturer Part Number: LTC4210
• Description: Dual Supply Hot Swap Controller for Advanced Mezzanine Card
• Manufacturer: LINER [Linear Technology]

As the TIMER capacitor is used for fault fi ltering 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
fi rst 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
APPLICATIONS INFORMATION
Figure 5. Overcurrent Fault on 12V Output
12V
12V
5V/DIV
5A/DIV
5V/DIV
5V/DIV
FAULT
I
LOAD
GATE
OUT
0.1ms/DIV
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short circuits or excessive load current on 12V supply. The
voltage across the external sense resistor is monitored by
the analog current limit (ACL) amplifi er 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 amplifi er 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 C
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 R
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.
LTC4223-1/LTC4223-2
G
/C
G
compensation network and enters into
T
• 6[ms/μF].
13
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