LTC4218CDHC-12#PBF Linear Technology, LTC4218CDHC-12#PBF Datasheet - Page 12

• Manufacturer Part Number: LTC4218CDHC-12#PBF
• Description: IC CTLR HOT SWAP 16-DFN
• Manufacturer: Linear Technology
• Type: Hot-Swap Controllerr

Specifications of LTC4218CDHC-12#PBF

• Applications: ATCA, MicroTCA™
• Internal Switch(s): No
• Voltage - Supply: 2.9 V ~ 26.5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-WFDFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
LTC4218
Monitor MOSFET Current
The current in the MOSFET passes through the sense
resistor. The voltage on the sense resistor is converted to
a current that is sourced out of the I
the I
sense resistor. This output current can be converted to a
voltage using an external resistor to drive a comparator
or ADC. The voltage compliance for the I
0V to INTV
A microcontroller with a built-in comparator can build a
simple integrating single-slope ADC by resetting a capaci-
tor that is charged with this current. When the capacitor
voltage trips the comparator and the capacitor is reset, a
timer is started. The time between resets will indicate the
MOSFET current.
Monitor OV and UV Faults
Protecting the load from an overvoltage condition is the
main function of the OV pin. In the LTC4218-12 an internal
resistive divider (driving the OV pin) connects to a compara-
tor to turn off the MOSFET when the V
15.05V. If the V
the switch will be allowed to turn on immediately. In the
LTC4218, the OV pin threshold is 1.23V when rising and
1.21V when falling out of overvoltage.
The UV pin functions as an undervoltage protection pin or
as an “on” pin. In the LTC4218-12 the MOSFET turns off
when V
rises above 9.88V for 100ms, the switch will be allowed
to turn on again. The LTC4218 UV turn on/off threshold
is 1.23V (rising) and 1.15V (falling).
In the case of an undervoltage or overvoltage, the MOSFET
turns off and there is indication on the PG status pin. When
the overvoltage is removed, the MOSFET’s gate ramps up
immediately.
Power Good Indication
In addition to setting the foldback current limit threshold,
the FB pin is used to determine a power good condition.
The LTC4218-12 uses an internal resistive divider on the
SOURCE pin to drive the FB pin. The PG comparator in-
dicates logic high when SOURCE pin rises above 10.5V. If
the SOURCE pin subsequently falls below 10.3V, the com-
12
SENSE
DD
amplifi er is 100μA from I
falls below 9.23V. If the V
CC
– 0.7V.
DD
pin subsequently falls back below 14.8V,
MON
MON
DD
DD
pin subsequently
for 15mV on the
voltage exceeds
pin. The gain of
MON
pin is from
parator toggles low. On the LTC4218, the PG comparator
drives high when the FB pin rises above 1.23V and low
when falls below 1.21V.
Once the PG comparator is high, the GATE pin voltage
is monitored with respect to the SOURCE pin. Once the
GATE minus SOURCE voltage exceeds 4.2V, the PG pin
goes high. This indicates to the system that it is safe to
load the Output while the MOSFET is completely turned
“on”. The PG pin goes low when the GATE is commanded
off (using the UV, OV or SENSE
the PG comparator drives low.
12V Fixed Version
In the LTC4218-12, the UV, OV and FB pins are driven by
internal dividers which may need to be fi ltered to prevent
false faults. By placing a bypass capacitor on these pins
the faults are delayed by the RC time constant. Use the R
value from the electrical table for this calculation.
In cases where the fi xed thresholds need a slight adjust-
ment, placing a resistor from the UV or OV pins to V
or GND will adjust the threshold up or down. Likewise,
placing a resistor between FB pin to OUT or GND adjusts
the threshold. Again, use the R
table for this calculation.
An example in Figure 4 raises the UV turn-on voltage from
9.88V to 10.5V. Increasing the UV level requires adding a
resistor between UV and ground. The resistor, (R
can be calculated using electrical table parameters as
follows:
R
SHUNT1
Figure 4. Adjusting LTC4218-12 Thresholds
=
(
V
R
NEW
( )
IN
LTC4218-12
• V
– V
OLD
OLD
4218 F04
)
=
IN
V
OV
UV
+
(
DD
value from the electrical
/SENSE
10.5 – 9.88
18k • 9.88
R
R
SHUNT2
SHUNT1
–
pins) or when
)
= 287k
SHUNT1
4218fe
DD
IN
),