LTC4217 Linear Technology, LTC4217 Datasheet - Page 12

• Manufacturer Part Number: LTC4217
• Description: 2A Integrated Hot Swap Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC4217
a resistor between the I
the Functional Diagram the voltage at the I
the clamp circuit) sets the CS amplifi er’s built-in offset
voltage. This offset voltage directly determines the active
current limit value. With the I
the I
effi cient reference. This voltage is set to 0.618V at room
temperature which corresponds to a 2A current limit at
room temperature.
An external resistor placed between the I
forms a resistive divider with the internal 20k sourcing
resistor. The divider acts to lower the voltage at the I
pin and therefore lower the current limit threshold. The
overall current limit threshold precision is reduced to ±11%
when using a 20k resistor to half the threshold.
Using a switch (connected to ground) in series with this
external resistor allows the active current limit to change
only when the switch is closed. This feature can be used
when the startup current exceeds the typical maximum
load current.
Monitor MOSFET Temperature
The voltage at the I
ing temperature. The temperature profi le of the I
is shown in Typical Performance Characteristics. Using a
comparator or ADC to measure the I
an indicator of the MOSFET temperature.
There is an over-temp circuit in the LTC4217 that monitors
an internal voltage similar to the I
the die temperature exceeds 145°C the circuit turns off
the MOSFET until the temperature drops to 125°C.
Monitor MOSFET Current
The current in the MOSFET passes through a sense resistor.
The voltage on the sense resistor is converted to a current
that is sourced out of the I
amplifi er is 50μA from I
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
A microcontroller with a built-in comparator can build a
simple integrating single-slope ADC by resetting a capaci-
12
SET
pin is determined by a positive temperature co-
SET
MON
pin increases linearly with increas-
SET
MON
pin is from 0V to INTV
pin and ground. As shown in
MON
SET
for 1A of MOSFET current.
pin open, the voltage at
pin. The gain of I
SET
SET
pin voltage. When
SET
voltage provides
pin and ground
SET
CC
pin (via
SET
– 0.7V.
SENSE
SET
pin
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 LTC4217-12 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
LTC4217 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 LTC4217-12 the MOSFET turns off
when V
rises above 9.88V for 100ms, the switch will be allowed
to turn on again. The LTC4217 UV turn on/off threshold
is 1.23V (rising) and 1.15V (falling).
In the cases 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 at the rate determined by the INRUSH
block.
Power Good Indication
In addition to setting the foldback current limit threshold,
the FB pin is used to determine a power good condition.
The LTC4217-12 uses an internal resistive divider on the
OUT pin to drive the FB pin. The PG comparator indicates
logic high when OUT pin rises above 10.5V. If the OUT pin
subsequently falls below 10.3V the comparator toggles low.
On the LTC4217 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 OUT pin. Once the GATE
minus OUT voltage exceeds 4.2V the PG pin goes high.
This indicates to the system that it is safe to load the OUT
pin while the MOSFET is completely turned “on”. The PG
pin goes low when the GATE is commanded off (using
the UV, OV or SENSE pins) or when the PG comparator
drives low.
DD
falls below 9.23V. If the V
DD
pin subsequently falls back below 14.8V,
DD
DD
pin subsequently
voltage exceeds
4217fb