LTC4218CGN-TRPBF(RevB) Просмотр технического описания (PDF) - Linear Technology
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LTC4218CGN-TRPBF Datasheet PDF : 16 Pages
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LTC4218
APPLICATIONS INFORMATION
Using a switch (connected to ground) in series with the
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 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 IMON pin. The gain of
the ISENSE amplifier is 100μA from IMON for 15mV on the
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 IMON pin is from
0V to INTVCC – 0.7V.
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 VDD voltage exceeds
15.05V. If the VDD pin subsequently falls back below 14.8V,
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 VDD falls below 9.23V. If the VDD pin subsequently
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.
Powergood Indication
In addition to setting the foldback current limit threshold,
the FB pin is used to determine a powergood condition.
The LTC4218-12 uses an internal resistive divider on
the SOURCE pin to drive the FB pin. The PG comparator
indicates logic high when SOURCE pin rises above 10.5V.
If the SOURCE pin subsequently falls below 10.3V, the
comparator toggles low. On the LTC4218 the PG compara-
tor 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+/SENSE– pins) or when
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 filtered to prevent
false faults. By placing a bypass capacitor on these pins
the faults are delayed by the RC time constant. Use the RIN
value from the electrical table for this calculation.
In cases where the fixed thresholds need a slight adjust-
ment, placing a resistor from the UV or OV pins to VDD
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 RIN value from the electrical
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, (RSHUNT1),
can be calculated using electrical table parameters as
follows:
( ) RSHUNT1 =
R(IN) • VOLD
VNEW – VOLD
=
18k
(10.5
• 9.88
– 9.88)
=
287k
4218fb
12
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