LTC4218IGN-PBF データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LTC4218IGN-PBF

Hot Swap Controller

Linear Technology 

LTC4218

Applications Information

to ±11% when using a 20k resistor to half the threshold.

This pin’s 20k sourcing impedance allows noise to couple

to this pin and disturb the current limit threshold. Place

a 0.1µF capacitor between the ISET pin and ground when

a board trace is connected to this pin.

Using a switch (connected to ground) in series with RSET

allows the active current limit to change only when the

switch is closed. This feature can be used to program a

reduced running current while the maximum current limit

is used at start-up.

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 1.235V when rising and

1.215V 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 1.235V (rising) and 1.155V (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-

parator toggles low. On the LTC4218, the PG comparator

drives high when the FB pin rises above 1.23V and low

when falls below 1.215V.

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),

12

For more information www.linear.com/LTC4218

4218fh

Share Link: