LTC4232 datasheet(13/18 Pages) AD

13 / 18 page

LT4200

Rev. 0

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APPLICATIONS INFORMATION

reduced running current while the maximum available

current limit is used at start-up.

Monitor MOSFET Temperature

The voltage at the ISET pin increases linearly with increas-

ing temperature. The temperature profile of the ISET pin is

shown in the Typical Performance Characteristics section.

Using a comparator or ADC to measure the ISET voltage

provides an accurate indication of the MOSFET tempera-

ture. The ISET voltage follows Equation 5.

VISET =

RSET

RSET +RISET

•(T

+ 273°C) • 2.093 [mV/°C] (5)

The MOSFET temperature is calculated using RISET of 20k

(Equation 6).

RSET + 20k

(

)• VISET

RSET • 2.093 [mV/°C]

– 273

°C

(6)

When RSET is not present, T is given by Equation 7.

VISET

2.093 [mV/°C]

– 273

°C

(7)

There is an overtemperature circuit in the LT4200 that

monitors an internal voltage similar to the ISET pin volt-

age. When the die temperature exceeds 155°C the cir-

cuit turns off the MOSFET until the temperature drops

to 135°C.

Monitor MOSFET Current

The current in the MOSFET passes through an internal

0.3mΩ sense resistor. The voltage on the sense resistor is

converted to a current that is sourced out of the IMON pin.

The gain of ISENSE amplifier is 2µA/A referenced from the

MOSFET current. 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 Figure 1 an external resis-

tive divider (driving the OV pin) connects to a comparator

to turn off the MOSFET when the VDD voltage exceeds

15.2V. If the VDD pin subsequently falls back below 14.9V,

the switch will be allowed to turn on immediately. In the

LT4200 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 Figure 1 application the MOSFET

turns off when VDD falls below 9.23V. If the VDD pin sub-

sequently rises above 9.88V for 48ms, the switch will

be allowed to turn on again. The LT4200 UV turn-on/off

threshold are 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 sta-

tus pin. When the overvoltage is removed, the MOSFET’s

gate ramps up immediately at the rate determined by the

INRUSH circuit.

Power Good Indication

In addition to setting the foldback current limit threshold,

the FB pin is used to determine a power good condition.

The Figure 1 application uses an external resistive divider

on the OUT pin to drive the FB pin. On the LT4200 the

PG comparator drives high when the FB pin rises above

1.235V and low when it falls below 1.215V.

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.

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