LTC4234

13

4234fa

For more information www.linear.com/LTC4234

providesanaccurateindicationoftheMOSFETtemperature.

•(T

+273°C)•2.093[mV/°C]

T

=

(

– 273

°C

T

=

2.093[mV/°C]

– 273

°C

There is an overtemperature circuit in the LTC4234 that

When the die temperature exceeds 155°C the circuit turns

off the MOSFET until the temperature drops to 135°C.

Monitor MOSFET Current

The current in the MOSFET passes through an internal

0.7mΩ sense resistor. The voltage on the sense resistor is

MOSFET current. This output current can be converted to

a voltage using an external resistor to drive a comparator

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

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

LTC4234 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 MOS-

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

be allowed to turn on again. The LTC4234 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 status 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 LTC4234 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

drives low.

Design Example

Consider the following design example (Figure 5):

= 10.5V. A current limit fault triggers an automatic restart

of the power-up sequence.

APPLICATIONS INFORMATION