LTC4211 datasheet(15/22 Pages) LINER

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LTC4235

4235f

For more information www.linear.com/LTC4235

applicaTions inForMaTion

During the undervoltage fault condition, FAULT will not

be pulled low but PWRGD will be pulled high as HGATE

is pulled low. The ideal diode function controlled by the

ideal diode MOSFET is not affected by the undervoltage

(UV) fault condition.

Power Good Monitor

Internal circuitry monitors the MOSFET gate overdrive

between the HGATE and OUT pins. Also, an internal resis-

tive divider that connects to OUT is used to determine a

power good condition. The power good comparator drives

high when the OUT pin rises above 10.5V, and drives low

when OUT falls below 10.33V. The power good status

for the input supply is reported via an open-drain output,

PWRGD. It is normally pulled high by an external pull-up

resistor or the internal 10µA pull-up.

The PWRGD pin pulls low when the OUT power good

comparatorishighandtheHGATEdriveexceeds4.2V.The

PWRGD pin goes high when the HGATE is turned off by the

ON or EN pins, or when the OUT power good comparator

drives low, or when INTVCC enters undervoltage lockout.

Current Sense Monitor

Thecurrentthroughtheexternalsenseresistorismonitored

by a LTC4235’s current sense amplifier at the SENSE+

and SENSE– pins (see Figure 5). The amplifier uses auto-

zeroing circuitry to achieve an offset below 150µV over

temperature, sense voltage and input supply voltage. The

frequency of the auto-zero clock is 10kHz. An internal

resistor RIN is connected between the amplifier’s negative

input terminal and SENSE+ pin. The sense amplifier loop

forcesthenegativeinputterminaltohavethesamepotential

as SENSE– and that develops a potential across RIN to be

the same as the sense voltage VSENSE. A corresponding

current, VSENSE/RIN, will flow through RIN. The high

impedance inputs of the sense amplifier will not conduct

this input current, allowing it to flow through an internal

MOSFET to a resistor ROUT connected between the IMON

and GND pins. The IMON output voltage is equal to (ROUT/

RIN) • VSENSE.TheresistorratioROUT/RINdefinesthevoltage

gain of the sense amplifier and is set to 100 with RIN =

200Ω and ROUT = 20k. Full scale input sense voltage to

the sense amplifier is 25mV, corresponding to an output

of 2.5V. The output clamps at 3.5V if the allowable input

sense voltage range is exceeded.

IMON Output Filtering

A capacitor connected in parallel with ROUT will give a

low pass response. This will reduce unwanted noise at

the output, and may also be useful as a charge reservoir

to keep the output steady while driving a switching circuit

such as an ADC (see Figure 5). This output capacitor

COUT in parallel with ROUT will create a pole in the output

response at:

fC =

2• π •ROUT •COUT

0.1µF

10µF

IMON VOUT

ILOAD

VCC

RIN

200

0.1µF

12V

VSENSE

ROUT

20k

2-WIRE I2C

INTERFACE

LTC4235

4235 F05

HGATE

LOAD

SENSE+

REF+

REF–

SENSE–

GND

REG

SCL

SDA

LTC2451

VOUT = ––––– • VSENSE = 100 • VSENSE

ROUT

RIN

Figure 5. High Side Current Monitor with LTC2451 ADC

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