LTC4236 datasheet(17/24 Pages) LINER

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LTC4236

4236f

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applicaTions inForMaTion

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

REG Pin Bypassing

The LTC4236 has an internally regulated supply near

SENSE+ for internal bias of the current sense amplifier. It

is not intended for use as a supply or bias pin for external

circuitry. A 0.1µF capacitor should be connected between

theREGandSENSE+pins.Thiscapacitorshouldbelocated

very near to the device and close to the REG pin for the

best performance.

REG and IMON Start-Up

The start-up current of the current sense amplifier when

the LTC4236 is powered on consists of two parts: the

first is the current necessary to charge the REG bypass

capacitor,whichisnominally0.1µF.SincetheREGvoltage

charges to approximately 4.1V below the SENSE+ voltage,

this can require a significant amount of start-up current.

The second source is the output current that flows into

ROUT, which upon start-up may temporarily drive the

IMON output high for less than 2ms. This is a temporary

conditionwhichwillceasewhenthesenseamplifiersettles

into normal closed-loop operation.

CPO and DGATE Start-Up

The CPO pin voltage is initially pulled up to a diode below

theIN1orD2SRCpinwhenfirstpoweredup(seeFigure 1).

However, for application with back-to-back MOSFETs in

IN2 power path, CPO2 starts off at 0V since D2SRC is

near ground (see Figure 8). CPO starts ramping up 7µs

after INTVCC clears its undervoltage lockout level. Another

40µs later, DGATE also starts ramping up with CPO. The

CPO ramp rate is determined by the CPO pull-up current

into the combined CPO and DGATE pin capacitances. An

internal clamp limits the CPO pin voltage to 12V above

the IN1 or D2SRC pin, while the final DGATE pin voltage

is determined by the gate drive amplifier. An internal 12V

clamp limits the DGATE1 and DGATE2 pin voltages above

IN1 and D2SRC respectively.

CPO Capacitor Selection

TherecommendedvalueofthecapacitorbetweentheCPO1

and IN1, and CP02 and D2SRC pins is approximately 10×

the input capacitance CISS of the ideal diode MOSFET. A

larger capacitor takes a correspondingly longer time to

chargeupbytheinternalchargepump.Asmallercapacitor

suffers more voltage drop during a fast gate turn-on event

as it shares charge with the MOSFET gate capacitance.

MOSFET Selection

The LTC4236 drives N-channel MOSFETs to conduct the

load current. The important features of the MOSFETs are

on-resistanceRDS(ON),themaximumdrain-sourcevoltage

BVDSS and the threshold voltage.

The gate drive for the ideal diode and Hot Swap MOSFET

is guaranteed to be greater than 5V when the supply

voltages at IN1 and IN2 are between 2.9V and 7V. When

the supply voltages at IN1 and IN2 are greater than 7V,

the gate drive is guaranteed to be greater than 10V. The

gate drive is limited to 14V. An external Zener diode can

be used to clamp the potential from the MOSFET’s gate

to source if the rated breakdown voltage is less than 14V.

The maximum allowable drain-source voltage BVDSS

must be higher than the supply voltage including supply

transients as the full supply voltage can appear across the

MOSFET. If an input or output is connected to ground,

the full supply voltage will appear across the MOSFET.

The RDS(ON) should be small enough to conduct the

maximumloadcurrent,and alsostaywithintheMOSFET’s

power rating.

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