LTC4211 datasheet(16/22 Pages) LINER

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LTC4235

4235f

For more information www.linear.com/LTC4235

applicaTions inForMaTion

REG Pin Bypassing

The LTC4235 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 LTC4235 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 and DGATE pin voltages are initially pulled up

to a diode below the IN pin when first powered up. CPO

starts ramping up 7µs after INTVCC clears its undervolt-

age 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 IN pin, while the final DGATE

pin voltage is determined by the gate drive amplifier. An

internal 12V clamp limits the DGATE pin voltage above IN.

CPO Capacitor Selection

The recommended value of the capacitor between the CPO

and IN pins is approximately 10× the input capacitance

CISS of the ideal diode MOSFET. A larger capacitor takes

a correspondingly longer time to charge up by the internal

charge pump. A smaller capacitor suffers more voltage

drop during a fast gate turn-on event as it shares charge

with the MOSFET gate capacitance.

MOSFET Selection

The LTC4235 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 10V and 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 maximum

load current, and also stay within the MOSFET’s power

rating.

Supply Transient Protection

When the capacitances at the input and output are very

small, rapid changes in current during input or output

short-circuit events can cause transients that exceed the

24V absolute maximum ratings of the IN and OUT pins.

To minimize such spikes, use wider traces or heavier

trace plating to reduce the power trace inductance. Also,

bypass locally with a 10µF electrolytic and 0.1µF ceramic,

or alternatively clamp the input with a transient voltage

suppressor (Z1, Z2). A 100Ω, 0.1µF snubber damps the

response and eliminates ringing (See Figure 10).

Design Example

As a design example for selecting components, consider a

12V system with a 7A maximum load current for the two

supplies (see Figure 1).

First, select the appropriate value of the current sense

resistor RSforthe12Vsupply.Calculatethesenseresistor

value based on the maximum load current ILOAD(MAX) and

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