LTC4261 datasheet(42/82 Pages) AD

42 / 82 page

LTC4284

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Sense resistors for each channel are selected assuming

they will carry the maximum channel current, or 33.3A

in this example. Selection is a matter of total cost, sense

voltage (configurable from 15mV to 30mV in 1mV steps),

allowable dissipation, availability of discrete resistance

values, using multiple devices to reduce the sensing

errors associated with high current density at the interface

between the PCB and resistor, and using multiple devices

to ballast current flow across a wide path, between 2 or

more connectors, or between 2 or more MOSFETs. These

factors are iterated until an acceptable solution is found.

First, determine the number of resistors needed to handle

the total sense power of each channel. Compute the total

sense power starting with the minimum sense voltage

or 15mV:

PS(CH) = ∆VSENSE(MIN) • ICH(MAX) =

15mV • 33.3A = 500mW

Second, compute the number of resistors needed to

handle this power. For example, 1206 resistors are rated

for 250mW dissipation. A conservative design is half as

much, or 125mW.

NRS(CH) =

PS(CH)

125mW

500mW

125mW

= 4

Thus at least four parallel 1206 resistors are needed for

each channel. Third, compute the resistance value:

RS(CH)=

VILIM(MIN)

ICH(MAX)

= 15mV

33.3A

=450µΩ

Four resistors of 1.8mΩ each would give the correct

sense resistance. Fourth, use the closest next-larger avail-

able sense resistor value and adjust the sense voltage as

needed to restore the current. In this case, a 2mΩ sense

resistor value is selected and the sense voltage is adjusted

to 16mV. Recompute the numbers:

RS(CH) =

2mΩ

= 500µΩ

ICH(MAX) =

16mV

500µΩ

= 32A

PS(CH) = 16mV • 32A = 512mW

The power dissipation of each resistor package is now

512mW/4 = 128mW. The total current limit is now 32A •

2 = 64A, close enough to the optimum value of 66.7A. The

above process might be iterated for several combinations

of different resistor counts, different package sizes, and

even combinations of mixed resistor values.

When a specific design is actually built, there can be small

inaccuracies in the current sensing owing to contact and

copper trace resistances. An immediate remedy without

changing sense resistors is to readjust the sense voltage

in 1mV steps. For instance, moving sense voltage from

16mV to 17mV gives a 6.25% increase in current.

Step 2. Select resistive dividers for DRNS (drain sense),

RTNS (RTN sense) and VOUTTH (output low threshold).

DRNS and RTNS serve multiple purposes. First, they are

the inputs to a differential amplifier that measures the

attenuated load voltage for dV/dt control at startup (see

Inrush Control). In the event of an output overload or

short-circuit, the current limit foldback profile in normal

operation depends upon the differential input between

RTNS and DRNS that represents the output voltage across

the load. The current limit starts to fold back when RTNS –

DRNS drops below 0.9V and reaches the minimum when

RTNS – DRNS drops to zero (see Current Limit Foldback).

Additionally, in current limit the DRNS input monitors the

MOSFET’s VDS and uses this information to scale the TMR

pull-up current accordingly. When not in current limit,

DRNS monitors VDS and serves as one input to a mul-

tiplier which generates the TMR pull-up current. Finally,

RTNS and DRNS also serve as inputs to the ADCs so that

the input voltage and MOSFET drain voltage can be read

remotely. RTNS and DRNS have a maximum useable input

voltage of 2.8V, so resistive dividers are required.

To select resistive dividers for RTNS and DRNS, compute

the divider ratio r using the maximum supply voltage:

VS(MAX)

1.8V

72V

1.8V

= 40

where 1.8V is the operating point of DRNS at which the

TMR pull-up current is tested and specified. The resulting

LTC4261 Datasheet(PDF) 42 Page - Analog Devices

LTC4261 Datasheet(HTML) 42 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82

Datasheet Download

Link URL