MAX5978ETJ+【PDF 17/38 页】IC CTLR HOT-SWAP 32TQFN

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0 to 16V, Hot-Swap Controller with 10-Bit

Current, Voltage Monitor, and 4 LED Drivers

Setting Circuit-Breaker Thresholds

To select and set the device slow-trip and fast-trip com-

parator thresholds, use the following procedure:

1) Select one of four ratios between the fast-trip thresh-

old and the slow-trip threshold: 200%, 175%, 150%,

or 125%. A system that experiences brief but large

transient load currents should use a higher ratio,

whereas a system that operates continuously at high-

er average load currents might benefit from a smaller

ratio to ensure adequate protection. The ratio is set by

writing to the ifast2slow register. (The default setting

on power-up is 200%.)

2) Determine the slow-trip threshold V

TH,ST

based on the

anticipated maximum continuous load current during

normal operation, and the value of the current-sense

resistor. The slow-trip threshold should include some

margin (possibly 20%) above the maximum load cur-

rent to prevent spurious circuit-breaker shutdown and

to accommodate passive component tolerances:

TH,ST

= R

SENSE

x I

LOAD,MAX

x 120%

3) Calculate the necessary fast-trip threshold V

TH,FT

based on the ratio set in step 1:

TH,FT

= V

TH,ST

x (ifast2slow ratio)

4) Select one of the three maximum current-sense

ranges: 25mV, 50mV, or 100mV. The current-sense

range is initially set upon power-up by the state of the

IRNG input, but can be altered at any time by writ-

ing to the status2 register. For maximum accuracy

and best measurement resolution, select the lowest

current-sense range that is larger than the V

TH,FT

value calculated in step 3.

5) Program the fast-trip and slow-trip thresholds by writ-

ing an 8-bit value to the dac_fast register. This 8-bit

value is determined from the desired V

TH,ST

value

that was calculated in step 2, the threshold ratio from

step 1, and the current-sense range from step 4:

DAC = V

TH,ST

x 255 x (ifast2slow ratio)/

(IRNG current-sense range)

The device provides a great deal of system flexibil-

ity because the current-sense range, DAC setting, and

threshold ratio can be changed on the fly for systems

that must protect a wide range of interchangeable

load devices, or for systems that control the allocation

of power to smart loads. Table 5 shows the specified

ranges for the fast-trip and slow-trip thresholds for all

combinations of current-sense range and threshold ratio.

When an overcurrent event causes the device to shut

down the power channel, the open-drain FAULT output

alerts the system. Figure 2 shows the operation and fault-

management flowchart.

Table 4a. ifast2slow Register Format

Table 4b. Setting Fast-Trip to Slow-Trip Threshold Ratio

Description:

Current threshold fast-to-slow setting bits

Resister Title:

ifast2slow

0x30

RESET

VALUE

R/W

Unused

FS1

FS0

0x0F

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

FS1

FS0

FAST-TRIP TO SLOW-TRIP RATIO (%)

125

150

175

200

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