Rainbow Electronics MAX5970 User Manual

Page 17

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

Current and Voltage Monitor and 4 LED Drivers

MAX5970

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from 40% to 100% of the selected full-scale current-
sense range. The slow-trip threshold follows the fast-trip
threshold as one of four programmable ratios, set by the
ifast2slow register (see Tables 5a and 5b).
The fast-trip threshold is always higher than the slow-trip
threshold, and the fast-trip comparator responds very
quickly to protect the system against sudden, severe
overcurrent events. The slower response of the slow-
trip comparator varies depending upon the amount of
overdrive beyond the slow-trip threshold. If the overdrive
is small and short-lived, the comparator does not shut
down the affected channel. As the overcurrent event
increases in magnitude, the response time of the slow-
trip comparator decreases. This scheme provides good
rejection of noise and spurious overcurrent transients
near the slow-trip threshold while aggressively protect-
ing the system against larger overcurrent events that
occur as a result of a load fault.

Setting Circuit-Breaker Thresholds

To select and set the MAX5970 slow-trip and fast-trip
comparator 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
higher 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
current 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 four maximum current-sense

ranges: 25mV, 50mV, or 100mV. The current-sense
range is initially set upon power-up by the state of
the associated IRNG_ input, but can be altered at any
time by writing 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_chx 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)

Table 5a. ifast2slow Register Format

Table 5b. Setting Fast-Trip to Slow-Trip Threshold Ratio

Description:

Current threshold fast to slow setting bits

ifast2slow

0x30

R/W

RESET
VALUE

—

Ch2_FS1

Ch2_FS0

Ch1_FS1

Ch1_FS0

0x0F

bit 7

bit 6

bit 5

bit 4

bit 3

bit 2

bit 1

bit 0

Chx_FS1

Chx_FS0

FAST-TRIP TO SLOW-TRIP RATIO (%)

125

150

175

200