Rainbow Electronics MAX5961 User Manual

Page 20

MAX5961

0 to 16V, Quad, Hot-Swap Controller
with 10-Bit Current and Voltage Monitor

______________________________________________________________________________________

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 over-
drive beyond the slow-trip threshold. If the overdrive is
small and short-lived, the comparator will not shut down
the affected channel. As the overcurrent event increas-
es in magnitude, the response time of the slow-trip com-
parator 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 (see Figure 2).

Setting Circuit-Breaker Thresholds

To select and set the MAX5961 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 cur-
rent-sense resistor. The slow-trip threshold should
include some margin (possibly 20%) above the max-
imum load current to prevent spurious circuit-break-
er 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 associated ILIM_ 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

writing an 8-bit value to the

dac_chx

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)/(ILIM_ current

sense range)

The MAX5961 provides a great deal of system flexibility
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 6 shows the specified
ranges for the fast-trip and slow-trip thresholds for all
combinations of current-sense range and threshold
ratio. The fast-trip DAC can be programmed to values
below 0x66 (40% of the current-sense range), but
accuracy is not specified for operation below 40%.

MAX5961 fig02

[(V

SENSE_

- V

MON_

) - V

TH,ST

] (V)

TURN-OFF TIME (ms)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

25mV SENSE RANGE;

DAC = 191, V

TH,ST

= 9.36mV

Figure 2. Slow-Comparator Turn-Off Time vs. Overdrive