MIC2589/2595
Micrel
Toaccommodateworst-casetolerancesinthesenseresistor
(for a ±1% initial tolerance, allow ±3% tolerance for variations
over time and temperature) and circuit breaker threshold
voltages, a slightly more detailed calculation must be used to
determine the minimum and maximum hot swap load cur-
rents.
current, the controller’s no-load detection loop is enabled. In
this loop, an internal current source, I
, will charge an
CNLD
external capacitor C
. An expression for the controller’s
NLD
no-load time-out delay is given by:
C
NLD
t
= V
×
NLD
CNLD
I
As the MIC2589/95’s minimum current-limit threshold volt-
age is 40mV, the minimum hot swap load current is deter-
mined where the sense resistor is 3% high:
CNLD
where V
= 1.24V (typ); I
= 25µA (typ); and C
is
CNLD
CNLD
NLD
an external capacitor connected from Pin 6 to V . Once the
EE
40mV
38.8mV
voltageonC
reachesitsno-loadthresholdvoltage,V
,
NLD
CNLD
IHOT_SWAP(min) =
=
the loop times out and the controller will shut down until it is
resetmanually(MIC2589/95)oruntilitperformsanauto-retry
operation (MIC2589R/95R).
1.03 ×RSENSE(nom) RSENSE(nom)
Keep in mind that the minimum hot swap load current should
be greater than the application circuit’s upper steady-state
load current boundary. Once the lower value of R
Undervoltage/Overvoltage Detection (MIC2589 and
MIC2589R)
has
SENSE
beencalculated, itisgoodpracticetocheckthemaximumhot
swaploadcurrent(I (max))whichthecircuitmaylet
The MIC2589 and the MIC2589R have “UV” and “OV” input
pins that can be used to detect input supply rail undervoltage
and overvoltage conditions. Undervoltage lockout prevents
energizing the load until the supply input is stable and within
tolerance. In a similar fashion, overvoltage turnoff prevents
damage to sensitive circuit components should the input
voltage exceed normal operational limits. Each of these pins
is internally connected to analog comparators with 20mV of
hysteresis. When the UV pin falls below its V
the OV pin is above its V
immediately pulled low. The GATE pin will be held low until
the UV pin is above its V threshold or the OV pin is below
HOT_SWAP
pass in the case of tolerance build-up in the opposite direc-
tion. Here, the worst-case maximum is found using a
V
(max) of 60mV and a sense resistor 3% low in value:
TRIP
60mV
0.97 ×R
61.9mV
R (nom)
SENSE
I
(max) =
=
HOT_SWAP
(nom)
SENSE
In this case, the application circuit must be sturdy enough to
operate over an ~1.6-to-1 range in hot swap load currents.
For example, if an MIC2595 circuit must pass a minimum hot
threshold or
UVL
threshold, the GATE pin is
OVH
swap load current of 4A without nuisance trips, R
SENSE
UVH
38.8mV
its V
threshold. The circuit’s UV and OV threshold voltage
OVL
= 9.7mΩ
should be set to
, and the nearest 1%
4A
standard value is 9.76mΩ. At the other tolerance extremes,
levels are programmed using the resistor divider R1, R2, and
R3asshowninthe“TypicalApplication”wheretheequations
to set the trip points are shown below. For the following
I
(max) for the circuit in question is then simply:
HOT_SWAP
example,thecircuit’sUVthresholdissettoV =37Vandthe
UV
61.9mV
I
(max) =
= 6.3A
OV threshold is set at V = 72V, values commonly used in
HOT_SWAP
OV
9.76mΩ
Central Office power distribution applications.
With a knowledge of the application circuit’s maximum hot
swap load current, the power dissipation rating of the sense
resistor can be determined using P = I × R. Here, The I is
R1+R2 +R3
(
)
V
= V
(typ)×
(typ)×
2
UV
UVL
R2 +R3
(
)
I
(max) = 6.3A and the R is R
(min) =
HOT_SWAP
SENSE
(0.97)(R
(nom)) = 9.47mΩ. Thus, the sense resistor’s
SENSE
R1+R2 +R3
(
)
maximum power dissipation is:
V
= V
OVH
OV
R3
2
P
= (6.3A) × (9.47mΩ) = 0.376W
MAX
Given V , V , and any one resistor value, the remaining
A 0.5Ω sense resistor is a good choice in this application.
UV
OV
two resistor values can be determined. A suggested value for
R3 is that which will provide approximately 100µA of current
throughthevoltagedividerchainatV =V .Thisyieldsthe
No-Load Detection
For those applications in which a minimum load current will
always be present, the no-load detect capability of the
MIC2589/89R/95/95R family offers system designer the abil-
ity to perform a shutdown operation on such fault conditions,
suchasanunscheduledorunexpectedremovalofPCboards
from the system or on-board fuse failure.
DD
UV
following as a starting point:
VOVH(typ)
1.223V
R3 =
=
= 12.23kΩ
100µA
100µA
The closest standard 1% value for R3 = 12.4kΩ. Solving for
R2 and R1 yields:
As long as the minimum current drawn by the load is at least
V
TRIP
20% of the maximum output current (defined by
),
R
SENSE
the output of the hot swap controllers will remain enabled. If
the output current falls below 12% of the maximum output
March 2004
13
M9999-031504
MICREL [ MICREL SEMICONDUCTOR ]
