MIC2589 Micrel Semiconductor, MIC2589 Datasheet - Page 13

• Manufacturer Part Number: MIC2589
• Description: (MIC2589 / MIC2595) SINGLE-CHANNEL NEGATIVE HIGH VOLTAGE HOT SWAP POWER CONTROLLERS/SEQUENCERS
• Manufacturer: Micrel Semiconductor

To accommodate worst-case tolerances in the sense resistor
(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.
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:
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
been calculated, it is good practice to check the maximum hot
swap load current (I
pass in the case of tolerance build-up in the opposite direc-
tion. Here, the worst-case maximum is found using a
V
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
swap load current of 4A without nuisance trips, R
should be set to
standard value is 9.76m . At the other tolerance extremes,
I
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
I
(0.97)(R
maximum power dissipation is:
A 0.5
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,
such as an unscheduled or unexpected removal of PC boards
from the system or on-board fuse failure.
As long as the minimum current drawn by the load is at least
20% of the maximum output current (defined by
the output of the hot swap controllers will remain enabled. If
the output current falls below 12% of the maximum output
March 2004
MIC2589/2595
I
I
HOT_SWAP
HOT_SWAP
HOT_SWAP
HOT_SWAP
TRIP
(max) of 60mV and a sense resistor 3% low in value:
P
I
HOT_SWAP
MAX
sense resistor is a good choice in this application.
SENSE
(max) for the circuit in question is then simply:
(min)
(max)
(max) = 6.3A and the R is R
= (6.3A)
(nom)) = 9.47m . Thus, the sense resistor’s
(max)
1.03 R
HOT_SWAP
38.8mV
0.97 R
2
4A
(9.47m ) = 0.376W
61.9mV
9.76m
40mV
60mV
SENSE
SENSE
(max)) which the circuit may let
9.7m
(nom)
(nom)
6.3A
, and the nearest 1%
2
R
R
R. Here, The I is
SENSE
SENSE
38.8mV
SENSE
61.9mV
SENSE
R
(nom)
(nom)
V
SENSE
(min) =
TRIP
SENSE
has
),
13
current, the controller’s no-load detection loop is enabled. In
this loop, an internal current source, I
external capacitor C
no-load time-out delay is given by:
where V
an external capacitor connected from Pin 6 to V
voltage on C
the loop times out and the controller will shut down until it is
reset manually (MIC2589/95) or until it performs an auto-retry
operation (MIC2589R/95R).
Undervoltage/Overvoltage Detection (MIC2589 and
MIC2589R)
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
its V
levels are programmed using the resistor divider R1, R2, and
R3 as shown in the “Typical Application ” where the equations
to set the trip points are shown below. For the following
example, the circuit’s UV threshold is set to V
OV threshold is set at V
Central Office power distribution applications.
Given V
two resistor values can be determined. A suggested value for
R3 is that which will provide approximately 100 A of current
through the voltage divider chain at V
following as a starting point:
The closest standard 1% value for R3 = 12.4k . Solving for
R2 and R1 yields:
OVL
R3 =
t
V
V
NLD
UV
OV
UV
CNLD
threshold. The circuit’s UV and OV threshold voltage
, V
V
NLD
V
V
OVH
100 A
V
OV
= 1.24V (typ); I
UVL
OVH
CNLD
reaches its no-load threshold voltage, V
, and any one resistor value, the remaining
(typ)
(typ)
(typ)
NLD
I
C
CNLD
OV
UVH
1.223V
. An expression for the controller’s
100 A
NLD
R1 R2 R3
R1 R2 R3
OVH
= 72V, values commonly used in
R2 R3
threshold or the OV pin is below
CNLD
R3
threshold, the GATE pin is
12.23k
= 25 A (typ); and C
DD
CNLD
= V
UV
UV
, will charge an
UVL
. This yields the
= 37V and the
EE
M9999-031504
threshold or
. Once the
Micrel
NLD
CNLD
is
,