MIC2595-2YM Micrel Inc, MIC2595-2YM Datasheet - Page 17

• Manufacturer Part Number: MIC2595-2YM
• Description: Negative Voltage Hot-Swap Controller -
• Manufacturer: Micrel Inc
• Type: Hot-Swap Controllerr

Specifications of MIC2595-2YM

• Applications: General Purpose
• Internal Switch(s): No
• Voltage - Supply: -19 V ~ -80 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

December 2005
Micrel
During startup, the CFILTER pin will begin to charge
once the GATE crosses 2.5V. In order to avoid false-
tripping of the circuit breaker by allowing the
overcurrent filter to time out, the overcurrent delay
must be set to exceed the time it takes to ramp the
GATE output above 5.5V (i.e., charge the output load
capacitance).
An initial value for C
time it will take for the MIC2589/MIC2595 to
completely charge up the output capacitive load.
Assuming the load is enabled by the PWRGDX (or
/PWRGDX) signal(s) of the controller, the turn-on
delay time is derived from I = C × (dv/dt):
Using
MIC2589/MIC2595, an expression relating C
the circuit’s turn-on delay time is:
Substituting the variables above with the specification
limits of the MIC2589/MIC2595, an expression for the
worst-case value for C
For example, in a system with a C
maximum (V
current on a nominal –48V buss of 2.5A, the nominal
circuit design equations steps are:
1. Choose I
2. Select an
3. Using I
C
C
C
t
TURN
FILTER
FILTER(max
FILTER(max
1% standard value is 13.0mΩ);
turn-on time is calculated:
−
ON
current limit is defined as the overcurrent fault
timer, t
equation.
The value of C
allow the circuit’s minimum regulated value of
I
the time it takes to charge the total load
capacitance.
t
OUT
FLT
=
parametric
=
CHARGE
(
t
)
)
to equal I
TURN
=
C
DD
LIMIT
=
=
LOAD
FLT
C
R
t
t
– V
FILTER
TURN
TURN
SENSE
= I
, and is determined by the following
V
I
−
= I
CFILTER(CH
ON
CFILTER
EE
HOT_SWAP
×
I
FILTER
LIMIT
LIMIT
−
−
) = 72V, and a maximum load
(
×
FILTER
ON
ON
LIMIT
V
×
values
I
=
DD
FILTER
CFILTER
V
= 3A, the application circuit
×
×
38.8mV
CFILTER(tr
is found by calculating the
for somewhat longer than
−
⎛
⎜
⎝
⎛
⎜
⎝
is given by:
(nom) = 3A (2.5A + 20%);
115
3A
135µA
ARGE)
1.17V
V
should be selected to
EE
)
×
)
specific
10
=
ip)
⎞
⎟
⎠
LOAD
12
−
6
9 .
sec
µF
m
= 1500µF, a
Ω
⎞
⎟
⎠
to
FILTER
(closest
the
to
17
Allowing for capacitor tolerances and a nominal 40ms
turn-on time, an initial worst-case value for C
C
The closest standard ±10% tolerance capacitor value
is 4.7µF and would be a good initial starting value for
prototyping.
Whenever the hot swap controller is not in current
limit, CFILTER is discharged to VEE by an internal
4µA current source.
For the MIC2589R/MIC2595R devices, the circuit
breaker automatically resets after approximately 25
t
condition still exists, capacitor C
charge up to V
Capacitor C
internal 4µA current source until the voltage across
C
another start cycle is initiated. This will continue until
the fault condition is removed or input power is
removed/cycled. The duty cycle of the auto-restart
function is therefore fixed at 4.25% and the period of
the auto-restart cycle is given by:
The auto-restart period for the example above where
the worst-case C
t
No-Load Detection
For applications in which a minimum load current will
always be present, the no-load detect capability of the
MIC2589 product family offers system designers the
ability 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 current limit (defined by
remain enabled. If the output current falls below 20%
of the actual current limit, the controller’s no-load
detection loop is enabled. In this loop, an internal
current source, I
C
out delay is given by:
t
t
t
R
FLT
AUTO-RESTART
RETRY
RETRY
TURN
FILTER(WORST-CASE)
FILTER
V
NLD
SENSE
TRIP
. An expression for the controller’s no-load time-
time constants (23.75 × t
−
ON
goes below V
=
=
), the output of the hot swap controller will
t
t
=
FLT
FLT
= 1.27s
FILTER
(
1500µF
CFILTER(TRIP),
+
+
CNLD
FILTER
t
[
C
FLT_AUTO
= 40ms×(115×10
3A
FILTER
will then be discharged by an
, will charge an external capacitor
×
was determined to be 4.7µF is:
72V
CFILTER(RETRY)
×
tripping the circuit breaker.
)
(
V
=
I
CFILTER(TR
CFILTER(pu
3
6
FLT_AUTO
ms
MIC2589/MIC2595
–6
, at which time
µF/sec) = 4.6µF
FILTER
(use 40ms)
). If the fault
[phone]
M9999-120505
ll
IP)
−
down
will again
-
FILTER
V
CFILTER
)
is:
(
retry
)
)
]