MIC2596/2597 Micrel
MIC2596/2597 8 April 2001
Application Information
Thermal Shutdown and Power Dissipation
Thermal shutdown protection is employed to protect the
internal power MOSFETs from damage. Whenever the junc-
tion temperature T
J
of the channel in current limit exceeds
145°C the output is immediately shut off without affecting the
other channel. A channel will automatically turn on again
when its T
J
falls below 135°C. The junction temperature is
related to the internal power dissipation of the MIC2596
(MIC2597). The equation for junction temperature is:
T
J
= [(θ
JA
· P
D
) + T
A
] where:
T
J
is the junction temperature,
P
D
is the total power dissipation of the part, and
T
A
is the ambient temperature.
P
D
is determined by adding the power dissipated by each
MOSFET to the power dissipated by the internal circuitry
(P
CHIP
). The equation for P
D
is thus:
P
D
= P
CHIP
+ P
FET1
+ P
FET2
= (V
EE
x I
EE
) + [(I
1
2
) x R
DS(ON)1
] + [(I
2
2
) x R
DS(ON)2
]
where I
1
and I
2
are the continuous output currents of chan-
nels 1 and 2.
For example, to compute the maximum continuous output
current per channel of the TSSOP package at V
EE
= –48V, T
A
= 70°C, and T
J(CONTINUOUS)
= 125°C:
R
θ(J-A)
= 90°C/W
P
D(MAX)
= (125°C - 70°C)/(90°C/W) = 0.611W
0.611W = (–48V x –5mA) + (2 x I
MAX
2
x 2.5Ω)
0.371W = 2 x 2.5Ω x I
MAX
2
0.371W/(2 x 2.5Ω) = I
MAX
2
= 0.0742 A
2
I
MAX
= 272mA per channel
Similarly, for the TSE package, at T
A
= 85°C and
T
J(CONTINUOUS)
= 125°C:
R
θ(J-A)
= 38°C/W
P
D(MAX)
= (125°C - 85°C)/(38°C/W) = 1.05W
1.05 W = (–48V x –5mA) + (2 x I
MAX
2
x 2.5Ω)
0.81W = 2 x 2.5Ω x I
MAX
2
0.81W/(2 x 2.5Ω) = I
MAX
2
= 0.162 A
2
I
MAX
= 402mA per channel
Note that in each case the assumption has been made that
the load currents will be the same on both channels.
External Components
A small number of passive components are used for each
channel of the MIC2596/MIC2597 to program such values as
maximum DC output current and the short circuit “trip” inter-
val. Calculating values for these parts is a straightforward
exercise, once the nomenclature for and effect of each such
part is understood. This section addresses each program-
mable pin by showing a sample calculation for that pin.
R
LIMIT
A resistor from I
LIMIT
to V
EE
sets the maximum DC operating
current of the channel. The formula for calculating this
resistance is R
LIMIT(NOMINAL)
= (1A·2000Ω)/I
LIMIT
. As an
example, if the maximum DC current from one channel of an
MIC2596 was to be 0.15A, the nominal value of R
LIMIT
for that
channel would be (1A· 2000Ω)/0.15A = 13.3kΩ. It is usually
necessary, however, to allow for device tolerances: using a
13.3kΩ resistor and the minimum Data Sheet value Current
Limit Factor of (1A·1700Ω)/R
LIMIT
could restrict the part to
delivering only 127mA. Therefore, it is necessary to use
R
LIMIT
= (1A·1700Ω)/I
LIMIT
to find R
LIMIT
’s minimum value:
1700/0.15A = 11.3kΩ. This revised value should then be
tested against the other extreme of the IC’s Data Sheet
tolerance. 11.3kΩ could program a steady-state DC current
as high as (1A·2300Ω)/11.3kΩ = 203mA maximum. The
system must be designed to accommodate this maximum
current, or R
LIMIT
can be made adjustable over the range
necessary to maintain a precise 150mA DC current limit
(11.3kΩ - 15.3kΩ). In order to minimize error budget issues,
the use of a 1% tolerance resistor for R
LIMIT
is generally
recommended.
C
TIMER
A capacitor from C
TIMER
to V
EE
sets the length of time for
which an overcurrent fault is allowed to exist on a channel
before the channel goes into shutdown. C
TIMER
is normally
pulled down to V
EE
by a small current (1.9µA nominal).
During an overcurrent condition, the pulldown current is
replaced by a charging current of 72µA nominal. The output
will be disabled once the voltage on C
TIMER
becomes 1.32V
greater than V
EE
. Given these numbers, it’s easy to program
the time an MIC2597 will tolerate an output overload before
“tripping” and shutting its output off, using the formula C
TIMER
= (72µA·T
OL
/1.32V). For example, if it’s desired to allow
50msec for the load capacitance to charge up before the
MIC2597 declares a “fault,” then C
TIMER
= (72µA·50msec/
1.32V) = 2.7µF.
For the MIC2596, there is a slight modification to the above
formula, due to the MIC2596’s auto-retry feature. When an
overcurrent condition occurs, C
TIMER
will (as with the MIC2597)
charge at a 72µA rate towards 1.32V. Once that threshold is
reached, the output will be turned off. However, instead of
being latched off as with the MIC2597, it will turn on again
when the voltage across C
TIMER
is discharged back to 0.24V
by the 1.9µA internal pulldown. The first fault timeout period
following power-on will therefore be T
OL
= (C
TIMER
·1.32V/
72µA), but the following retry intervals will be of duration T
OL
= [C
TIMER
·(1.3V-0.24V)/72µA] = (C
TIMER
·1.06V/72µA). Re-
arranging, we get: C
TIMER
= (72µA·T
OL
/1.06V). Again using
50msec as an example for the desired fault timeout, this gives
C
TIMER
= (72µA·50msec/1.06V) = 3.4µF. In this case, 3.3µF
would be a good choice for C
TIMER
. The maximum voltage
to which C
TIMER
will charge is less than 2V, so a 4.7V voltage
rating on the capacitor provides ample safety margin.
Note that, for the MIC2596, the ratio of C
TIMER
charge and
discharge currents are always 38:1. This means that in an
overload fault condition, the part will attempt to restart the
load with a duty cycle of approximately 2.5%, which is low
enough to protect the IC and the system, yet high enough to
prevent undue restart delays.
