MIC2596A/2697A Micrel
MIC2596A/2697A 8 December 2000
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 TJ 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 TJ falls below 135°C. The junction temperature is
related to the internal power dissipation of the MIC2596A
(MIC2597A). The equation for junction temperature is:
TJ = [(θJA · PD) + TA] where:
TJ is the junction temperature,
PD is the total power dissipation of the part, and
TA is the ambient temperature.
PD is determined by adding the power dissipated by each
MOSFET to the power dissipated by the internal circuitry
(PCHIP). The equation for PD is thus:
PD = PCHIP + PFET1 + PFET2
= (VEE x IEE) + [(I12) x RDS(ON)1] + [(I22) x RDS(ON)2]
where I1 and I2 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 VEE = –48V, TA
= 70°C, and TJ(CONTINUOUS) = 125°C:
Rθ(J-A) = 90°C/W
PD(MAX) = (125°C - 70°C)/(90°C/W) = 0.611W
0.611W = (–48V x –5mA) + (2 x IMAX2 x 2.5Ω)
0.371W = 2 x 2.5Ω x IMAX2
0.371W/(2 x 2.5Ω) = IMAX2 = 0.0742 A2
IMAX = 272mA per channel
Similarly, for the TSE package, at TA = 85°C and
TJ(CONTINUOUS) = 125°C:
Rθ(J-A) = 38°C/W
PD(MAX) = (125°C - 85°C)/(38°C/W) = 1.05W
1.05 W = (–48V x –5mA) + (2 x IMAX2 x 2.5Ω)
0.81W = 2 x 2.5Ω x IMAX2
0.81W/(2 x 2.5Ω) = IMAX2 = 0.162 A2
IMAX = 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 MIC2596A/MIC2597A to program such values
as maximum DC output current and the short circuit “trip”
interval. Calculating values for these parts is a straightfor-
ward exercise, once the nomenclature for and effect of each
such part is understood. This section addresses each
programmable pin by showing a sample calculation for that
pin.
RLIMIT
A resistor from ILIMIT to VEE sets the maximum DC operating
current of the channel. The formula for calculating this
resistance is RLIMIT(NOMINAL) = (1A·2000Ω)/ILIMIT. As an
example, if the maximum DC current from one channel of an
MIC2596A was to be 0.15A, the nominal value of RLIMIT 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)/RLIMIT could restrict the
part to delivering only 127mA. Therefore, it is necessary to
use RLIMIT = (1A·1700Ω)/ILIMIT to find RLIMIT’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 RLIMIT 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 RLIMIT is generally
recommended.
CTIMER
A capacitor from CTIMER to VEE sets the length of time for
which an overcurrent fault is allowed to exist on a channel
before the channel goes into shutdown. CTIMER is normally
pulled down to VEE 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 CTIMER becomes 1.32V
greater than VEE. Given these numbers, it’s easy to program
the time an MIC2597A will tolerate an output overload before
“tripping” and shutting its output off, using the formula CTIMER
= (72µA·TOL/1.32V). For example, if it’s desired to allow
50msec for the load capacitance to charge up before the
MIC2597A declares a “fault,” then CTIMER = (72µA·50msec/
1.32V) = 2.7µF.
For the MIC2596A, there is a slight modification to the above
formula, due to the MIC2596A’s auto-retry feature. When an
overcurrent condition occurs, CTIMER will (as with the
MIC2597A) 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 MIC2597A, it will turn
on again when the voltage across CTIMER is discharged back
to 0.24V by the 1.9µA internal pulldown. The first fault
timeout period following power-on will therefore be TOL =
(CTIMER·1.32V/72µA), but the following retry intervals will be
of duration TOL = [CTIMER·(1.3V-0.24V)/72µA] =
(CTIMER·1.06V/72µA). Rearranging, we get: CTIMER =
(72µA·TOL /1.06V). Again using 50msec as an example for
the desired fault timeout, this gives CTIMER =
(72µA·50msec/1.06V) = 3.4µF. In this case, 3.3µF would be
a good choice for CTIMER. The maximum voltage to which
CTIMER will charge is less than 2V, so a 4.7V voltage rating on
the capacitor provides ample safety margin.
Note that, for the MIC2596A, the ratio of CTIMER 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