Micrel, Inc.
MIC2544A/2548A
Giving one a maximum ILIMIT variation over-temperature
of:
Setting ILIMIT
The MIC2544A/2548A’s current limit is user
programmable and controlled by a resistor connected
between the ILIMIT pin and ground. The value of this
resistor is determined by the following equation:
ILIMIT_MIN
ILIMIT_TYP
1.0A
ILIMIT_MAX
0.79A (-21%)
1.19A (+19%)
CurrentLimitFactor(CLF)
ILIMIT
=
RSET
or
I_OUT
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
R_SET
I_OUT_Min
I_OUT_Max
0.260
0.390
0.520
0.650
0.720
0.840
0.960
1.080
1.200
1.320
1.440
1.560
1.680
1.800
1150
767
575
460
383
329
288
256
230
209
192
177
164
153
0.140
0.210
0.280
0.350
0.480
0.560
0.640
0.720
0.800
0.880
0.960
1.040
1.120
1.200
CurrentLimitFactor(V)
ILIMIT (A)
RSET
=
For example: Set ILIMIT = 1.0A
Looking in the Electrical specifications we will find CLF
at ILIMIT = 1 A.
Min
Typ
Max
Units
184
230
276
V
1.1
1.2
1.3
1.4
1.5
Table 1. CLF at ILIMIT = 1A
For the sake of this example, we will say the typical
value of CLF at an IOUT of 1A is 230V. Applying the
equation above:
230V
RSET(Ω) =
= 230Ω
1.0A
Table 2. MIC2544A/2548A RSET Table
R
SET = 232Ω
(the closest standard 1% value)
Designers should be aware that variations in the
measured ILIMIT for a given RSET resistor, will occur
because of small differences between individual ICs
(inherent in silicon processing) resulting in a spread of
Short-Circuit Protection
In the event of a short circuit, the output current will fold
back to approximately 70% of the short-circuit current-
limit.
ILIMIT values. In the example above a typical value of CLF
was used to calculate RSET. One can determine ILIMIT’s
spread by using the minimum and maximum values of
Fault Flag
FLG is an N-channel, open-drain MOSFET output. The
fault flag is active (low) for current-limit or thermal
shutdown conditions. The flag output MOSFET is
capable of sinking a 10mA load to typically 100mV
above ground. The FLG response delay time tD is
CLF and the calculated value of RSET
.
184V
ILIMIT_MIN
=
= 0.79A
232Ω
276V
ILIMIT _ MAX
=
= 1.19A
232Ω
typically 5µs.
M9999-043010
April 2010
15
MICREL [ MICREL SEMICONDUCTOR ]
