MIC2042/2043
Micrel
Programmable Current Limit
The MIC2042/43 is designed to prevent damage to the
external load by limiting the maximum amount of current it
can draw. The current limit is programmed by an external
resistor (R
SET
) connected from ILIM to ground and becomes
active when the output voltage is at least 200mV below the
voltage at the input to the device. The limiting current value
is defined by the current limit factor (CLF) divided by R
SET
,
and the MIC2042/43 will limit from 0.5A to 3A with a set point
accuracy of
±22%.
In programming the nominal current limit,
the value of R
SET
is determined using the following equation:
R
SET
=
CLF
I
LIMIT
=
Functional Description
The MIC2042 and MIC2043 are high-side N-Channel switches
equipped with programmable current limit up to 3A for use in
general purpose power distribution applications. The switches,
available with active-high or active-low enable inputs, provide
output slew-rate control and circuit protection via thermal
shutdown and an optional output latch during overcurrent
conditions.
Input and Output
V
BIAS
supplies power to the internal circuitry of the switch and
must be present for the switch to operate. V
IN
is connected to
the drain of the output MOSFET and sources power to the
switched load. V
IN
must be less than or equal to V
BIAS
. V
OUT
is the source terminal of the output MOSFET and attaches to
the load. In a typical circuit, current flows from V
IN
to V
OUT
toward the load. If V
OUT
is greater than V
IN
, current will flow
from V
OUT
to V
IN
since the switch is bi-directional when the
device is enabled. When disabled (OFF), the switch will block
current flow from either direction.
Enable Input
Enable, the ON/OFF control for the output switch, is a digital
input available as an active-high (–1) or active-low (–2)
signal. The EN pin, referenced to approximately 0.5
×
VBIAS,
must be driven to a clearly defined logic high or logic low.
Failure to observe this requirement, or allowing EN to float,
will cause the MIC2042/43 to exhibit unpredictable behavior.
EN should not be allowed to go negative with respect to
ground, nor allowed to exceed VBIAS. Failure to adhere to
these conditions may result in damage to the device.
Undervoltage Lockout
When the switch is enabled, undervoltage lockout (UVLO)
monitors the input voltage, V
IN
, and prevents the output
MOSFET from turning on until V
IN
exceeds a predetermined
level, nominally set at 1.45V. The UVLO threshold is adjust-
able and can be varied by applying an external resistor divider
to the UVLOIN pin from VIN to GND. The resistive divider
network is required when the input voltage is below 1.5V. The
UVLO threshold is internally preset to 1.45V if the UVLOIN
pin is left open. See
“Applications Information”
section.
(
390A
×
Ω
)
I
LIMIT
(1)
And given the
±22%
tolerance of the current limit factor (CLF),
the external resistor is bound by:
103Ω
≤
R
SET
≤
970Ω
(2)
The graphs below (Figure 3) display the current limit factor
characteristic over the full temperature range at the indicated
voltage. These curves can be used as a point of reference in
determining the maximum variation in the device’s current
limit over the full temperature range. For example: With
V
IN
= V
BIAS
= 3.0V and a nominal 2A current limit
(R
SET
= 192Ω), the low and high current limit settings for the
MIC2042/43 would be 1.66A and 2.34A, respectively, as
shown on the 3V graph using the 192Ω reference point.
When current limiting occurs, the MIC2042 and MIC2043
respond differently. Upon first reaching the limiting current
both devices restrict current flow, allowing the load voltage to
drop below V
IN
. If the VIN-to-VOUT differential voltage ex-
ceeds 200mV, then a fault condition is declared and the fault
delay timer is started. If the fault condition persists longer than
the delay period, typically 28ms, then the /FAULT output
asserts low. At this point, the MIC2042 will continue to supply
current to the load at the limiting value (I
LIMIT
), whereas the
MIC2043 will latch off its output.
4
3.5
3
I
LIM
(A)
Current Limit
vs. R
SET
–40°C to +85°C
V
IN
= V
BIAS
= 5V
4
3.5
3
I
LIM
(A)
Current Limit
vs. R
SET
–40°C to +85°C
V
IN
= V
BIAS
= 3V
4
3.5
3
I
LIM
(A)
Current Limit
vs. R
SET
–40°C to +85°C
V
IN
= V
BIAS
= 1.6V
2.5
2
1.5
1
0.5 CLF (LO)
0
0 120 240 360 480 600 720 840 960
R
SET
(Ω)
CLF (HI)
2.5
2
1.5
1
0.5
0
CLF (LO)
0 120 240 360 480 600 720 840 960
R
SET
(Ω)
CLF (HI)
2.5
2
1.5
1
0.5
0
CLF (LO)
CLF (HI)
0 120 240 360 480 600 720 840 960
R
SET
(Ω)
Figure 3. Current Limit Factor
January 2005
11
M0512-112603
MICREL [ MICREL SEMICONDUCTOR ]
