January 2005
11
MIC2044/2045
MIC2044/2045
Micrel
Functional Description
The MIC2044 and MIC2045 are high-side N-Channel switches
equipped with programmable current limit up to 6A 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
VBIAS supplies power to the internal circuitry of the switch
and must be present for the switch to operate. VIN is con-
nected to the drain of the output MOSFET and sources power
to the switched load. VIN must be less than or equal to VBIAS.
VOUT is the source terminal of the output MOSFET and
attaches to the load. In a typical circuit, current flows from VIN
to VOUT toward the load. If VOUT is greater than VIN, current
will flow from VOUT to VIN 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 MIC2044/45 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.”
Programmable Current Limit
The MIC2044/45 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 (Pin 6) 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 MIC2044/45 will limit from 1A to 6A
with a set point accuracy of
±
21%. In programming the
nominal current limit, the value of R
SET
is determined using
the following equation:
R
CLF
I
380A
I
SET
LIMIT
LIMIT
=
=
×
Ω
(
)
(1)
And given the
±
21% tolerance of the current limit factor (CLF),
the external resistor is bound by:
50
Ω
≤
R
SET
≤
460
Ω
(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 4A current limit (R
SET
= 95
Ω
),
the low and high current limit settings for the MIC2044/45
would be 3.15A and 4.85A, respectively, as shown on the 3V
graph using the 95
Ω
reference point.
When current limiting occurs, the MIC2044 and MIC2045
respond differently. Upon first reaching the limiting current
both devices restrict current flow, allowing the load voltage to
drop below V
IN
. If the V
IN
to V
OUT
differential voltage exceeds
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 32ms, then the /FAULT output asserts
low. At this point, the MIC2044 will continue to supply current
to the load at the limiting value (I
LIMIT
), whereas the MIC2045
will latch off its output.
0
1
2
3
4
5
6
7
8
0
50 100 150 200 250 300 350 400 450
I
LIMIT
(A)
R
SET
(
Ω
)
Current Limit
vs. R
SET
–40
°
C to +85
°
C
V
IN
= V
BIAS
= 3V, 5V
CLF (LO)
CLF (HI)
0
1
2
3
4
5
6
7
8
0
50 100 150 200 250 300 350 400 450
I
LIMIT
(A)
R
SET
(
Ω
)
Current Limit
vs. R
SET
–40
°
C to +85
°
C
V
IN
= V
BIAS
= 1.6V
CLF (LO)
CLF (HI)
Figure 3. Current Limit Factor
