October 2003

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

MOSFET from turning on until V

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

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

with a set point accuracy of

±21%. In programming the

nominal current limit, the value of R

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

(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

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

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

will latch off its output.

0

1

2

3

4

5

6

7

8

0

50 100 150 200 250 300 350 400 450

R

Current Limit

vs. R

SET

–40

°C to +85°C

V

CLF (LO)

CLF (HI)

0

1

2

3

4

5

6

7

8

0

50 100 150 200 250 300 350 400 450

R

Current Limit

vs. R

SET

–40

°C to +85°C

V

CLF (LO)

CLF (HI)

Figure 3. Current Limit Factor