MIC2025 Micrel Semiconductor, MIC2025 Datasheet - Page 9

• Manufacturer Part Number: MIC2025
• Description: Single-Channel Power Distribution Switch Preliminary Information
• Manufacturer: Micrel Semiconductor

Block Diagram
Functional Description
Input and Output
IN is the power supply connection to the logic circuitry and the
drain of the output MOSFET. OUT is the source of the output
MOSFET. In a typical circuit, current flows from IN to OUT
toward the load. If V
from OUT to IN since the switch is bidirectional when en-
abled. The output MOSFET and driver circuitry are also
designed to allow the MOSFET source to be externally forced
to a higher voltage than the drain (V
switch is disabled. In this situation, the MIC2025/75 avoids
undesirable current flow from OUT to IN.
Thermal Shutdown
Thermal shutdown is employed to protect the device from
damage should the die temperature exceed safe margins
due mainly to short circuit faults. Each channel employs its
own thermal sensor. Thermal shutdown shuts off the output
MOSFET and asserts the FLG output if the die temperature
reaches 140 C. The MIC2025 will automatically reset its
output should the die temperature cool down to 120 C. The
MIC2025 output and FLG signal will continue to cycle on and
off until the device is disabled or the fault is removed. Figure
2 depicts typical timing. If the MIC2075 goes into thermal
shutdown, its output will latch off and a pull-up current source
is activated. This allows the output latch to automatically reset
when the load (such as a USB device) is removed. The output
can also be reset by toggling EN. Refer to Figure 1 for details.
Depending on PCB layout, package, ambient temperature,
etc., it may take several hundred milliseconds from the
incidence of the fault to the output MOSFET being shut off.
The worst-case scenario of thermal shutdown is that of a
short-circuit fault and is shown in the in the “Function Char-
acteristics: Thermal Shutdown Response” graph.
March 2000
MIC2025/2075
OUT
EN
is greater than V
CHARGE
PUMP
OSC.
OUT
IN
, current will flow
> V
SHUTDOWN
THERMAL
IN
) when the
GND
9
UVLO
Power Dissipation
The device’s junction temperature depends on several fac-
tors such as the load, PCB layout, ambient temperature and
package type. Equations that can be used to calculate power
dissipation of each channel and junction temperature are
found below.
Total power dissipation of the device will be the summation of
P
the following equation can be used:
where:
Current Sensing and Limiting
The current-limit threshold is preset internally. The preset
level prevents damage to the device and external load but still
allows a minimum current of 500mA to be delivered to the
load.
The current-limit circuit senses a portion of the output MOS-
FET switch current. The current-sense resistor shown in the
block diagram is virtual and has no voltage drop. The reaction
to an overcurrent condition varies with three scenarios:
Switch Enabled into Short-Circuit
If a switch is enabled into a heavy load or short-circuit, the
switch immediately enters into a constant-current mode,
reducing the output voltage. The FLG signal is asserted
indicating an overcurrent condition. See the Short-Circuit
Response graph under Functional Characteristics.
D
for both channels. To relate this to junction temperature,
CONTROL
P
T
T
T
JA
GATE
J
J
A
D
= P
= junction temperature
= ambient temperature
= R
= is the thermal resistance of the package
REFERENCE
D
DS(on)
1.2V
RESPONSE
JA
DELAY
FLAG
+ T
CURRENT
I
OUT
LIMIT
A
2
IN
OUT
FLG
MIC2025/2075
Micrel