MIC2525 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – USB Power Control Switch Not Recommended for New Designs

MIC2525
Functional Description
The MIC2525-1 and MIC2525-2 are high-side switches with
active-high and active-low enable inputs, respectively. Fault
conditions turn off or inhibit turn-on of the output transistor,
depending upon the type of fault, and activate the open-drain
error flag transistor making it sink current to ground.
Input and Output
IN (input) is the power supply connection to the logic circuitry
and the drain of the output MOSFET. OUT (output) is the
source of the output MOSFET. In a typical circuit, current
flows through the switch from IN to OUT toward the load. If
V_OUT is greater than V_IN when the switch is enabled, current
will flow from OUT to IN since the MOSFET is bidirectional
when on.
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_OUT > V_IN) when the output is off. In
this situation, the MIC2525 avoids undesirable current flow
from OUT to IN. If V_IN < 2.5V, UVLO disables the switch.
Thermal Shutdown
Thermal shutdown shuts off the output MOSFET and signals
the fault flag if the die temperature exceeds 135°C. 10°C of
hysteresis prevents the switch from turning on until the die
temperature drops to 125°C. Overtemperature detection
functions only when the switch is enabled.
Current Limit Induced Thermal Shutdown
Internal circuitry increases the output MOSFET on-resis-
tance until the series combination of the MOSFET on-resis-
tance and the load impedance limit current to typically 750mA.
The increase in power dissipation, in most cases, will cause
the MIC2525 to go into thermal shutdown. When this is
undesirable, thermal shutdown can be avoided by externally
responding to the fault and disabling the current limited
channel before the shutdown temperature is reached. The
delay between the flag indication of a current limit fault and
thermal shutdown will vary with ambient temperature, board
layout, and load impedance, but is typically several hundred
milliseconds. The USB controller must therefore recognize a
fault and disable the switch within this time. If the fault is not
removed or the switch is not disabled within this time, then the
device will enter into a thermal oscillation of about 2Hz. This
does not cause any damage to the device. Refer to “Func-
tional Characteristics: Thermal Shutdown Response.”
Micrel
Undervoltage Lockout
UVLO (undervoltage lockout) prevents the output MOSFET
from turning on until V_IN exceeds approximately 2.5V. In the
undervoltage state, the FLAG will be low. After the switch
turns on, if the voltage drops below approximately 2.3V,
UVLO shuts off the output MOSFET and signals fault flag.
Undervoltage detection functions only when the switch is
enabled.
Current Sensing and Limiting
The current-limit threshold is preset internally. The preset
level prevents damage to the output MOSFET and external
load but allows a minimum current of 0.5A through the output
MOSFET.
The current-limit circuit senses a portion of the output 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 powered on or enabled into a heavy load or short-
circuit, the switch immediately goes into a constant-current
mode, reducing the output voltage. The fault flag goes low
until the load is reduced. See the “Functional Characteristics:
Short Circuit Response, Enabled into Short Circuit” photo.
Short Circuit Applied to Output
When a heavy load is applied, a large transient current may
flow until the current limit circuitry responds. Once this
occurs, the device limits current to less than the short-circuit
current limit specification. See the “Short Circuit Transient
Response, Short Applied to Output” graph.
Current-Limit Response
The MIC2525 current-limit profile exhibits a small foldback
effect of approximately 500mA. Once this current-limit thresh-
old is exceeded the device enters constant-current mode.
This constant current is specified as the short circuit current
limit in the “Electrical Characteristics” table. It is important to
note that the MIC2525 will deliver load current up to the
current-limit threshold which is typically 1.6A. Refer to “Func-
tional Characteristics: Current-Limit Response” photo for
details.
Fault Flag
FLG is an N-channel, open-drain MOSFET output. The fault-
flag is active (low) for one or more of the following conditions:
undervoltage (while 2V < V_IN < 2.7), current limit, or thermal
shutdown. The flag output MOSFET is capable of sinking a
10mA load to typically 100mV above ground.
MIC2525
8
January 2000