MIC2524 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – Quad USB Power Control Switch Not Recommended for New Designs

MIC2524/2527
Functional Description
The MIC2524/7-1 and MIC2524/7-2 are quad high-side
switches with active-high and active-low enable inputs, re-
spectively. Fault conditions turn off or inhibit turn-on of one or
more of the output transistors, depending upon the type of
fault, and activate the open-drain error flag transistors mak-
ing them 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. OUTx (output) is the
source of its respective 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, 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 MIC2524/7 prevents reverse current flow.
If V_IN < 2.5V, UVLO disables both switches.
Thermal Shutdown
Thermal shutdown shuts off the affected output MOSFETs
and signals all fault flags 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 at least one 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 850mA.
The increase in power dissipation, in most cases, will cause
the MIC2524/7 to go into thermal shutdown, disabling af-
fected channels. 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 indica-
tion 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 appropriate
channel 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 “Functional Char-
acteristics: 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 at least one
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 of each channel.
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 will respond. 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 MIC2524/7 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 MIC2524/7 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. Multiple FLG
pins may be “wire NORed” to a common pull-up resistor.
MIC2524/2527
8
June 1999