MIC2526
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:
Functional Description
The MIC2526-1 and MIC2526-2 are dual high-side switches
with active-high and active-low enable inputs, respectively.
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 making 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
when a 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 MIC2526 avoids undesirable current flow
from OUT to IN. 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 MIC2526 to go into thermal shutdown, disabling affected
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 indication of a current
limit fault and thermal shutdown will vary with ambient tem-
perature, board layout, and load impedance, but is typically
several hundred milliseconds. The USB controller must there-
fore 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 Characteristics:
Thermal Shutdown Response.”
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 MIC2526 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 MIC2526 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.
MIC2526
8
February 2000
MICREL [ MICREL SEMICONDUCTOR ]
