MIC2527 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – Quad USB Power Control Switch

MIC2527
Functional Description
The MIC2527-1 and MIC2527-2 are quad high-side switches
with active-high and active-low enable inputs, respectively.
Fault conditions turn off or inhibit turn-on one or both 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.
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 MIC2527 avoids undesirable drain-to-body
diode current flow by grounding the body when the switch is
off. (The conventional method for optimum turn-on threshold
has the source connected to the body. This would allow a
large current to flow when V_source > V_drain + 0.6V.) If V_IN <
2.5V, UVLO disables both switches. If V_IN < 2.3V, reverse
current may flow from OUT to IN regardless of enable state.
If OUT is greater than IN when a switch is disabled, current
will flow from OUT to IN when the switch is enabled.
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.
Undervoltage Lockout
UVLO (undervoltage lockout) prevents the output MOSFET
from turning on until IN (input voltage) exceeds 2.5V typical.
In the undervoltage state the FLAG will be low. After the
switch turns on, if the voltage drops below 2.3V typical, UVLO
shuts off the output MOSFET and signals the fault flag until
V_IN drops below 2V. Undervoltage detection functions only
when at least one switch is enabled.
Micrel
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. For further protection, there is
typically 150mA foldback in the output current after the
current limit threshold is exceeded.
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 current limit
threshold varies with output voltage, with a 10% to 15% fold
back. The reaction to an overcurrent condition varies with
three scenarios:
Switch Enable into Heavy Load
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 output voltage. The fault flag goes low until
the load is reduced or thermal shutdown occurs. See the
“Short Circuit Response Enable into Short Circuit” graph.
Heavy Load Applied Suddenly to Enabled Switch
When a heavy load is applied, a large current may flow from
the output capacitor and the switch. The current limit circuit
may shut the switch off briefly, then fold back into constant-
current mode. The fault flag falls for 10µs to 20µs during the
initial current flow, goes inactive for 10µs to 20µs while the
switch is off, and then goes low again while the switch is in
constant-current mode until the load is reduced or thermal
shutdown occurs. See the “Short Circuit Response, Short
Applied to Output” graph.
Gradual Load Increase
If the load current is slowly increased above 500mA, current
limiting does not occur until approximately 125% of the short-
circuit current limit. Current above this threshold causes the
same reaction as a suddenly applied load: the fault flag falls
for 10µs to 20µs during the initial current flow, goes inactive
for 10µs to 20µs while the switch is off, and then goes low
again while the switch is in constant-current mode until the
load is reduced or thermal shutdown occurs. See the “Current
Limit Response with Ramped Load” graph.
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 pullup resistor.
May 7, 1998
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MIC2527