MIC2027_06 Datasheet, PDF (13/17 Pages) Micrel Semiconductor – Quad USB Power Distribution Switch

Micrel, Inc.
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_OUT is greater than
V_IN, current will flow from OUT to IN, since the switch is
bidirectional when enabled. 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 switch is disabled. In this
situation, the MIC2027/77 prevents 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 and the
overheated channel is in current limit. The other
channels are not affected. If however, the die
temperature exceeds 160°C, all channels will be shut off.
Upon determining a thermal shutdown condition, the
MIC2077 will latch the output off and activate a pull-up
current source. When the load is removed, this current
source will pull the output up and reset the latch.
Toggling EN will also reset the latch.
The MIC2027 will automatically reset its output when the
die temperature cools down to 120°C. The MIC2027
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.
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. This time will be shortest in the
case of a dead short on the output.
Power Dissipation
The device’s junction temperature depends on several
factors 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.
P_D = R_DS(on) × I_OUT2
Total power dissipation of the device will be the
summation of P_D for all channels. To relate this to
junction temperature, the following equation can be
used:
T_J = P_D × θ_JA + T_A
MIC2027/2077
where:
T_J = junction temperature
T_A = ambient temperature
θ_JA = is the thermal resistance of the package
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
MOSFET 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, limiting the output voltage. The FLG signal is
asserted indicating an overcurrent condition.
Short-Circuit Applied to Enabled Output
When a heavy load or short-circuit is applied to an
enabled switch, 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.
Current-Limit Response—Ramped Load
The MIC2027/77 current-limit profile exhibits a small
fold-back effect of about 100mA. Once this current-limit
threshold is exceeded the device switches into a
constant current mode. It is important to note that the
device will supply current up to the current-limit
threshold.
Fault Flag
The FLG signal is an N-channel open-drain MOSFET
output. FLG is asserted (active-low) when either an
overcurrent or thermal shutdown condition occurs. In the
case of an overcurrent condition, FLG will be asserted
only after the flag response delay time, t_D, has elapsed.
This ensures that FLG is asserted only upon valid
overcurrent conditions and that erroneous error reporting
is eliminated. For example, false overcurrent conditions
can occur during hot-plug events when a highly
capacitive load is connected and causes a high transient
inrush current that exceeds the current-limit thresh-old.
The FLG response delay time t_D is typically 3ms.
Undervoltage Lockout
Undervoltage lockout (UVLO) prevents the output
MOSFET from turning on until V_IN exceeds approx-
imately 2.5V. Undervoltage detection functions only
when the switch is enabled.
September 2006
13
M9999-091506