MIC2027 Datasheet, PDF (12/16 Pages) Micrel Semiconductor – Quad USB Power Distribution Switch Preliminary Information

MIC2027/2077
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 effected. 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 fac-
tors 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
Micrel
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 MOS-
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 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 indicat-
ing 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 foldback
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 overcur-
rent 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 approximately 2.5V. Under-
voltage detection functions only when the switch is enabled.
MIC2027/2077
12
June 2000