MIC2536
Micrel
Equations that can be used to calculate power dissipation
and die temperature are found below:
Functional Description
The MIC2536-1 and MIC2536-2 are dual high-side switches
with active-high and active-low enable inputs, respectively.
Faultconditionsturnofforinhibitturn-onofoneormoreofthe
output transistors, depending upon the type of fault, and
activate the open-drain error flag transistors making them
sink current to ground.
Calculation of power dissipated by each channel can be
accomplished by the following equation:
2
P = R
× (I
)
D
DS(on)
OUT
Total power dissipation of the device will be the summation
of P for both channels. To relate this to junction
D
Input and Output
temperature, the following equation can be used:
IN (input) is the power supply connection to the logic circuitry
and the drain of each output MOSFET. OUTx (output) is the
source of each respective MOSFET. In a typical circuit,
current flows through the switch from IN to OUTx toward the
T = P × θ + T
A
j
D
JA
where:
T = junction temperature
j
T = ambient temperature
load. If V
is greater than V , current will flow from OUT
A
OUT
IN
to IN during an on-condition since the MOSFET is bidirec-
tional when enabled.
θ
= is the thermal resistance of the package
JA
Current Sensing and Limiting
The output MOSFET and driver circuitry are also designed to
allow the MOSFET source to be externally forced to a higher
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 150mA through the
output MOSFET of each channel.
voltage than the drain (V
> V ) when the output is
OUTx
IN
disabled. In this situation, the MIC2536 prevents reverse
current flow.
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 the following three sce-
narios:
Thermal Shutdown
Each output MOSFET has its own thermal sensor. If either or
both channels reach 135°C, affected channel(s) will be shut
down and flag(s) asserted. 10°C of hysteresis prevents the
switches from turning on until the die temperature drops to
125°C. Overtemperature detection functions only when at
least one switch is enabled.
Switch Enabled into Short Circuit
If a switch is enabled into a heavy load or short circuit, the
switch immediately goes into a constant-current mode, re-
ducing the output voltage. The FLG is asserted indicating an
overcurrent condition.
The MIC2536 will automatically reset its output when the die
temperature cools to approximately 125°C. The MIC2536
outputandFLGsignalwillcontinuetocycleonandoffuntilthe
device is disabled or the fault is removed.
Short Circuit Applied to 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 maximum short-circuit current-limit
specification.
Depending on PCB layout, package, ambient temperature,
etc., it may take several hundred milliseconds from the
occurrence of the fault to the output MOSFET being shut off.
Delay to reach thermal shutdown will be shortest with a dead
short on the output.
Current-Limit Response Ramped Load
Current-Limit Induced Thermal Shutdown
The MIC2536 current-limit profile exhibits a small foldback
effectofapproximately100mA.Oncethiscurrent-limitthresh-
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 MIC2536 will deliver load current up to the
current-limit threshold before entering current-limited opera-
tion.
Internal circuitry increases the output MOSFET on-resis-
tance until the series combination of the MOSFET on-resis-
tance and the load impedance limits output current to ap-
proximately 275mA. The resulting increase in power dissipa-
tion may cause the shorted channel to go into thermal
shutdown. In addition, even though individual channels are
thermally isolated, it is possible they may shut down when an
adjacent channel is shorted. When this is undesirable, ther-
malshutdowncanbeavoidedbyexternallyrespondingtothe
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 temperature, board layout, and load
impedance, but is typically several seconds. The USB con-
troller must therefore recognize a fault and disable the
appropriate channel within this time.
Fault Flag
FLGx is an open-drain N-channel MOSFET output. Fault
flags are active (low) for current-limit or thermal shutdown. In
the case where an overcurrent condition occurs, FLG will be
asserted only after the flag response delay time, t has
D
elapsed. This ensures that FLG is asserted only upon valid
overcurrent conditions and that erroneous error reporting is
eliminated. 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 threshold. The flag response delay time is typi-
cally 12ms.
Power Dissipation
Power dissipation depends on several factors such as the
load, PCB layout, ambient temperature and package type.
MIC2536
8
March 2000
MICREL [ MICREL SEMICONDUCTOR ]
