MIC2536
Micrel
Equations that can be used to calculate power dissipation
and die temperature are found below:
Calculation of power dissipated by each channel can be
accomplished by the following equation:
P
D
= R
DS(on)
×
(I
OUT
)
2
Total power dissipation of the device will be the summation
of P
D
for both channels. To relate this to junction
temperature, the following equation can be used:
T
j
= P
D
× θ
JA
+ T
A
where:
T
j
= junction temperature
T
A
= ambient temperature
Functional Description
The MIC2536-1 and MIC2536-2 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 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
load. If V
OUT
is greater than V
IN
, current will flow from OUT
to IN during an on-condition since the MOSFET is bidirec-
tional 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
OUTx
> V
IN
) when the output is
disabled. In this situation, the MIC2536 prevents reverse
current flow.
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.
The MIC2536 will automatically reset its output when the die
temperature cools to approximately 125°C. The MIC2536
output and FLG signal will continue to cycle on and off until the
device is disabled or the fault is removed.
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 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 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-
mal 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 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.
Power Dissipation
Power dissipation depends on several factors such as the
load, PCB layout, ambient temperature and package type.
March 2000
9
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 output MOSFET and external
load but allows a minimum current of 150mA 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 the following three sce-
narios:
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.
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.
Current-Limit Response Ramped Load
The MIC2536 current-limit profile exhibits a small foldback
effect of approximately 100mA. 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 MIC2536 will deliver load current up to the
current-limit threshold before entering current-limited opera-
tion.
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
D
has
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.
MIC2536
MICREL [ MICREL SEMICONDUCTOR ]
