CM2026
D
UAL
USB P
OWER
C
ONTROL
S
WITCH
FUNCTIONAL DESCRIPTION
The CM2026 is dual high-side switch with active-high 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 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. If
V
OUT
is greater than V
IN
when a switch is enabled, current will
flow from OUT to IN since the MOSFET is bi-directional when
on.
The output MOSFET and driver circuitry are also designed to
allow the MOSFET source to be externally forced to a higher
voltage than drain (V
OUT
> V
IN
) when the output is off. In this
situation, the CM2026 avoids undesirable current flow from
OUT to IN. If V
IN
< 2.5V, UVLO disables both switches.
Thermal Shutdown
Thermal shutdown shuts off the affected output MOSFETs
and signals all fault flags if the die temperature exceeds 135
℃.
10℃ of hysteresis prevents the switch from turning on
until the die temperature drops to 125℃. Overtemperature
detection functions only when at least one switch is enabled.
Current Limit Induced Thermal Shutdown
Internal circuitry increases the output MOSFET on-resistance
until the series combination of the MOSFET on-resistance and
the load impedance limit current to typically 850mA. The
increase in power dissipation, in most cases, will cause the
CM2026 to go into thermal shutdown, disabling affected
channels. When this is undesirable, thermal 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 hundred milliseconds, The USB controller
must therefore recognize a fault and disable the appropriate
channel with this time. If the fault is not removed or the switch
is not disabled with the time, then the device will enter into a
thermal oscillation of about 2Hz. This does not cause any
damage to the device. Refer to "Functional Characteristics:
Thermal Shutdown Response."
Undervoltage Lockout
UVLO (undervoltage lockout) prevents the output MOSFET
from turning on until V
IN
exceeds approximately 2.5V. In the
undervoltage state, the FLAG will be low. After the switch
turns on, if the voltage drops below approximately 2.3V, UVLO
shuts off the output MOSFET and signals fault flag.
Undervoltage detection functions only when at least one
switch is enabled.
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.
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 three scenarios:
Switch Enabled into Short Circuit
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 the output voltage. The fault flag goes low
until the load is reduced.
Short Circuit Applied to Output
When a heavy load is applied, a large transient current may
flow until the current limit circuitry will respond. Once this
occurs, the device limits current to less than the short-circuit
current limit specification.
Current-Limit Response
The CM2026 current-limit profile exhibits a small foldback
effect of approximately 500mA. Once this current-limit
threshold is exceeded the device enters constant current
mode.
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
<2.7V), current limit, or thermal
shutdown. The flag output MOSFT is capable of sinking a
10mA load to typically 100mV above ground. Multiple FLG
pins may be "wire NORed" to a common pull-up resistor.
2002/05/29
Rev. 2
Champion Microelectronic Corporation
Page 4
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
