MIC2026A_09 Datasheet, PDF (12/16 Pages) Micrel Semiconductor – Dual-Channel Power Distribution Switch

Micrel, Inc.
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) x I_OUT2
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
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 a 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, reducing the output voltage. The FLG
signal is asserted indicating an overcurrent condition.
MIC2026A/2076A
• 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 the short-circuit
current limit specification.
• Current-Limit Response
The MIC2026A/2076A current-limit response is often
called the foldback current-limit. The foldback
current-limit is the current limit reached when the
output current is increased slowly rather than
abruptly. An approximation of slowly is tens of
milliamps per second. The foldback current-limit is
typical 200 mA higher than the short-circuit current-
limit. When the foldback current-limit is reached, the
output current will abruptly decrease to the short-
circuit current-limit.
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 threshold for
up to 1ms. 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 2.45V. Undervoltage detection functions
only when the switch is enabled.
July 2009
Figure 1. MIC2076A Fault Timing: Output Reset by Removing Load
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M9999-072309-B
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