MIC2073 Datasheet, PDF (12/16 Pages) Micrel Semiconductor – Dual-Channel Power Distribution Switch

Micrel, Inc.
Power Dissipation
The device’s junction temperature depends upon 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) × 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 resistor 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.
MIC2073/2074
• 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 MIC2073/2074 current-limit profile exhibits a
small foldback effect of about 200mA. 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 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 VIN exceeds
approximately 2.5V. Undervoltage detection functions
only when the switch is enabled.
V_EN
V_OUT
I_LIMIT
I_LOAD
I_OUT
V_FLG
Short-Circuit Fault
Load and Fault Removed
(Output Reset)
3ms typ.
delay
Thermal
Shutdown
Reached
Figure 1. MIC2074-2 Fault Timing: Output Reset by Removing Load
January 2009
12
M9999-011209