Micrel, Inc.
MIC2026/2076
Current-Limit Response—Ramped Load
TJ = PD × θJA + TA
where:
TJ = junction temperature
The MIC2026/76 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.
TA = ambient temperature
θJA = is the thermal resistance of the package
Current Sensing and Limiting
Fault Flag
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 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, tD, 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 tD is typically
3ms.
The current-limit circuit senses a portion of the output
MOSFET 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 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.
Undervoltage Lockout
Short-Circuit Applied to Enabled Output
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.
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.
Load and Fault Removed
(Output Reset)
Short-Circuit Fault
VEN
VOUT
ILIMIT
ILOAD
IOUT
Thermal
Shutdown
Reached
VFLG
3ms typ.
delay
Figure 1. MIC2076-2 Fault Timing: Output Reset by Removing Load
Short-Circuit Fautl
VEN
Load/Fault
Removed
VOUT
ILIMIT
ILOAD
IOUT
Thermal
Shutdown
Reached
3ms typ.
delay
VFLG
Figure 2. MIC2026-2 Fault Timing
M9999-060410-B
June 2010
12
MICROCHIP [ MICROCHIP ]
