MIC2026 Datasheet, PDF (11/16 Pages) Micrel Semiconductor – Dual-Channel Power Distribution Switch Preliminary Information

MIC2026/2076
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 MOS-
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 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.
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 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.
Micrel
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 overcur-
rent 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 thresh-
old 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 2.5V. Under-
voltage detection functions only when the switch is enabled.
V_EN
V_OUT
I_LIMIT
I_LOAD
I_OUT
V_FLG
Short-Circuit Fault
3ms typ.
delay
Thermal
Shutdown
Reached
Load and Fault Removed
(Output Reset)
Figure 1. MIC2076-2 Fault Timing: Output Reset by Removing Load
V_EN
V_OUT
I_LIMIT
I_LOAD
I_OUT
V_FLG
March 2000
Short-Circuit Fault
Load/Fault
Removed
3ms typ.
delay
Thermal
Shutdown
Reached
Figure 2. MIC2026-2 Fault Timing
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MIC2026/2076