MIC2095 Datasheet, PDF (19/25 Pages) Micrel Semiconductor – Current-Limiting Power Distribution Switches

Micrel, Inc.
I_LIMIT vs. I_OUT Measured
When in current limit, the switches are designed to act
as a constant-current source to the load. As the load
tries to pull more than the maximum current, V_OUT drops
and the input-to-output voltage differential increases. As
the (V_IN − V_OUT) voltage differential increases, the IC
internal temperature also increases. To limit the IC’s
power dissipation, the current limit is reduced as a
function of output voltage.
This folding back of I_LIMIT can be generalized by plotting
I_LIMIT as a function of V_OUT, as shown in Figures 3 and 4.
The slope of V_OUT between I_OUT = 0V and I_OUT = I_LIMIT
(where I_LIMIT is a normalized 1A) is determined by R_ON of
the switch and I_LIMIT.
Normalized Output Current
vs. Output Voltage (5V)
1.2
1.0
0.8
0.6
0.4
0.2
0
0123456
OUTPUT VOLTAGE (V)
Figure 3. Normalized Output Current vs. Output Voltage
Normalized Output Current
vs. Output Voltage (2.5V)
1.2
1.0
0.8
0.6
0.4
0.2
0
0 0.5 1.0 1.5 2.0 2.5 3.0
OUTPUT VOLTAGE (V)
Figure 4. Normalized Output Current vs. Output Voltage
MIC2095/97/98/99
When measuring I_OUT it is important to remember voltage
dependence, otherwise the measurement data may
appear to indicate a problem when none really exists.
This voltage dependence is illustrated in Figures 5 and
6.
In Figure 5, output current is measured as V_OUT is pulled
below V_IN, with the test terminating when V_OUT is 1V
below V_IN. Observe that once I_LIMIT is reached I_OUT
remains constant throughout the remainder of the test. In
Figure 6 this test is repeated but with (V_IN − V_OUT) is 4V.
Figure 5. I_OUT in Current Limiting for V_OUT = 4V
Figure 6. I_OUT in Current Limiting for V_OUT = 1V
Under Voltage Lock Out (UVLO)
The switches have an Under Voltage Lock Out (UVLO)
feature that will shut down the switch in a reproducible
manner when the input power supply voltage goes too
low. The UVLO circuit disables the output until the
supply voltage exceeds the UVLO threshold. Hysteresis
in the UVLO circuit prevents noise and finite circuit
impedance from causing chatter during turn-on and turn-
off. While disable by the UVLO circuit, the output switch
(power MOSFET) is OFF and no circuit functions, such
as FAULT/ or ENABLE, are considered to be valid or
operative.
September 2010
19
M9999-092010-C