MIC20XX_1108 Datasheet, PDF (26/30 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
To prevent false triggering of the VUVLO feature, the
MIC20X6 includes a delay timer to blank out
momentary excursions below the VUVLO trip point. If
V_IN stays below the VUVLO trip point for longer than
32ms (typical), then the load is disengaged and the
MIC20X6 will wait 128ms before reapplying power to
the load. If V_IN remains below the VUVLO trip point,
then the load will be powered for the 32ms blanking
period and then again disengaged. This is illustrated in
the scope plot below. If V_IN remains above the VUVLO
trip point MIC20X6 resumes normal operation.
Figure 9. VUVLO Operation
VUVLO and Kickstart™ operate independently in the
MIC2016. If the high-current surge allowed by
Kickstart™ causes V_IN to dip below the VUVLO trip
point for more than 32ms, VUVLO will disengage the
load, even though the Kickstart™ timer has not timed
out.
I_{IN_LOAD}
Input
Supply
+
VIN
VOUT
R1
MIC20X6
+
VUVLO
R2
Figure 10. VUVLO Application Circuit
MIC20xx Family
Calculating VUVLO Resistor Divider Values
The VUVLO feature is designed to keep the internal
switch off until the voltage on the VUVLO pin is greater
than 0.25V. A resistor divider network connected to the
VUVLO and VIN pins is used to set the input trip
voltage V_TRIP (see Figure 10). The value of R2 is
chosen to minimize the load on the input supply I_DIV and
the value of R1 sets the trip voltage V_TRIP.
The value of R2 is calculated using:
R2 = ^VVUVLO
I_DIV
The vale of R1 is calculated using:
R1
=
R2
×
⎜⎜⎝⎛
V_TRIP
V_VUVLO
− 1_{⎟⎟⎠⎞}
Where for both equations:
V_VUVLO = 0.25V
When working with large value resistors, a small
amount of leakage current from the VUVLO terminal
can cause voltage offsets that degrade system
accuracy. Therefore, the maximum recommended
resistor value for R2 is 100kΩ.
Using the divider loading current I_DIV of 100µA, the
value of R2 can be estimated by:
R2 = ^0.25V = 2.5kΩ
100µA
Now the value of R1 can be calculated by:
R1 = 2.5kΩ × ⎜⎛ ^4.75V − 1⎟⎞ = 45k
⎝ ^0.25V ⎠
where:
V_TRIP = 4.75V (for a 5V supply)
V_VUVLO = 0.25V
August 2011
26
M9999-080211-D