MIC2007YML datasheet(21/25 Pages) MICREL

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Micrel, Inc.

MIC20XX Family

January 2009

M9999-012109-A

require dual current limits on their peripheral ports. The

MIC20X6 is designed for systems demanding two

primary current limiting levels but without the use of a

control signal to select between current limits.

To better understand how the MIC20X6 provides this,

imagine a system whose main power supply supports

heavy loads during normal operation, but in sleep mode

is reduced to only few hundred milliamps of output

current. In addition, this system has several USB ports

which must remain active during sleep. In normal

operation, each port can support a 500mA peripheral,

but in sleep mode their combined output current is

limited to what the power supply can deliver minus

whatever the system itself is drawing.

If a peripheral device is plugged in which demands more

current than is available, the system power supply will

sag, or crash. The MIC20X6 prevents this by monitoring

both the load current and VIN. During normal operation,

when the power supply can source plenty of current, the

MIC20X6 will support any load up to its factory

programmed current limit. When the weaker, standby

supply is in operation, the MIC20X6 monitors VIN and

will shut off its output should VIN dip below a

predetermined value. This predetermined voltage is user

programmable and set by the selection of the resistor

divider driving the VUVLO pin.

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 VIN 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

VIN 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 VIN 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 VIN 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.

VIN

VOUT

VUVLO

Input

Supply

IIN_LOAD

Calculating VUVLO resistor divider values

Selection of R1 and R2 is driven by the input voltage at

which VUVLO should go into effect and the allowed

loading of the input supply. The VUVLO pin input

voltage is the result of the voltage division of VIN by the

voltage divider comprised of R1 and R2. We know

VVUVLO_TH = 250 mV, then by choosing a VIN trip voltage

(VTRIP) we know the voltage divider ratio formed by R1

and Then an R2.is chosen such that the series

resistance R1 + R2 results in a small IIN_LOAD.

And then the VUVLO trip voltage as it relates to the

comparator threshold and the resistor divider:

()

TRIP

VUVLO_TH

Rearranging these:

() 2

−

Choose an R2 that minimizes the IIN_LOAD current yet at

the same time is less than input impedance of the

VUVLO pin. The VUVLO pin internally is connected to a

comparator with an extremely high input impedance. It is

recommended that R2 not exceed 1 MΩ. R2 can then be

calculated from the equation above.

For example:

VTRIP = 4.75V for a 5V supply

VVUVLO_TH = 250mV

R2 = 750kΩ

Substituting these values into the equation above:

05263

0.25

4.75

TRIP

VUVLO_TH

()

−

667

750

05263

R1 = 41,667kΩ

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