January 2005

13

M9999-012004

MIC2040/2041

Micrel

Applications Information

Input and Output

Supply Bypass Filtering

The need for input supply bypass is brought about due to

several factors, most notably the input/output inductance

along the power path, operating current and current limit, and

output capacitance. A 0.1µF to 0.47µF bypass capacitor

positioned very close to the VIN pin to GND of the device is

strongly recommended to filter high frequency oscillations

due to inductance. Also, a sufficient bypass capacitor posi-

tioned close to the input source to the switch is strongly

advised in order to suppress supply transient spikes and to

limit input voltage droop. Inrush current increases with larger

output capacitance, thus the minimum value of this capacitor

will require experimental determination for the intended appli-

cation and design. A good starting point is a capacitor

between 4.7µF to 15µF. Without these bypass capacitors, an

extreme overload condition such as a short circuit, or a large

capacitive load, may cause either the input supply to exceed

the maximum rating of 6V and possibly cause damage to the

internal control circuitry or allow the input supply to droop and

fall out of regulation and/or below the minimum operating

voltage of the device.

Output Capacitance

When the MIC2040 die exceeds the overtemperature thresh-

old of approximately 140°C, the device can enter into a

thermal shutdown mode if the die temperature falls below

120°C and then rises above 140°C in a continuous cycle.

With the V

MIC2040 will reset the /FAULT while in an overtemperature

fault condition if the output voltage is allowed to swing below

ground. The inductance present at the output must be neu-

tralized by capacitance in order to ensure that the output does

not fall below ground. In order to counter the board parasitic

inductance and the inductance of relatively short-length

(<1 ft., 16 - 20 gauge wire), a minimum output capacitance of

22µF is strongly recommended and should be placed close

to the V

more than a foot of cable, an additional 10µF/ft. is recom-

mended.

Reverse Current Block

The MIC2040/41 provides reverse current flow block through

the output MOSFET if the voltage at VOUT is greater than VIN

when the device is disabled. The VBIAS supply has a limited

reverse current flow if the voltage at VOUT is pulled above

VBIAS when the device is disabled. The reverse current for

VBIAS can be completely blocked by inserting a Schottky

diode from the VBIAS pin (cathode) to the supply (anode).

However, the minimum voltage of 1.6V must be supplied to

VBIAS after accounting for the voltage drop across the diode.

UVLO Threshold Setting With Low Input Voltages

When the switching voltage is below 1.6V, the device’s

standard UVLO threshold (1.45V nominal) will hinder the

output MOSFET in switching VIN to VOUT. In this case, the

use of the UVLOIN pin is required to override the standard

UVLO threshold and set a new, lower threshold for the lower

input operating voltages. An external resistive divider net-

work connected at the UVLOIN pin is used to set the new

threshold. Due to the ratio of the internal components, the

total series resistance of the external resistive divider should

not exceed 200kΩ. The circuit shown in Figure 4 illustrates an

application that switches 0.8V while the device is powered

from a separate 2.5V power supply. The UVLO threshold is

set by the following equation.

V

0.23V

1

R2

R3

+













(3)

In substituting the resistor values from Figure 4, the resulting

UVLO threshold (V

switching application. When using the UVLOIN pin to set a

new UVLO threshold, an optional 0.1µF to 1.0µF capacitor

from UVLOIN to GND may be used as a glitch filter in order

to avoid nuisance tripping of the UVLO threshold. If the

UVLOIN pin is not in use, this pin should be left open

(floating). The use of a pull-down resistor to ground will offset

the ratio of the internal resistive divider to this pin resulting in

a shift in the UVLO threshold. To bypass (disable) UVLO,

connect the UVLOIN pin directly to the VIN pin of the

MIC2040/41.

7,9

8

1

3

5

8,10

2

220

Ω

C1

0.1

µF

10

MIC2040-1BM

VBIAS

VIN

EN

/FAULT

UVLOIN

VOUT

ILIM

GND

R1

47k

Ω

R4

75k

Ω

1%

R5

36.5k

Ω

1%

22

µF

0.8V @1.25A

R6

47k

Ω

Digital

Output

Signal

C3

10

µF

C2

0.1

µF

0.8V

2.5V

R2

95.3k

Ω

1%

R3

59k

Ω

1%

Note:

Both V

Both V

Undervoltage Lockout = 0.6V.

Figure 4. Lower UVLO Setting