January 2005

13

M0512-112603

MIC2042/2043

Micrel

Applications Information

Input and Output

Supply Bypass Filtering

The need for input supply bypass is necessary 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 positioned

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 applica-

tion 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 capaci-

tive 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 MIC2042 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 VOUT and /FAULT outputs cycling on and off, the

MIC2042 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

power cable (≤ 1ft., 16 to 20 gauge wire), a minimum output

capacitance of 22µF is strongly recommended and should be

placed close to the VOUT pin of the MIC2042. For applica-

tions that use more than a foot of cable, an additional

10µF/ft. is recommended.

Reverse Current Block

The MIC2042/43 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 pin has a

limited reverse current flow if the voltage at VOUT is pulled

above VBIAS when the device is disabled. A graph of the

V

acteristics” plots. The reverse current for V

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.

Output Slew-Rate Adjustment

The output slew-rate for the MIC2042/43 can be slowed down

by the capacitor (16V rating, minimum; 25V suggested)

between SLEW and GND. The slew-rate control circuitry is

independent of the load capacitance and exhibits a non-

linear response. See the “Functional Characteristics” plots.

Table 1 shows the rise time for various standard capacitor

values. Additionally, the output turn-on time must be less than

the nominal flag delay of 28ms in order to avoid nuisance

tripping of the /FAULT output. This limit is imposed by the

current limiting circuitry which monitors the (VIN – VOUT)

differential voltage and concludes a fault condition is present

if the differential voltage exceeds 200mV for more than the

flag delay period. For the MIC2043, the /FAULT will assert

and the output will latch off if the output is not within 200mV

of the input before the flag delay times out. When using the

active-low (–2) option with the EN input tied to ground, slew

control is functional during initial start-up but does not func-

tion upon resetting the input power to the device. In order for

the SLEW control to operate during consecutive system

restarts, the EN pin must reset (toggle OFF to ON).

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 voltage. An external resistive divider network con-

nected 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

+













(4)

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

MIC2042/43.

C

Rise Time (ms)

5V

3V

0.01

3

4.75

0.033

10.5

15

0.047

14

21

0.1

32

46

Table 1. Typical Output Rise Time for Various C

SLEW