October 2003

13

MIC2044/2045

MIC2044/2045

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 0.1

µF 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 4.7

µF 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 MIC2044 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 VOUT cycling on and off, the MIC2044 will reset the

/FAULT while in an overtemperature fault condition if V

allowed to swing below ground. The inductance present at

the output must be neutralized 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 - 20 gauge wire),

a minimum output capacitance of 22

µF is strongly recom-

mended and should be placed close to the VOUT pin of the

MIC2044. For applications that use more than a foot of cable,

an additional 10

µF/ft. is recommended.

Reverse Current Blocking

The MIC2044/45 provides reverse current flow blocking

through the output MOSFET if the voltage at VOUT is greater

than VIN when the device is disabled. The V

a limited reverse current flow if the voltage at VOUT is pulled

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

V

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.

Output Slew-Rate Adjustment

The output slew-rate for the MIC2044/45 can be slowed down

by the use of a capacitor (16V rating, minimum; 25V sug-

gested) 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”

section. 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 32ms 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 MIC2045, 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 function 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

200k

Ω. 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 0.1

µF 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

MIC2044/45.

Conditions: V

C

C

Rise Time (ms)

5V

3V

0.02

4.4

6.6

0.033

7.5

11.25

0.047

11

16

0.1

24

31.5

Table 1. Typical Output Rise Time for Various C

SLEW

(V