MIC2040 Datasheet, PDF (13/15 Pages) Micrel Semiconductor – Single Channel Low Voltage Power Distribution Switch

MIC2040/2041
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_OUT and /FAULT outputs cycling on and off, the
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_OUT pin of the MIC2040. For applications that use
more than a foot of cable, an additional 10µF/ft. is recom-
mended.
Micrel
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_UVTH
=
0.23V
×
₁₊
R2
R3

(3)
In substituting the resistor values from Figure 4, the resulting
UVLO threshold (V_UVTH) is calculated as 0.6V for this 0.8V
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.
V_DD
2.5V
V_IN
0.8V
C3
10µF
R2
95.3kΩ
1%
R1
47kΩ
R3
59kΩ
1%
C2
C1
0.1µF 0.1µF
R_SET
220Ω
MIC2040-1BM
8 _{VBIAS VOUT} 8,10
7,9 _VIN
1
EN
3 _UVLOIN
5 _{ILIM /FAULT} 2
GND
10
R6
47kΩ
R4
75kΩ
1%
R5
36.5kΩ
1%
V_OUT
C_LOAD 0.8V @1.25A
22µF
Digital
Output
Signal
Note:
Both V_IN pins (7, 9) must be externally tied together.
Both V_OUT pins (8, 10) must be externally tied together.
Undervoltage Lockout = 0.6V.
Figure 4. Lower UVLO Setting
January 2005
13
M9999-012004