MIC2505/2506
Micrel
U2 is off, this feature prevents current flow from the load (5V)
backward through U2 to the 3.3V supply. (If a discrete
MOSFET and driver were used, the MOSFET’s internal body
diode would short the 5V load to the 3.3V supply.)
Applications Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor from IN to GND, located at
the device is strongly recommended to control supply tran-
sients. Without a bypass capacitor, an output short may
cause sufficient ringing on the input (from supply lead induc-
tance) to destroy the internal control circuitry.
FLG will be active (low) on any switch that is off whenever the
load voltage is greater than the open load threshold (approxi-
mately 1V) except for MIC2505-1 and MIC2505-2.
+5V
+3.3V
Logic-High = 5V Output
Logic-Low = 3.3V Output
Input transients must not exceed the absolute maximum
supply voltage (V
= 7.5V) even for a short duration.
IN max
U1
U2
MIC2505BM
MIC2505BM
2.7V to 7.5V
1N4148
1
8
1
2
3
4
8
(optional)
CTL
OUT
IN
CTL
OUT
IN
MIC2505BM
2
3
4
7
7
6
FLG
FLG
1
2
3
4
8
ON
OFF
CTL
OUT
IN
6
GND
GATE
OUT
IN
GND
GATE
OUT
IN
7
FLG
5
5
6
0.1µF
0.1µF
GND
GATE
OUT
IN
0.1µF to 1µF
5
Figure 1. Supply Bypassing
Figure 2. 5V/3.3V Switch Concept
The bypass capacitor may be omitted only if board design
precautions are followed, such as using extremely short
supply leads or power and ground planes.
This circuit’s function would otherwise require a dual driver,
two MOSFETs, plus two diodes (or a dual driver plus four
MOSFETs).
Control Input
CTL must be driven logic high or logic low, or be pulled high
orlowforaclearlydefinedinput. Floatingtheinputmaycause
unpredictableoperation.Addadiodeclampifnegativespikes
may occur. See Figure 2.
Hot Plug-In Applications (Soft-Start)
The MIC2505/6 family can be used to protect the socket-side
andcard-sideofasupplycircuitfromtransientscausedwhen
a capacitive load is connected to an active supply.
Open-Load Detection
The switch presents a high impedance when off, and slowly
becomes a low impedance as it turns on. This reduces the
inrush current and related voltage drop that result from
charging a capacitive load.
Refer to “Typical Applications” (first page). Open-load detec-
tion is available only on the MIC2505 and MIC2506. For USB
power distribution applications, the open-load detection fea-
ture is not included in the MIC2505-1 or -2 versions.
The optional open-load detection resistor supplies a small
pull-up current to the load when the output switch is off. (A
100k resistor will draw 50µA from a 5V supply.) Normally, the
load dominates, pulling OUT low. If the load is absent, the
optionalresistorpullsOUThigh,activatingthefaultflagifCTL
is off.
Power Control
Circuitry
3.3V
MIC2505BM
1
2
3
4
8
CTL
OUT
IN
7
FLG
Capacitive
Load
6
GND
GATE
OUT
IN
5
When a load is switched off with CTL, capacitance on the
output may cause the open-load function to pull the flag low
until the capacitor is discharged below approximately 2.4V.
0.1µF
GND
Socket
Card
Omit the pull-up resistor when open load detection is not
required and for minimum off-state supply current.
Figure 3. Hot Plug-In Concept
Power Bus Switch
A gate capacitor may be added to the MIC2505 to slow the
turn on time even more, reducing the inrush current. See
“MIC2505 Turn-On Delay” graph. The UVLO feature insures
The MIC2505/6 family features a MOSFET reverse current
flow prevention circuit. This prevents current from flowing
backwards (from OUT to IN) when CTL is disabled as long as
that each time the card is removed and V = 0 that the gate
IN
of the output switch is discharged to zero volts. A controlled
turn-onisexecutedeachtimeaboardispluggedin, evenwith
multiple insertions.
V
is above UVLO minimum. In Figure 2, when U1 is on and
IN
April 1998
9
MIC2505/2506
MICREL [ MICREL SEMICONDUCTOR ]
