April 1998

9

MIC2505/2506

MIC2505/2506

Micrel

Applications Information

Supply Filtering

A 0.1

µF 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.

Input transients must not exceed the absolute maximum

ON

OFF

MIC2505BM

CTL

OUT

FLG

GND

OUT

GATE

IN

IN

18

27

36

45

0.1µF to 1µF

2.7V to 7.5V

Figure 1. Supply Bypassing

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.

Control Input

CTL must be driven logic high or logic low, or be pulled high

or low for a clearly defined input. Floating the input may cause

unpredictable operation. Add a diode clamp if negative spikes

may occur. See Figure 2.

Open-Load Detection

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 50

µA from a 5V supply.) Normally, the

load dominates, pulling OUT low. If the load is absent, the

optional resistor pulls OUT high, activating the fault flag if CTL

is off.

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.

Omit the pull-up resistor when open load detection is not

required and for minimum off-state supply current.

Power Bus Switch

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

V

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.)

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.

Logic-High = 5V Output

Logic-Low = 3.3V Output

U1

MIC2505BM

CTL

OUT

FLG

GND

OUT

GATE

IN

IN

+5V

18

27

36

45

0.1µF

U2

MIC2505BM

CTL

OUT

FLG

GND

OUT

GATE

IN

IN

18

27

36

45

+3.3V

0.1µF

1N4148

(optional)

Figure 2. 5V/3.3V Switch Concept

This circuit’s function would otherwise require a dual driver,

two MOSFETs, plus two diodes (or a dual driver plus four

MOSFETs).

Hot Plug-In Applications (Soft-Start)

The MIC2505/6 family can be used to protect the socket-side

and card-side of a supply circuit from transients caused when

a capacitive load is connected to an active supply.

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.

MIC2505BM

CTL

OUT

FLG

GND

OUT

GATE

IN

IN

18

27

36

45

0.1µF

Capacitive

Load

3.3V

Power Control

Circuitry

GND

Card

Socket

Figure 3. Hot Plug-In Concept

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

that each time the card is removed and V

of the output switch is discharged to zero volts. A controlled

turn-on is executed each time a board is plugged in, even with

multiple insertions.