LTC4075/LTC4075X

14

4075Xfa

Figure 5. Low Loss Input Reverse Polarity Protection

WALL

ADAPTER

DCIN

LTC4075

DRAIN-BULK

DIODE OF FET

4075 F05

makes them especially susceptible to this problem. To

bypass the USB pin and the wall adapter input, add a 1

Ω

resistor in series with a ceramic capacitor to lower the

effective Q of the network and greatly reduce the ringing.

A tantalum, OS-CON, or electrolytic capacitor can be used

in place of the ceramic and resistor, as their higher ESR

reduces the Q, thus reducing the voltage ringing.

The oscilloscope photograph in Figure 4 shows how

serious the overvoltage transient can be for the USB

and wall adapter inputs. For both traces, a 5V supply is

hot-plugged using a three foot long cable. For the top

trace, only a 4.7µF capacitor (without the recommended

1

Ω series resistor) is used to locally bypass the input.

This trace shows excessive ringing when the 5V cable

is inserted, with the overvoltage spike reaching 10V. For

the bottom trace, a 1

Ω resistor is added in series with the

4.7µF capacitor to locally bypass the 5V input. This trace

shows the clean response resulting from the addition of

the 1

Ω resistor.

Even with the additional 1

Ωresistor,baddesigntechniques

and poor board layout can often make the overvoltage

problem even worse. System designers often add extra

inductance in series with input lines in an attempt to mini-

mize the noise fed back to those inputs by the application.

In reality, adding these extra inductances only makes the

overvoltage transients worse. Since cable inductance is

one of the fundamental causes of the excessive ringing,

adding a series ferrite bead or inductor increases the ef-

fective cable inductance, making the problem even worse.

For this reason, do not add additional inductance (ferrite

beads or inductors) in series with the USB or wall adapter

inputs. For the most robust solution, 6V transorbs or zener

diodes may also be added to further protect the USB and

wall adapter inputs. Two possible protection devices are

the SM2T from STMicroelectronics and the EDZ series

devices from ROHM.

Always use an oscilloscope to check the voltage wave-

forms at the USBIN and DCIN pins during USB and wall

adapter hot-plug events to ensure that overvoltage

transients have been adequately removed.

Reverse Polarity Input Voltage Protection

In some applications, protection from reverse polarity

voltage on the input supply pins is desired. If the sup-

ply voltage is high enough, a series blocking diode can

be used. In other cases where the voltage drop must be

kept low, a P-channel MOSFET can be used (as shown in

Figure 5).

APPLICATIO S I FOR ATIO

Figure 4. Waveforms Resulting from Hot-Plugging a

5V Input Supply

4.7µF ONLY

2V/DIV

4.7µF + 1

Ω

2V/DIV

20µs/DIV

3455 F04