LTM2883 datasheet(17/24 Pages) AD

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LTM2884

Rev. D

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APPLICATIONS INFORMATION

a 100mA input current, the VCC2 load current must be

less than 25mA. This characteristic of an isolated supply

may limit the use of the LTM2884 in bus powered hub

applications or downstream connection to a bus powered

hub. Connect VCC to an external supply to mitigate this

concern.

Hot Plug Protection

The VCC and VBUS inputs are bypassed with low ESR

ceramic capacitors. During a hot plug event, the supply

inputs can overshoot the supplied voltage due to cable

inductance. When using external power supply sources

greater than 10V that can be hot plugged, add an addi-

tional 2.2µF tantalum capacitor with greater than 1Ω of

ESR, or a ceramic capacitor with a series 1Ω resistor

to the VCC input to reduce the possibility of exceeding

absolute maximum ratings. Refer to Application Note 88,

“Ceramic Capacitors Can Cause Overvoltage Transients,”

for a detailed discussion of this problem.

PC Board Layout

The high integration of the LTM2884 makes PCB layout

simple. However, to optimize its electrical isolation char-

acteristics, EMI, and thermal performance, some layout

considerations are necessary. The PCB layout in Figure 5

is a recommended configuration for a low EMI USB appli-

cation. The following considerations optimize the perfor-

mance of the LTM2884:

• Under loaded conditions, VCC and GND current exceed

700mA, VCC2 and GND2 current is up to 500mA. Use

sufficient copper on the PCB to ensure resistive losses

do not cause the supply voltage to drop below the mini-

mum allowed level. The heavy copper traces will also

help to reduce thermal stress and improve thermal

conductivity.

• Input and output decoupling is not required on periph-

eral or hub inputs. Add additional low ESR capaci-

tance to reduce noise induction on the power supply

connections. Hub/bus splitter outputs require an addi-

tional 100µF of low ESR capacitance.

• Do not place copper between the inner columns of

pads on the top or bottom of the PCB. This area must

remain open to withstand the rated isolation voltage

and maintain the creepage distance.

RF, Magnetic Field Immunity

The isolator µModule technology used within the

LTM2884 has been independently evaluated, and suc-

cessfully passed the RF and magnetic field immunity test-

ing requirements per European Standard EN 55024, in

accordance with the following test standards:

EN 61000-4-3

Radiated, Radio-Frequency,

Electromagnetic Field Immunity

EN 61000-4-8

Power Frequency Magnetic

Field Immunity

EN 61000-4-9

Pulsed Magnetic Field Immunity

Tests were performed using an unshielded test card

designed per the data sheet PCB layout recommenda-

tions. Specific limits per test are detailed in Table 2.

Table 2. Test Frequency Field Strength

EN 61000-4-3, Annex D, 80MHz to 1GHz

1.4MHz to 2GHz

2GHz to 2.7GHz

10V/m

3V/m

1V/m

EN 61000-4-8, Level 4 50Hz and 60Hz

30A/m

EN 61000-4-8, Level 5 60Hz

100A/m*

EN 61000-4-9, Level 5 Pulse

1000A/m

*Non IEC Method

EMI

Radiated emissions have been measured for the LTM2884

using a gigahertz transverse electromagnetic (GTEM) cell

with and without a USB cable attached. The performance

shown in Figure 6 was achieved with the layout structure

in Figure 5. Results are corrected per IEC 61000-4-20.

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