voltage of the part at the power pin is 5.25V and this means

the supply line loss is 0.25V. So at 5.5V supply voltage the

part can drive laser diodes with a forward voltage in access

of 4V.

Application Hints

SUPPLY SEQUENCING

As the LMH6525/6526 is fabricated in the CMOS7 process,

latch-up concerns are minimal. Be aware that applying a low

impedance input to the part when it has no supply voltage

will forward bias the ESD diode on the input pin and then

source power into the part’s V

sustained operation with active inputs and no supply voltage,

all the active inputs should have series resistors to limit the

current into the input pins to levels below a few milliamperes.

DECOUPLING

The LMH6525/6526 has very high output currents changing

within a nanosecond. This makes decoupling especially im-

portant. High performance, low impedance ceramic capaci-

tors should be located as close as possible to the supply

pins. The LMH6525/6526 needs two decoupling capacitors,

one for the analog power and ground V

for the power side supply and ground (3xV

The high level of output current dictates the power side

decoupling capacitor should be 0.1 microfarads minimum.

Larger values may improve rise times depending on the

layout and trace impedances of the connections. The capaci-

tors should have direct connection across the supply pins on

the top layer, preferably with small copper-pour planes.

These planes can connect to the bottom side ground and/or

power planes with vias but there should be a topside low

impedance path with no vias if possible. (see also Figure 4

Decoupling Capacitors).

OVERSHOOT

As the LMH6525/6526 has fast rise times of less then a

nanosecond, any inductance in the output path will cause

overshoot. This includes the inductance in the laser diode

itself as well as any trace inductance. A series connection of

a resistor and a capacitor across the laser diode could be

helpful to reduce unwanted overshoot or to reduce the very

high peaks caused by the relaxation oscillations of a laser

diode when driven from below the knee voltage. But keep

always in mind that this causes a slower rise and/ or fall time.

Typical values are 10

Ω and 100 pF. The actual values re-

quired depend on the laser diode used and the circuit layout

and should be determined empirically.

THERMAL

General

The LMH6525/6526 is a very high current output device.

This means that the device must have adequate heat-

sinking to prevent the die from reaching its absolute maxi-

mum rating of 150˚C. The primary way heat is removed from

the LMH6525/6526 is through the Die Attach Pad, the large

center pad on the bottomside of the device. Heat is also

carried out of the die through the bond wires to the traces.

The outputs and the V

bond wires on this device so they will conduct about twice as

much heat to the pad. In any event, the heat able to be

transferred out the bond wires is far less than that which can

be conducted out of the die attach pad. Heat can also be

removed from the top of the part but the plastic encapsula-

tion has worse thermal conductivity then copper. This means

a heat sink on top of the part is less effective than the same

copper area on the circuit board that is thermally attached to

the Die Attach Pad.

PBC Heatsinks

In order to remove the heat from the die attach pad there

must be a good thermal path to large copper pours on the

circuit board. If the part is mounted on a dual-layer board the

simplest method is to use 6 or 8 vias under the die attach

pad to connect the pad thermally (as well as electrically, of

course) to the bottomside of the circuit board. The vias can

then conduct heat to a copper pour area with a size as large

as possible. Please see application note AN-1187 for guide-

lines about these vias and LLP packaging in general. Follow

the link below to view the mentioned application note. http://

www.national.com/an/AN/AN-1187.pdf

Derating

It is essential to keep the LMH6525/6526 die under 150˚C.

This means that if there is inadequate heat sinking the part

may overheat at maximum load while at maximum operating

ambient of 85˚C. How much power (current) the part can

deliver to the load at elevated ambient temperatures is

purely dependent on the amount of heat sinking the part is

provided with.

LAYOUT

Inputs

Critical inputs are the LVDS lines. These are two coupled

lines of a certain impedance, mostly 100

Ω. For some reason

those lines could have another value but in that case the

termination resistance must have the same value. The dif-

ferential input resistance of the LMH6525 and LMH6526 is

100

Ω and normally the impedance of the incoming transmis-

sion line matches that value. When using a flexible flat cable

it is important to know the impedance of two parallel wires in

that cable. Flex cables can have different pitch distances,

but a commonly used cable has a pitch of 0.5 mm. When

verified by TDR equipment, the measurements show an

impedance of about 142

Ω. It is possible to calculate the

impedance of such a cable when some parameters are

known. Needed parameters are the pitch (a) of the wires, the

20110123

FIGURE 1. Output Configuration

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