LTM8065 datasheet(17/28 Pages) AD

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LTM8080

Rev. 0

For more information www.analog.com

A final precaution regarding ceramic capacitors con-

cerns the maximum input voltage rating of the LTM8080.

A ceramic input capacitor combined with trace or cable

inductance forms a high-Q (underdamped) tank circuit. If

the LTM8080 circuit is plugged into a live supply, the input

voltage can ring twice its nominal value, possibly exceed-

ing the device’s rating. This situation is easily avoided; see

the Hot-Plugging Safely section.

Frequency Selection

The LTM8080 uses a constant-frequency PWM architec-

ture that can be programmed to switch from 200kHz to

2.2MHz by using a resistor tied from the RT pin to the

ground. Table 2 provides a list of RT resistor values and

their resultant frequencies.

Table 2. SW Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

221

0.3

143

0.4

110

0.5

86.6

0.6

71.5

0.7

60.4

0.8

52.3

0.9

46.4

1.0

40.2

1.2

33.2

1.4

27.4

1.6

23.7

1.8

20.5

18.2

2.2

16.2

Operating Frequency Trade-Offs

It is recommended that the user apply the optimal RT

value shown in Table 1 for the input and output operating

conditions. System level or other considerations, however,

may necessitate another operating frequency. While the

LTM8080 is flexible enough to accommodate a wide range

of operating frequencies, a haphazardly chosen one may

result in undesirable operation under certain operating or

fault conditions. A frequency that is too high can reduce

efficiency, generate excessive heat, or even damage the

LTM8080 if the output is overloaded or short-circuited. A

frequency that is too low can result in a final design that

has too much VBUS ripple or too large of a VBUS capacitor.

Maximum Load

The maximum practical continuous load that the LTM8080

can drive, while rated at 500mA per channel, actually

depends upon both the internal current limit and the inter-

nal temperature. The internal current limit is designed to

prevent damage to the LTM8080 in the case of overload

or short-circuit. The internal temperature of the LTM8080

depends upon operating conditions such as the ambi-

ent temperature, the power delivered, and the system’s

heat-sinking capability. For example, if the output voltage

is 8V and the ambient temperature is 85°C, the LTM8080

will provide at most 200mA from 36VIN, less than the

500mA continuous rating. See the derating curves in the

Typical Performance Characteristics section.

Load Sharing

The two LTM8080 channels may be paralleled to produce

a higher current. To do this, tie the VOUTn, VSNSn, and

VSETn pins of all the paralleled channels together. Connect

the VOUTn pins using small pieces of PCB trace (used as

a ballast resistor) to equalize currents in the LTM8080

outputs. PCB trace resistance in milliohms/inch is shown

in Table 3.

Table 3. PC Board Trace Resistance*

WEIGHT (oz)

WIDTH (10mil)

WIDTH (20mil)

54.3

27.1

13.6

*Trace resistance is measured in mΩ/in.

The slight worst-case offset of 2mV for each output min-

imizes the required ballast resistor value. Figure 1 shows

that LTM8080 two outputs, each using a 20mΩ PCB trace

ballast resistor, provide better than 20% accurate output

current sharing at full load. The two 20mΩ external resis-

tors only add 10mV of output regulation drop with a 1A

maximum current. A 3.3V output, only adds 0.3% to the

regulation accuracy. Tie the VSNSn pin directly to each

output capacitor, respectively. The Typical Applications

APPLICATIONS INFORMATION

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