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LTM8060

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

excessive heat or even damage the LTM8060 if the output

is overloaded or short-circuited. A frequency that is too

low can result in a final design that has too much output

ripple or too large of an output capacitor.

BIAS

n Pin Considerations

The BIAS

n pin is used to provide drive power for the

internal power switching stage and operate other internal

circuitry. For proper operation, it must be powered by at

least 3.2V. If the output voltage is programmed to 3.2V

or higher, BIAS

n may be simply tied to VOUTn. If VOUTn

is less than 3.2V, BIAS

n can be tied to VINn or some

other voltage source. If the BIAS

n pin voltage is too high,

the efficiency of the LTM8060 may suffer. The optimum

BIAS

n voltage is dependent upon many factors, such as

load current, input voltage, output voltage and switching

frequency. In all cases, ensure that the maximum volt-

age at the BIAS

n pin is less than 10V. If BIASn power is

applied from a remote or noisy voltage source, it may be

necessary to apply a decoupling capacitor locally to the

pin. A 1µF ceramic capacitor works well. The BIAS

n pin

may also be tied to GND at the cost of a small degrada-

tion in efficiency.

Maximum Load

The maximum practical continuous load that the LTM8060

can drive per channel, while rated at 3A, actually depends

upon both the internal current limit and the internal tem-

perature. The internal current limit is designed to prevent

damage to the LTM8060 in the case of overload or short-

circuit. The internal temperature of the LTM8060 depends

upon operating conditions such as the ambient tempera-

ture, the power delivered, and the heat sinking capabil-

ity of the system. For example, if VOUT1 of LTM8060 is

configured to regulate at 1.5V, and the other 3 channels

are turned off, VOUT1 may continuously deliver 6A from

24VIN if the ambient temperature is controlled to less

than 60°C. This is quite a bit higher than the 3A continu-

ous rating. Please see graphs in the Typical Performance

Characteristics section. Similarly, if all 4 channels of the

LTM8060 are delivering 3.3VOUT and the ambient tem-

perature is 100°C, each channel will deliver at most 1.5A

from 24VIN, which is less than the 3A continuous rating.

Table 2. Switching Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.2

200

0.3

137

0.4

100

0.5

76.8

0.6

64.9

0.7

54.9

0.8

46.4

0.9

41.2

1.0

35.7

1.2

27.4

1.4

23.2

1.6

19.6

1.8

16.9

2.0

14.7

2.2

12.7

2.4

11.3

2.6

10.2

2.8

9.09

3.0

8.06

Frequency Selection

The LTM8060 uses a constant frequency PWM architec-

ture that can be programmed to switch from 200kHz to

3MHz by using a resistor tied from the RT pin to ground.

Table 2 provides a list of RT resistor values and their resul-

tant frequencies. The resistors in the table are standard

1% E96 values.

Operating Frequency Trade-Offs

It is recommended that the user apply the optimal RT value

given in Table 1 for the input and output operating condi-

tion. When using the LTM8060 with different output volt-

ages, the higher frequency recommended by Table 1 will

usually result in the best operation. System level or other

considerations, however, may necessitate another operat-

ing frequency. While the LTM8060 is flexible enough to

accommodate a wide range of operating frequencies, a

haphazardly chosen one may result in undesirable opera-

tion under certain operating or fault conditions. A fre-

quency that is too high can reduce efficiency, generate

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