LTM4644 datasheet(17/28 Pages) AD

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LTM8051

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Frequency Selection

The LTM8051 uses a constant frequency PWM architec-

ture that can be programmed to switch from 300kHz 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 LTM8051 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 LTM8051 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

excessive heat or even damage the LTM8051 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 LTM8051 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 degradation

in efficiency.

Maximum Load

The maximum practical continuous load that the

LTM8051 can drive per channel, while rated at 1.2A,

actually depends upon both the internal current limit

and the internal temperature. The internal current limit is

designed to prevent damage to the LTM8051 in the case

of overload or short-circuit. The internal temperature of

the LTM8051 depends upon operating conditions such

as the ambient temperature, the power delivered, and

the heat sinking capability of the system. For example,

if VOUT1 of LTM8051 is configured to regulate at 1V, and

the other 3 channels are turned off, VOUT1 may continu-

ously deliver 3A from 24VIN if the ambient temperature

is controlled to less than 60°C. This is quite a bit higher

than the 1.2A continuous rating. Please see graphs in the

Typical Performance Characteristics section. Similarly,

if all 4 channels of the LTM8051 are delivering 8VOUT

and the ambient temperature is 100°C, each channel will

deliver at most 0.6A from 24VIN, which is less than the

1.2A continuous rating.

Table 2. Switching Frequency vs RT Value

fSW (MHz)

RT (kΩ)

0.3

113

0.4

86.6

0.5

68.1

0.6

54.9

0.7

46.4

0.8

40.2

0.9

34.8

1.0

30.9

1.2

24.9

1.4

21.0

1.6

17.8

1.8

15.0

2.0

13.3

2.2

11.5

2.4

10.2

2.6

9.09

2.8

8.06

3.0

7.15

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