LTM8024 datasheet(15/32 Pages) AD

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LTM4703

Rev. 0

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Frequency Selection

The LTM4703 uses a constant-frequency PWM architec-

ture, which can be programmed to switch from 300kHz

to 3MHz by using a resistor connected from the RT pin

to the ground. Table 2 shows a list of RT resistor values

and their resultant frequencies.

Table 2. Switching Frequency vs RT Value

fSW

(MHz)

(kΩ)

0.3

392

0.4

287

0.5

226

0.6

187

0.7

154

0.8

137

0.9

118

1.0

105

1.2

86.6

1.4

71.5

1.6

61.9

1.8

53.6

2.5

35.7

28.7

Operating Frequency Trade-Offs

It is recommended that you apply the optimal RT value

given in Table 2 for the input and output operating con-

ditions. System-level or other considerations, however,

may necessitate another operating frequency. While the

LTM4703 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

LTM4703 device if the output is overloaded or short-cir-

cuited. 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.

Maximum Load

The maximum practical continuous load that the LTM4703

can drive, while rated at 12A, actually depends upon both

the internal current limit and the internal temperature.

The internal current limit is designed to prevent damage

to the LTM4703 in the case of overload or short-circuit.

The internal temperature of the LTM4703 depends on the

operating conditions, such as the ambient temperature,

the power delivered, and the heat sinking capability of

the system. See the derating curve for VOUT = 1V in the

Typical Performance Characteristics section.

Load Sharing

Two or more LTM4703 devices may be paralleled to pro-

duce higher currents. To do this, connect the VIN, VOUT,

VOSNS, COMPa, and COMPb pins of all the paralleled

LTM4703 devices together. Figure 12 is an example of

two LTM4703 devices configured for load sharing.

The CLKOUT signal can be connected to the SYNC pin

of the following LTM4703 device to line up both the fre-

quency and the phase of the entire system. Connecting

the PHMODE pin to GND, INTVCC, or floating the pin gen-

erates a phase difference between LTM4703’s internal

clock and CLKOUT of 180°, 90°, or 120°, respectively,

which corresponds to a 2-phase, a 4-phase, or a 3-phase

operation. A total of 12 phases can be paralleled to run

simultaneously out-of-phase with respect to each other

by programming the PHMODE pin of each LTM4703 to

different voltage levels. During FCM and synchronization

modes, all devices operate at the same frequency. When

load sharing among a number (n) of units and using a

single RSET resistor, the value of the resistor is given

by Equation 2.

RSET =

VOUT

n•100µA

(2)

Minimum Input Voltage

The LTM4703 is a step-down converter; therefore, a min-

imum amount of headroom is required to keep the out-

put in regulation. Keep PVIN above 3V to ensure proper

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