LTM4613 datasheet(14/28 Pages) LINER

14 / 28 page

LTM8001

8001f

For more information www.linear.com/8001

APPLICATIONS INFORMATION

Programming Switching Frequency

The LTM8001 has an operational switching frequency

range between 200kHz and 1MHz. This frequency is

programmed with an external resistor from the RT pin to

ground. Do not leave this pin open under any condition.

See Table 2 for resistor values and the corresponding

switching frequencies.

Table 2. RT Resistor Values and Their Resultant Switching

Frequencies

SWITCHING FREQUENCY (MHz)

RT (kΩ)

39.2

0.75

53.6

0.5

82.5

0.3

140

0.2

200

Switching Frequency Trade-Offs

ItisrecommendedthattheuserapplytheoptimalRTresis-

tor value given in Table 1 for the input and output operating

condition. System level or other considerations, however,

may necessitate another operating frequency. While the

LTM8001 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

LTM8001 in some fault conditions. 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.

Switching Frequency Synchronization

The nominal switching frequency of the LTM8001 is

determined by the resistor from the RT pin to GND and

may be set from 200kHz to 1MHz. The internal oscillator

may also be synchronized to an external clock through

the SYNC pin. The external clock applied to the SYNC pin

must have a logic low below 0.8V and a logic high greater

than 1.2V. The input frequency must be 20% higher than

the frequency determined by the resistor at the RT pin.

The SYNC pin must be tied to GND if the synchroniza-

tion to an external clock is not required. When SYNC is

grounded, the switching frequency is determined by the

resistor at the RT pin.

Soft-Start

The soft-start function controls the slew rate of the power

supply output VOUT0 voltage during start-up. A controlled

output voltage ramp minimizes output voltage overshoot,

reduces inrush current from the VIN0 supply, and facili-

tates supply sequencing. A capacitor connected from the

SS pin to GND programs the slew rate. The capacitor is

charged from an internal 11μA current source to produce

a ramped output voltage.

Maximum Output Current Adjust

The LTM8001 features an adjustable accurate current

limit. To adjust the load current limit, an analog voltage

is applied to the ILIM pin. Varying the voltage between

0V and 1.5V adjusts the maximum current between the

minimum and the maximum current, 5.6A typical. Above

1.5V, the control voltage has no effect on the regulated

inductorcurrent.GraphsoftheoutputcurrentvsILIMvolt-

ages are given in the Typical Performance Characteristics

section. The LTM8001 provides a 2V reference voltage for

conveniently applying resistive dividers to set the current

limit. The current limit can be set as shown in Figure 1

with the following equation:

IMAX = 7.47

+R2

A convenient value of R1 may be 10k. In that case,

10 •IMAX

7.47 –IMAX

Ω

LTM8001

VREF

8001 F01

ILIM

Figure 1. Setting the Output Current Limit, IMAX

LTM4613 bảng dữ liệu(PDF) 14 Page - Linear Technology

LTM4613 bảng dữ liệu(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

bảng dữ liệu tải về

Link URL