LTM8049 datasheet(9/18 Pages) LINEAR

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LTM8049

8049f

For more information www.linear.com/LTM8049

OPERATION

The LTM8049 contains two stand-alone switching DC/

DC converters; either one may be configured as a SEPIC

(single-ended primary inductance converter) or inverting

power supply simply by tying VOUTN or VOUTP to GND,

respectively. It accepts an input voltage up to 20VDC. The

output is adjustable between 2.5V and 24V for the SEPIC,

and between –2.5V and –24V for the inverting configura-

tion. The LTM8049 can provide 1.5A at VIN = 12V when

VOUT = 5V or –5V at ambient room temperature.

As shown in the Block Diagram, the LTM8049 contains a

current mode controller, power switching element, power

coupled inductor, power Schottky diode and a modest

amount of input and output capacitance. The LTM8049

is a fixed frequency PWM regulator.

TheLTM8049switchingcanfreerunbyapplyingaresistorto

theRTpinorsynchronizetoanexternalsourceatafrequency

between 200kHz and 2.5MHz. To synchronize to an external

source, drive a valid signal source into the SYNC pin. See

SynchronizationintheApplicationsSectionformoredetails.

For most applications, the design process is straight

forward, summarized as follows:

1. Look at Table 1 and find the row that has the desired

input range and output voltage.

2. Apply the recommended CIN,COUT,RADJandRTvalues.

While these component combinations have been tested

for proper operation, it is incumbent upon the user to

verify proper operation over the intended system’s line,

load and environmental conditions. Bear in mind that the

maximum output current is limited by junction tempera-

ture, the relationship between the input and output voltage

magnitude and polarity and other factors. Please refer to

the graphs in the Typical Performance Characteristics

section for guidance.

Table 1 gives the recommended component values and

configuration for a single channel. Each channel may be

configured independently. The maximum frequency (and

attendant RT value) at which the LTM8049 should be al-

lowed to switch is given in Table 1 in the fMAX column,

while the recommended frequency (and RT value) for

The LTM8049 also features RUN and SS pins to control

the start-up behavior of the device. The RUN pin may also

be used to implement an accurate undervoltage lockout

function by applying a resistor network to the RUN pin.

The LTM8049 features frequency foldback to protect the

power switches during a fault or output current overload.

During start-up, frequency foldback also limits the current

the LTM8049 delivers to the load. The user must evaluate

the start-up behavior of the LTM8049 to ensure that it

properly powers up the load.

The LTM8049 is equipped with a thermal shutdown to

protect the device during momentary overload conditions.

It is set above the 125°C absolute maximum internal tem-

perature rating to avoid interfering with normal specified

operation, so internal device temperatures will exceed

the absolute maximum rating when the overtemperature

protection is active. Therefore, continuous or repeated

activation of the thermal shutdown may impair device

reliability.

optimal efficiency over the given input condition is given

in the fOPTIMAL column. Running the LTM8049 faster than

the recommended frequency may reduce the usable input

voltage range.

Capacitor Selection Considerations

The CIN and COUT capacitor values in Table 1 are the

minimum recommended values for the associated oper-

ating conditions. Applying capacitor values below those

indicated in Table 1 is not recommended, and may result

in undesirable operation. Using larger values is generally

acceptable, and can yield improved dynamic response, if

it is necessary. Again, it is incumbent upon the user to

verify proper operation over the intended system’s line,

load and environmental conditions.

Ceramic capacitors are small, robust and have very low

ESR. However, not all ceramic capacitors are suitable.

X5R and X7R types are stable over temperature and ap-

plied voltage and give dependable service. Other types,

including Y5V and Z5U have very large temperature and

voltage coefficients of capacitance. In an application

APPLICATIONS INFORMATION

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