LTM8021 datasheet(17/28 Pages) LINER

17 / 28 page

LTM8053

8053fc

For more information www.linear.com/LTM8053

APPLICATIONS INFORMATION

BIAS Pin Considerations

The BIAS pin is used to provide drive power for the in-

ternal 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 may be simply tied to AUX. If VOUT is less

than 3.2V, BIAS can be tied to VIN or some other voltage

source. If the BIAS pin voltage is too high, the efficiency

of the LTM8053 may suffer. The optimum BIAS voltage

is dependent upon many factors, such as load current,

input voltage, output voltage and switching frequency. In

all cases, ensure that the maximum voltage at the BIAS pin

is less than 19V. If BIAS 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 pin may also be left open

at the cost of a small degradation in efficiency. If unused

or when generating a negative output, connect BIAS to

LTM8053 GND.

Maximum Load

ThemaximumpracticalcontinuousloadthattheLTM8053

can drive, while rated at 3.5A, actually depends upon both

the internal current limit and the internal temperature.

The internal current limit is designed to prevent damage

to the LTM8053 in the case of overload or short-circuit.

The internal temperature of the LTM8053 depends upon

operating conditions such as the ambient temperature,

the power delivered, and the heat sinking capability of

the system. For example, if the LTM8053 is configured

to regulate at 1.2V, it may continuously deliver 5A from

12VIN if the ambient temperature is controlled to less than

60°C; this is quite a bit higher than the 3.5A continuous

rating. Please see the derating curve for VOUT = 1.2V in

theTypicalPerformanceCharacteristicssection.Similarly,

if the output voltage is 15V and the ambient temperature

is 85°C, the LTM8053 will deliver at most 2A from 24VIN,

which is less than the 3.5A continuous rating.

Load Sharing

Two or more LTM8053s may be paralleled to pro-

duce higher currents. To do this, tie the VIN, VOUT and

SHARE pins of all the paralleled LTM8053s together.

To ensure that paralleled modules start up together, the

TR/SSpinsmaybetiedtogether,aswell.Ifitisinconvenient

to tie the TR/SS pins together, make sure that the same

valued soft-start capacitors are used for each µModule

regulator. An example of two LTM8053s configured for

load sharing is given in the Typical Applications section.

For closer load sharing, synchronize the LTM8053s to an

external clock source. When load sharing among n units

and using a single RFB resistor, the value of the resistor is:

RFB =

241.53

n VOUT – 0.97

(

)

where RFB is in k.

Burst Mode Operation

To enhance efficiency at light loads, the LTM8053 auto-

matically switches to Burst Mode operation which keeps

the output capacitor charged to the proper voltage while

minimizingtheinputquiescentcurrent.DuringBurstMode

operation, the LTM8053 delivers single cycle bursts of

current to the output capacitor followed by sleep periods

where most of the internal circuitry is powered off and

energy is delivered to the load by the output capacitor.

During the sleep time, VIN and BIAS quiescent currents

are greatly reduced, so, as the load current decreases

towards a no load condition, the percentage of time that

the LTM8053 operates in sleep mode increases and the

averageinputcurrentisgreatlyreduced,resultinginhigher

light load efficiency.

Burst Mode operation is enabled by tying SYNC to GND.

Minimum Input Voltage

The LTM8053 is a step-down converter, so a minimum

amount of headroom is required to keep the output in

regulation. Keep the input above 3.4V to ensure proper

operation. Voltage transients or ripple valleys that cause

the input to fall below 3.4V may turn off the LTM8053.

Output Voltage Tracking and Soft-Start

TheLTM8053allowstheusertoprogramitsoutputvoltage

ramp rate by means of the TR/SS pin. An internal 2A pulls

up the TR/SS pin to about 2.4V. Putting an external capaci-

tor on TR/SS enables soft starting the output to reduce

LTM8021 датащи(PDF) 17 Page - Linear Technology

LTM8021 датащи(HTML) 17 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

датащи скачать

ссылки URL