LT8302 datasheet(14/20 Pages) LINER

14 / 20 page

LTM8068

8068fb

For more information www.linear.com/LTM8068

APPLICATIONS INFORMATION

5. Connect all of the GND connections to as large a cop-

per pour or plane area as possible on the top layer.

Avoid breaking the ground connection between the

external components and the LTM8068.

6. Use vias to connect the GND copper area to the board’s

internal ground planes. Liberally distribute these GND

vias to provide both a good ground connection and

thermal path to the internal planes of the printed circuit

board. Pay attention to the location and density of the

thermal vias in Figure 1. The LTM8068 can benefit

from the heat sinking afforded by vias that connect

to internal GND planes at these locations, due to their

proximity to internal power handling components.

The optimum number of thermal vias depends upon

the printed circuit board design. For example, a board

might use very small via holes. It should employ more

thermal vias than a board that uses larger holes.

Minimum Load

Due to the nature of the flyback regulator in general, and

the LTM8068 control scheme specifically, the LTM8068

requires a minimum load for proper operation. Otherwise,

the output may go out of regulation if the load is too

light. The most common way to address this is to place a

resistor across the output. The Minimum Load Current vs

VOUT Over Full Output Voltage Range graph in the Typical

Performance Characteristics section of may be used as

a guide in selecting the resistor. Note that this graph

describes room temperature operation. If the end appli-

cation operates at a colder temperature, the minimum

load requirement may be higher and the minimum load

condition must be characterized for the lowest operating

temperature.

If it is impractical to place a resistive load permanently

across the output, a resistor and Zener diode may be

used instead, as shown in Figure 2. While the minimum

load resistor mentioned in the prior paragraph will always

draw current while the LTM8068 output is powered, the

series resistor-Zener diode combination will only draw

current if the output is too high. When using this circuit,

take care to ensure that the characteristics of the Zener

diode are appropriate for the intended application’s tem-

perature range.

Figure 2: Use a Resistor and Zener Diode to Meet

the Minimum Load Requirement

LTM8068

VOUTP

VOUTN

8068 F02

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of the LTM8068. However, these capaci-

tors can cause problems if the LTM8068 is plugged into

a live supply (see Linear Technology Application Note 88

for a complete discussion). The low loss ceramic capacitor

combined with stray inductance in series with the power

source forms an underdamped tank circuit, and the volt-

age at the VIN pin of the LTM8068 can ring to more than

twice the nominal input voltage, possibly exceeding the

LTM8068’s rating and damaging the part. If the input

supply is poorly controlled or the user will be plugging

the LTM8068 into an energized supply, the input network

should be designed to prevent this overshoot. This can be

accomplished by installing a small resistor in series to VIN,

but the most popular method of controlling input voltage

overshoot is adding an electrolytic bulk capacitor to the

VIN net. This capacitor’s relatively high equivalent series

resistance damps the circuit and eliminates the voltage

overshoot. The extra capacitor improves low frequency

ripple filtering and can slightly improve the efficiency of the

circuit, though it can be a large component in the circuit.

Thermal Considerations

The LTM8068 output current may need to be derated if

it is required to operate in a high ambient temperature.

The amount of current derating is dependent upon the

input voltage, output power and ambient temperature. The

temperature rise curves given in the Typical Performance

Characteristics section can be used as a guide. These

curves were generated by the LTM8068 mounted to

a 58cm2 4-layer FR4 printed circuit board. Boards of

other sizes and layer count can exhibit different thermal

behavior, so it is incumbent upon the user to verify proper

LT8302 датащи(PDF) 14 Page - Linear Technology

LT8302 датащи(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

датащи скачать

ссылки URL