LT1934IS6 查看數據表（PDF） - Linear Technology

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LT1934IS6

Micropower Step-Down Switching Regulators in ThinSOT and DFN

Linear Technology 

LT1934/LT1934-1

APPLICATIONS INFORMATION

in series with the input to eliminate the voltage overshoot

(it also reduces the peak input current). A 0.1μF capacitor

improves high frequency filtering. This solution is smaller

and less expensive than the electrolytic capacitor. For high

input voltages its impact on efficiency is minor, reducing

efficiency less than one half percent for a 5V output at full

load operating from 24V.

Voltage overshoot gets worse with reduced input capaci-

tance. Figure 7d shows the hot plug response with a 1μF

ceramic input capacitor, with the input ringing above 40V.

The LT1934-1 can tolerate a larger input resistance, such

as shown in Figure 7e where a 4.7Ω resistor damps the

voltage transient and greatly reduces the input current

glitch on the 24V supply.

High Temperature Considerations

The die temperature of the LT1934 must be lower than the

maximum rating of 125°C. This is generally not a concern

unless the ambient temperature is above 85°C. For higher

temperatures, care should be taken in the layout of the

circuit to ensure good heat sinking of the LT1934. The

maximum load current should be derated as the ambient

temperature approaches 125°C.

The die temperature is calculated by multiplying the LT1934

power dissipation by the thermal resistance from junction

to ambient. Power dissipation within the LT1934 can be

estimated by calculating the total power loss from an

efficiency measurement and subtracting the catch diode

loss. The resulting temperature rise at full load is nearly

independent of input voltage. Thermal resistance depends

on the layout of the circuit board, but a value of 150°C/W

is typical for the TSOT-23 and 75°C/W for the DFN.

The temperature rise for an LT1934 (TSOT-23) producing

5V at 250mA is approximately 25°C, allowing it to deliver

full load to 100°C ambient. Above this temperature the

load current should be reduced. For 3.3V at 250mA the

temperature rise is 15°C. The DFN temperature rise will

be roughly one-half of these values.

Finally, be aware that at high ambient temperatures the

external Schottky diode, D1, is likely to have significant

leakage current, increasing the quiescent current of the

LT1934 converter.

Outputs Greater Than 6V

For outputs greater than 6V, tie a diode (such as a 1N4148)

from the SW pin to VIN to prevent the SW pin from ringing

above VIN during discontinuous mode operation. The 12V

output circuit in Typical Applications shows the location of

this diode. Also note that for outputs above 6V, the input

voltage range will be limited by the maximum rating of

the BOOST pin. The 12V circuit shows how to overcome

this limitation using an additional Zener diode.

1934fe

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