LT1934/LT1934-1
15
1934fe
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 fi ltering. This solution is smaller
and less expensive than the electrolytic capacitor. For high
input voltages its impact on effi ciency is minor, reducing
effi ciency 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
effi ciency 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 signifi cant
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 V
IN
to prevent the SW pin from ringing
above V
IN
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.
