lt3470a Linear Technology Corporation, lt3470a Datasheet - Page 15

• Manufacturer Part Number: lt3470a
• Description: Micropower Buck Regulator With Integrated Boost And Catch Diodes
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypass capacitor of LT3470A. However, these capacitors
can cause problems if the LT3470A 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 under damped tank circuit, and the volt-
age at the V
nominal input voltage, possibly exceeding the LT3470A’s
rating and damaging the part. If the input supply is poorly
controlled or the user will be plugging the LT3470A into an
energized supply, the input network should be designed
to prevent this overshoot. Figure 6 shows the waveforms
that result when an LT3470A circuit is connected to a 24V
supply through six feet of 24-gauge twisted pair. The fi rst
plot is the response with a 2.2μF ceramic capacitor at the
input. The input voltage rings as high as 35V and the input
current peaks at 20A. One method of damping the tank
circuit is to add another capacitor with a series resistor to
the circuit. In Figure 6b an aluminum electrolytic capacitor
has been added. This capacitor’s high equivalent series
resistance damps the circuit and eliminates the voltage
overshoot. The extra capacitor improves low frequency
ripple fi ltering and can slightly improve the effi ciency of the
circuit, though it is likely to be the largest component in the
circuit. An alternative solution is shown in Figure 6c. A 1Ω
resistor is added in series with the input to eliminate the
IN
pin of the LT3470A can ring to twice 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.
High Temperature Considerations
The die junction temperature of the LT3470A 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
LT3470A. The maximum load current should be derated
as the ambient temperature approaches the maximum
junction rating. The die temperature is calculated by
multiplying the LT3470A power dissipation by the thermal
resistance from junction to ambient. Power dissipation
within the LT3470A can be estimated by calculating the
total power loss from an effi ciency measurement. Thermal
resistance depends on the layout of the circuit board and
choice of package. The DFN package with the exposed
pad has a thermal resistance of approximately 80°C/W.
Finally, be aware that at high ambient temperatures the
internal Schottky diode will have signifi cant leakage current
(see Typical Performance Characteristics) increasing the
quiescent current of the LT3470A converter.
LT3470A
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3470af