LT3470AEDDB#TRMPBF Linear Technology, LT3470AEDDB#TRMPBF Datasheet - Page 14

• Manufacturer Part Number: LT3470AEDDB#TRMPBF
• Description: IC BUCK ADJ .25A 8DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3470AEDDB#TRMPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 16 V
• Current - Output: 250mA
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-DFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIONS INFORMATION
LT3470A
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 first
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 filtering and can slightly improve the efficiency 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
14
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 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.
High Temperature Considerations
The die junction temperature of the LT3470A must be
lower than the maximum rating of 125°C (150°C for
H-grade). This is generally not a concern unless the ambi-
ent 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 tem-
perature 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
efficiency 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 significant leakage current (see Typical
Performance Characteristics) increasing the quiescent
current of the LT3470A converter.
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