lt3505ems8e-trpbf Linear Technology Corporation, lt3505ems8e-trpbf Datasheet - Page 18

• Manufacturer Part Number: lt3505ems8e-trpbf
• Description: 1.2a, Step-down Switching Regulator In 3mm X 3mm Dfn
• Manufacturer: Linear Technology Corporation

LT3505
APPLICATIONS INFORMATION
damaging the part. If the input supply is poorly controlled
or the user will be plugging the LT3505 into an energized
supply, the input network should be designed to prevent
this overshoot.
Figure 10 shows the waveforms that result when an LT3505
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 9b
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
18
24V SUPPLY
IMPEDANCE
ENERGIZED
LOW
+
+
Figure 10. A Well Chosen Input Network Prevents Input Voltage Overshoot and
Ensures Reliable Operation When the LT3505 is Connected to a Live Supply
+
SIMULATES HOT PLUG
CLOSING SWITCH
AI.EI.
0.1µF
10µF
35V
STRAY
INDUCTANCE
DUE TO 6 FEET
(2 METERS) OF
TWISTED PAIR
+
I
IN
1Ω
V
IN
2.2µF
2.2µF
2.2µF
LT3505
LT3505
LT3505
(9a)
(9b)
(9c)
alternative solution is shown in Figure 9c. A 1Ω resistor
is added 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 only one percent for a 5V output at full
load operating from 24V.
Frequency Compensation
The LT3505 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular,
the LT3505 does not require the ESR of the output capaci-
tor for stability allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size.
Frequency compensation is provided by the components
tied to the V
20V/DIV
20V/DIV
20V/DIV
5A/DIV
5A/DIV
5A/DIV
V
V
V
I
I
I
IN
IN
IN
IN
IN
IN
C
pin, as shown in Figure 10. Generally a
DANGER!
RINGING V
ABSOLUTE MAXIMUM
RATING OF THE LT3505
20µs/DIV
20µs/DIV
20µs/DIV
IN
MAY EXCEED
3505 F10
3505fc