LT3500IDD#PBF Linear Technology, LT3500IDD#PBF Datasheet - Page 16

• Manufacturer Part Number: LT3500IDD#PBF
• Description: IC REG STP-DWN 2A 12-DFN
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3500IDD#PBF

• Topology: Step-Down (Buck) (1), Linear (LDO) (1)
• Function: Any Function
• Number Of Outputs: 2
• Frequency - Switching: 500kHz ~ 2.4MHz
• Voltage/current - Output 1: 0.8 ~ 38.9 V, 2A
• Voltage/current - Output 2: Adjustable, 13mA
• W/led Driver: No
• W/supervisor: No
• W/sequencer: No
• Voltage - Supply: 3 V ~ 36 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 12-DFN
• Current - Output: 2A
• Voltage - Output: 0.8 ~ 38.9 V
• Voltage - Input: 3 ~ 36 V
• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3500
the value of output capacitance required. Also, the current
mode control loop doesn’t require the presence of output
capacitor series resistance (ESR). For these reasons, you
are free to use ceramic capacitors to achieve very low
output ripple and small circuit size. Estimate output ripple
with the following equations:
where ∆I
inductor.
The RMS content of this ripple is very low, and the RMS
current rating of the output capacitor is usually not of
concern.
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this require-
ment becomes:
Finally, there must be enough capacitance for good transient
performance. The last equation gives a good starting point.
Alternatively, you can start with one of the designs in this
data sheet and experiment to get the desired performance.
This topic is covered more thoroughly in the section on
loop compensation.
APPLICATIONS INFORMATION
16
For ceramic capacitors and,
V
For electrolytic (tantalum and aluminum)
C
V
RIPPLE
RIPPLE
OUT1
> 10 • L
L
= ∆I
=
is the peak-to-peak ripple current in the
8 • Frequency • C
L
• ESR
V
I
OUT1
LIM
ΔI
L
2
OUT1
The high performance (low ESR), small size and robust-
ness of ceramic capacitors make them the preferred type
for LT3500 applications. However, all ceramic capacitors
are not the same. As mentioned above, many of the high
value capacitors use poor dielectrics with high tempera-
ture and voltage coeffi cients. In particular, Y5V and Z5U
types lose a large fraction of their capacitance with ap-
plied voltage and temperature extremes. Because the loop
stability and transient response depend on the value of
C
and X5R types. You can also use electrolytic capacitors.
The ESRs of most aluminum electrolytics are too large to
deliver low output ripple. Tantalum and newer, lower ESR
organic electrolytic capacitors intended for power supply
use, are suitable and the manufacturers will specify the
ESR. The choice of capacitor value will be based on the
ESR required for low ripple. Because the volume of the
capacitor determines its ESR, both the size and the value
will be larger than a ceramic capacitor that would give you
similar ripple performance. One benefi t is that the larger
capacitance may give better transient response for large
changes in load current.
Catch Diode
The diode D1 conducts current only during switch off
time. Use a Schottky diode to limit forward voltage drop to
increase effi ciency. The Schottky diode must have a peak
reverse voltage that is equal to regulator input voltage and
sized for average forward current in normal operation.
Average forward current can be calculated from:
OUT1
I
D(AVG)
, you may not be able to tolerate this loss. Use X7R
=
I
OUT1
V
IN
• V
(
IN
− V
OUT1
)
3500fc