lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 14

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LT3724
APPLICATIO S I FOR ATIO
14
Step-Down Converter: Input Capacitor Selection
A local input bypass capacitor is required for buck con-
verters because the input current is pulsed with fast rise
and fall times. The input capacitor selection criteria are
based on the bulk capacitance and RMS current capability.
The bulk capacitance will determine the supply input ripple
voltage. The RMS current capability is used to keep from
overheating the capacitor.
The bulk capacitance is calculated based on maximum
input ripple, ΔV
ΔV
100mV-200mV is a good starting point. Aluminum elec-
trolytic capacitors are a good choice for high voltage, bulk
capacitance due to their high capacitance per unit area.
The capacitor’s RMS current is:
If applicable, calculate it at the worst case condition, V
2V
by the manufacturer and should exceed the calculated
I
tance), ceramic capacitors are a good choice for high
voltage, high RMS current handling. Note that the ripple
current ratings from aluminum electrolytic capacitor manu-
facturers are based on 2000 hours of life. This makes it
advisable to further derate the capacitor or to choose a
capacitor rated at a higher temperature than required.
The combination of aluminum electrolytic capacitors and
ceramic capacitors is an economical approach to meeting
the input capacitor requirements. The capacitor voltage
rating must be rated greater than V
pacitors may also be paralleled to meet size or height
requirements in the design. Locate the capacitor very
close to the MOSFET switch and use short, wide PCB
traces to minimize parasitic inductance.
CIN(RMS)
OUT
IN
I
C
CIN RMS
IN BULK
is typically chosen at a level acceptable to the user.
. The RMS current rating of the capacitor is specified
(
(
. Due to their low ESR (Equivalent Series Resis-
)
)
=
=
I
OUT
IN
Δ
I
:
V
OUT MAX
IN
U
•
(
V
f
OUT IN
SW
U
)
•
(
V
(
•
V
V
V
IN MIN
IN
OUT
(
–
)
2
V
OUT
W
)
IN(MAX)
)
. Multiple ca-
U
IN
=
Step-Down Converter: Output Capacitor Selection
The output capacitance, C
design’s output voltage ripple, ΔV
requirements. ΔV
ESR. It is calculated by:
The maximum ESR required to meet a ΔV
requirement can be calculated by:
Worst-case ΔV
paralleled multiple capacitors to meet the ESR require-
ments. Increasing the inductance is an option to lower the
ESR requirements. For extremely low ΔV
LC filter stage can be added to the output of the supply.
Application Note 44 has some good tips on sizing an
additional output filter.
Output Voltage Programming
A resistive divider sets the DC output voltage according to
the following formula:
The external resistor divider is connected to the output of
the converter as shown in Figure 2. Tolerance of the
feedback resistors will add additional error to the output
voltage.
Example: V
R
R
2 10
Δ
ESR MAX
2
V
=
OUT
=
(
R
k
1
Ω
= Δ
⎛
⎜
⎝
OUT
1 231
⎛
⎜
⎝
)
V
.
1 231
=
I
OUT
.
L
12
OUT
= 12V; R1 = 10kΩ
V
•
V
OUT
V
OUT
⎛
⎜
⎝
V
ESR
(
occurs at highest input voltage. Use
Δ
–
−
V
1
is a function of ΔI
•
⎞
⎟
⎠
OUT
1
⎛
⎜
⎝
⎞
⎟ =
⎠
+
1
OUT
–
( •
)( )(
8
87 48
V
L f
IN MAX
, selection is based on the
V
f
.
SW
OUT
(
SW
k
OUT
1
•
Ω −
)
C
)
⎞
⎟
⎠
, and transient load
OUT
use
OUT
L
)
⎞
⎟
⎠
and the C
, an additional
86 6
OUT
.
k
Ω
design
1
3724fb
%
OUT