3724MPFE datasheet(14/24 Pages) LINER

14 / 24 page

LT3724

3724fc

APPLICATIONS INFORMATION

Step-Down Converter: Input Capacitor Selection

A local input bypass capacitor is required for buck convert-

ers 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, ΔVIN:

CIN(BULK) =

IOUT(MAX) •VOUT

VIN •fSW •VIN(MIN)

ΔVIN is typically chosen at a level acceptable to the user.

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:

ICIN(RMS) =IOUT

VOUT(VIN –VOUT)

(VIN)

If applicable, calculate it at the worst case condition,

VIN = 2VOUT. The RMS current rating of the capacitor

is speciﬁed by the manufacturer and should exceed the

calculated ICIN(RMS). Due to their low ESR (Equivalent

Series Resistance), ceramic capacitors are a good choice

for high voltage, high RMS current handling. Note that the

ripple current ratings from aluminum electrolytic capacitor

manufacturers 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 meet-

ing the input capacitor requirements. The capacitor volt-

age rating must be rated greater than VIN(MAX). Multiple

capacitors 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.

Step-Down Converter: Output Capacitor Selection

The output capacitance, COUT, selection is based on the

design’s output voltage ripple, ΔVOUT, and transient load

requirements. ΔVOUT is a function of ΔIL and the COUT

ESR. It is calculated by:

VOUT = IL •ESR +

(8 • fSW •COUT)

The maximum ESR required to meet a ΔVOUT design

requirement can be calculated by:

ESR(MAX)

( VOUT)(L)(fSW)

VOUT •1–

VOUT

VIN(MAX)

Worst-case ΔVOUT occurs at highest input voltage. Use

paralleled multiple capacitors to meet the ESR require-

ments. Increasing the inductance is an option to lower the

ESR requirements. For extremely low ΔVOUT, an additional

LC ﬁlter stage can be added to the output of the supply.

Application Note 44 has some good tips on sizing an ad-

ditional output ﬁlter.

Output Voltage Programming

A resistive divider sets the DC output voltage according

to the following formula:

=R1

VOUT

1.231V

–1

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: VOUT = 12V; R1 = 10kΩ

= 10k

12V

1.231V

= 87.48k

use 86.6k 1%

3724MPFE Datenblatt(PDF) 14 Page - Linear Technology

3724MPFE Datenblatt(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datenblatt Download

Link URL