LT3640 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LT3640

Dual Monolithic Buck Regulator with Power-On Reset and Watchdog Timer

Linear Technology 

LT3640

Applications Information

To guarantee sufficient output current, peak inductor cur-

rent must be lower than the switch current limit (ILIM).

The largest inductor current ripple occurs at the highest

VIN. To guarantee current capacity, use VIN(MAX) in the

above formula.

The inductance for the low voltage channel is:

L2

=

1.5

VOUT2

fS

For a selected inductance, the inductor current ripple can

be calculated:

∆IL2

=

VOUT2

L2 • fS

•





1–

VOUT2

VIN2





For robust operation in fault conditions, the inductor

saturation current should be higher than the upper limit

of the corresponding top switch current limit.

To keep the efficiency high, the inductor series resistance

(DCR) should be as small as possible (must be < 0.1Ω),

and the core material should be intended for the chosen

operation frequency. High efficiency converters generally

cannot afford the core loss found in low cost powdered

iron cores; instead use ferrite, molypermalloy or Kool Mµ

cores. Table 2 lists several vendors and suitable inductor

series.

Table 2. Inductor Vendors

PART SERIES

VENDOR

LQH55D

Murata

www.murata.com

SLF7045

SLF10145

TDK

www.componenttdk.com

D62CB, D63CB

D75C, D75F

TOKO

www.toko.com

CR54, CDRH74

CDRH6D38, CR75

Sumida

www.sumida.com

Of course, such a simple design guide will not always

result in the optimum inductors for the applications. A

larger value inductor provides a slightly higher maximum

load current and will reduce the output voltage ripple. A

larger value inductor also results in higher efficiency in the

condition of same DCR and same magnetic loss. However,

for a same series of inductors, a larger value inductor has

higher DCR. The trade-off between inductance and DCR

is not always obvious. Use experiments to find optimum

inductors.

Low inductance may result in discontinuous mode opera-

tion, which is okay, but reduces maximum load current.

For details of maximum output current and discontinuous

mode operation, see the Linear Technology Application Note

44. For duty cycles greater than 50%, there is a minimum

inductance required to avoid subharmonic oscillations.

See the Linear Technology Application Note 19.

Input Capacitor

Bypass the VIN pin of the LT3640 with a ceramic capacitor

of X7R (–55°C to 125°C) or X5R (–55°C to 85°C) type.

Buck converters draw pulse current from the input sup-

ply. The input capacitor is required to reduce the resulting

voltage ripple. Use a ceramic capacitor with:

CIN

≥

10µF

fS

where fS in the switching frequency in MHz.

A second precaution regarding the ceramic input capacitor

concerns the maximum input voltage rating of the LT3640.

A ceramic input capacitor combined with trace or cable

inductance forms a under damped tank circuit. If the LT3640

circuit is plugged into a live supply, the input voltage can

ring to twice its nominal value, possibly exceeding the

LT3640’s voltage rating. This situation can be easily avoided

(see the Linear Technology Application Note 80).

3640f

14

Share Link: