10

LTC3404

3404fb

GND

LTC3404

0.8V

R2

R1

3404 F03

Figure 3. Setting the LTC3404 Output Voltage

APPLICATIO S I FOR ATIO

inductor ripple current and consequent output voltage

ripple. Do not allow the core to saturate!

Kool M

μ (from Magnetics, Inc.) is a very good, low loss

core material for toroids with a “soft” saturation character-

istic. Molypermalloy is slightly more efficient at high

(>200kHz) switching frequencies but quite a bit more

expensive. Toroids are very space efficient, especially

when you can use several layers of wire, while inductors

wound on bobbins are generally easier to surface mount.

New designs for surface mount inductors are available

from Coiltronics, Coilcraft, Dale and Sumida.

In continuous mode, the source current of the top MOS-

large voltage transients, a low ESR input capacitor sized

for the maximum RMS current must be used. The maxi-

mum RMS capacitor current is given by:

CI

VV

V

V

IN

OMAX

OUT

IN

OUT

IN

−

()

monly used for design because even significant deviations

do not offer much relief. Note that the capacitor

manufacturer’s ripple current ratings are often based on

2000 hours of life. This makes it advisable to further derate

the capacitor, or choose a capacitor rated at a higher

temperature than required. Several capacitors may also be

paralleled to meet size or height requirements in the

design. Always consult the manufacturer if there is any

question.

series resistance (ESR). Typically, once the ESR require-

ment is satisfied, the capacitance is adequate for filtering.

The output ripple

Δ≅ Δ

+

⎛

⎝⎜

⎞

⎠⎟

V

I ESR

fC

OUT

L

OUT

1

8

and

is highest at maximum input voltage since

with input voltage. For the LTC3404, the general rule for

proper operation is:

The choice of using a smaller output capacitance

increases the output ripple voltage due to the frequency

dependent term but can be compensated for by using

capacitor(s) of very low ESR to maintain low ripple

optimized to provide stable high performance transient

response regardless of the output capacitor selected.

ESR is a direct function of the volume of the capacitor.

Manufacturers such as Taiyo-Yuden, AVX, Kemet, Sprague

and Sanyo should be considered for high performance

capacitors. The POSCAP solid electrolytic chip capacitor

available from Sanyo is an excellent choice for output bulk

capacitors due to its low ESR/size ratio. Once the ESR

When using tantalum capacitors, it is critical that they are

surge tested for use in switching power supplies. A good

choice is the AVX TPS series of surface mount tantalum,

available in case heights ranging from 2mm to 4mm. Other

capacitor types include KEMET T510 and T495 series and

Sprague 593D and 595D series. Consult the manufacturer

for other specific recommendations.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

VV

R

R

⎛

⎝⎜

⎞

⎠⎟

08

1

2

1

.

(2)

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 3.