7

LTC1504

EXTERNAL COMPONENT SELECTION

External components required by the LTC1504 fall into

three categories: input bypass, output filtering and com-

pensation. Additional components to set up soft start and

current limit are usually included as well. A minimum

LTC1504 circuit can be constructed with as few as four

external components; a circuit that utilizes all of the

LTC1504s functionality usually includes eight or nine

external components, with two additional feedback resis-

tors required for adjustable parts. See the Typical Applica-

tions section for examples of external component hookup.

Input Bypass

The input bypass capacitor is critical to proper LTC1504

operation. The LTC1504 includes a precision reference

and a pair of high power switches feeding from the same

the reference voltage and the LTC1504 will not regulate

accurately. Symptoms of inadequate bypassing include

poor load regulation and/or erratic waveforms at the SW

pin. If an oscilloscope won’t trigger cleanly when looking

at the SW pin, there isn’t adequate input bypass.

Ideally, the LTC1504 requires a low impedance bypass

right at the chip and a larger reservoir capacitor that can be

located somewhat farther away. This requirement usually

can be met with a ceramic capacitor right next to the

LTC1504 and an electrolytic capacitor (usually 10

µF to

100

µF,dependingonexpectedloadcurrent)locatedsome-

where nearby. In certain cases, the bulk capacitance

requirement can be met by the output bypass of the input

supply. Applications running at very high load currents or

at input supply voltages greater than 6V may require the

local ceramic capacitor to be 1

µF or greater. In some

cases, both the low impedance and bulk capacitance

requirements can be met by a single capacitor, mounted

very close to the LTC1504. Low ESR organic semiconduc-

tor (OS-CON) electrolytic capacitors or surge tested sur-

face mount tantalum capacitors can have low enough

impedance to keep the LTC1504 happy in some circuits.

Often the RMS current capacity of the input bypass capaci-

tors is more important to capacitor selection than value.

Buck converters like the LTC1504 are hard on input

capacitors, since the current flow alternates between the

full load current and near zero during every clock cycle. In

current flow in the input capacitor is half of the total load

current plus half the ripple current in the inductor—

perhaps 300mA in a typical 500mA load current applica-

tion. This current flows through the ESR of the input

bypass capacitor, heating it up and shortening its life,

sometimes dramatically. Many ordinary electrolytic ca-

pacitors that look OK at fist glance are not rated to

withstand such currents—check the RMS current rating

before you specify a device! If the RMS current rating isn’t

specified, it should not be used as an input bypass

capacitor. Again, low ESR electrolytic and surge tested

tantalums usually do well in LTC1504 applications and

have high RMS current ratings. The local ceramic bypass

capacitor usually has negligible ESR allowing it to with-

stand large RMS currents without trouble. Table 1 shows

typical surface mount capacitors that make acceptable

input bypass capacitors in LTC1504 applications.

Table 1. Representative Surface Mount Input Bypass Capacitors

PART

VALUE

ESR

MAX RMS

TYPE

HEIGHT

AVX

TPSC226M016R0375

22

µF 0.38Ω

0.54A

Tantalum

2.6mm

TPSD476M016R0150

47

µF 0.15Ω

0.86A

Tantalum

2.9mm

TPSE107M016R0125

100

µF 0.13Ω

1.15A

Tantalum

4.1mm

1206YC105M

1

µF

Low

>1A

X7R Ceramic 1.5mm

1210YG106Z

10

µF

Low

>1A

Y5V Ceramic 1.7mm

Sanyo

16SN33M

33

µF 0.15Ω

1.24A

OS-CON

7mm

16SN68M

68

µF

0.1

Ω

1.65A

OS-CON

7mm

16CV100GX

100

µF 0.44Ω

0.23A*

Electrolytic

6mm

16CV220GX

220

µF 0.34Ω

0.28A*

Electrolytic 7.7mm

Sprague

593D476X0016D2W

47

µF 0.17Ω

0.93A

Tantalum

2.8mm

593D107X0016E2W

100

µ 0.15Ω

1.05A

Tantalum

4mm

*Note: Use multiple devices in parallel or limit output current to prevent capacitor overload.

Inductor

The LTC1504 requires an external inductor to be con-

nected from the switching node SW to the output node

where the load is connected. Inductor requirements are

fairly straightforward; it must be rated to handle continu-

ous DC current equal to the maximum load current plus