LTC1504CS8-3.3 View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1504CS8-3.3

500mA Low Voltage Step-Down Synchronous Switching Regulator

Linear Technology 

LTC1504

APPLICATIONS INFORMATION

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

VCC pin. If VCC does not have adequate bypassing, the

switch pulses introduce enough ripple at VCC to corrupt

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 10µF to

100µF, depending on expected load current) located some-

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 1µF 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

the worst case (50% duty cycle or VOUT = 0.5VIN) the RMS

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

TPSD476M016R0150

TPSE107M016R0125

1206YC105M

1210YG106Z

22µF

47µF

100µF

1µF

10µF

0.38Ω

0.15Ω

0.13Ω

Low

Low

0.54A

0.86A

1.15A

>1A

>1A

Tantalum 2.6mm

Tantalum 2.9mm

Tantalum 4.1mm

X7R Ceramic 1.5mm

Y5V Ceramic 1.7mm

Sanyo

16SN33M

16SN68M

16CV100GX

16CV220GX

33µF

68µF

100µF

220µF

0.15Ω

0.1Ω

0.44Ω

0.34Ω

1.24A

1.65A

0.23A*

0.28A*

OS-CON

OS-CON

Electrolytic

Electrolytic

7mm

7mm

6mm

7.7mm

Sprague

593D476X0016D2W

593D107X0016E2W

47µF 0.17Ω

100µ 0.15Ω

0.93A

1.05A

Tantalum 2.8mm

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

7

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