LTC1504IS8-3.3#PBF Datasheet LTC1504CS8#PBF, LTC1504IS8#TRPBF, LTC1504IS8#PBF | P7-P9

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

pin. If V

does not have adequate bypassing, the

switch pulses introduce enough ripple at V

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 V

OUT

= 0.5V

) 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 22µF 0.38Ω 0.54A Tantalum 2.6mm

TPSD476M016R0150 47µF 0.15Ω 0.86A Tantalum 2.9mm

TPSE107M016R0125 100µF 0.13Ω 1.15A Tantalum 4.1mm

1206YC105M 1µF Low >1A X7R Ceramic 1.5mm

1210YG106Z 10µF Low >1A Y5V Ceramic 1.7mm

Sanyo

16SN33M 33µF 0.15Ω 1.24A OS-CON 7mm

16SN68M 68µF 0.1Ω 1.65A OS-CON 7mm

16CV100GX 100µF 0.44Ω 0.23A* Electrolytic 6mm

16CV220GX 220µF 0.34Ω 0.28A* Electrolytic 7.7mm

Sprague

593D476X0016D2W 47µF 0.17Ω 0.93A Tantalum 2.8mm

593D107X0016E2W 100µ 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

APPLICATIONS INFORMATION

WUU

LTC1504

APPLICATIONS INFORMATION

WUU

half the ripple current and its value should be chosen

based on the desired ripple current and/or the output

current transient requirements. Large value inductors

lower ripple current and decrease the required output

capacitance, but limit the speed that the LTC1504 can

change the output current, limiting output transient re-

sponse. Small value inductors result in higher ripple

currents and increase the demands on the output capaci-

tor, but allow faster output current slew rates and are often

smaller and cheaper for the same DC current rating. A

typical inductor used in an LTC1504 application might

have a maximum current rating between 500mA and 1A

and an inductance between 33µH and 220µH.

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost more

than powdered iron rod core inductors with similar electrical

characteristics. The choice of which style inductor to use

often depends more on the price vs size requirements and any

radiated field/EMI requirements than on what the LTC1504

requires to operate. Table 2 shows some typical surface

mount inductors that work well in LTC1504 applications.

Table 2. Representative Surface Mount Inductors

CORE CORE

PART VALUE MAX DC TYPE MATERIAL HEIGHT

CoilCraft

DT3316-473 47µH 1A Shielded Ferrite 5.1mm

DT3316-104 100µH 0.8A Shielded Ferrite 5.1mm

DO1608-473 47µH 0.5A Open Ferrite 3.2mm

DO3316-224 220µH 0.8A Open Ferrite 5.5mm

Coiltronics

CTX50-1 50µH 0.65A Toroid KoolMµ

4.2mm

CTX100-2 100µH 0.63A Toroid KoolMµ 6mm

CTX50-1P 50µH 0.66A Toroid Type 52 4.2mm

CTX100-2P 100µH 0.55A Toroid Type 52 6mm

Sumida

CDRH62-470 47µH 0.54A Shielded Ferrite 3mm

CDRH73-101 100µH 0.50A Shielded Ferrite 3.4mm

CD43-470 47µH 0.54A Open Ferrite 3.2mm

CD54-101 100µH 0.52A Open Ferrite 4.5mm

Output Capacitor

The output capacitor affects the performance of the

LTC1504 in a couple of ways: it provides the first line of

defense during a transient load step and it has a large effect

on the compensation required to keep the LTC1504 feed-

back loop stable. Transient load response of an LTC1504

circuit is controlled almost entirely by the output capacitor

and the inductor. In steady load operation, the average

current in the inductor will match the load current. When

the load current changes suddenly, the inductor is sud-

denly carrying the wrong current and requires a finite

amount of time to correct itself—at least several switch

cycles with typical LTC1504 inductor values. Even if the

LTC1504 had psychic abilities and could instantly assume

the correct duty cycle, the rate of change of current in the

inductor is still related to its value and will not change

instantaneously.

Until the inductor current adjusts to match the load cur-

rent, the output capacitor has to make up the difference.

Applications that require exceptional transient response

(2% or better for instantaneous full-load steps) will re-

quire relatively large value, low ESR output capacitors.

Applications with more moderate transient load require-

ments can often get away with traditional standard ESR

electrolytic capacitors at the output and can use larger

valued inductors to minimize the required output capaci-

tor value. Note that the RMS current in the output capacitor

is slightly more than half of the inductor ripple current—

much smaller than the RMS current in the input bypass

capacitor. Output capacitor lifetime is usually not a factor

in typical LTC1504 applications.

Large value ceramic capacitors used as output bypass

capacitors provide excellent ESR characteristics but can

cause loop compensation difficulties. See the Loop Com-

pensation section.

Loop Compensation

Loop compensation is strongly affected by the output

capacitor. From a loop stability point of view, the output

inductor and capacitor form a series RLC resonant circuit,

with the L set by the inductor value, the C by the value of

the output capacitor and the R dominated by the output

capacitor’s ESR. The amplitude response and phase shift

due to these components is compensated by a network of

Rs and Cs at the COMP pin to (hopefully) close the

feedback loop in a stable manner. Qualitatively, the L and

Kool Mµ is a registered trademark of Magnetics, Inc..

LTC1504

APPLICATIONS INFORMATION

WUU

C of the output stage form a 2nd order roll-off with 180° of

phase shift; the R due to ESR forms a single zero at a

somewhat higher frequency that reduces the roll-off to

first order and reduces the phase shift to 90°.

If the output capacitor has a relatively high ESR, the zero

comes in well before the initial phase shift gets all the way

to 180° and the loop only requires a single small capacitor

from COMP to GND to remain stable (Figure 4a). If, on the

other hand, the output capacitor is a low ESR type to

maximize transient response, the ESR zero can increase in

frequency by a decade or more and the output stage phase

shift can get awfully close to 180° before it turns around

and comes back to 90°. Large value ceramic, OS-CON

electrolytic and low impedance tantalum capacitors fall

into this category. These loops require an additional zero

to be inserted at the COMP pin; a series RC in parallel with

a smaller C to ground will usually ensure stability. Figure

4b shows a typical compensation network which will

optimize transient response with most output capacitors.

Adjustable output parts can add a feedforward capacitor

across the feedback resistor divider to further improve

phase margin. The typical applications in this data sheet

COMP

*ADJUSTABLE PARTS ONLY

1504 • F04a

LTC1504

FB2

FB1

OUT

COMP

*ADJUSTABLE PARTS ONLY

1504 • F04b

LTC1504

FB2

FB1

OUT

Figure 4a. Minimum Compensation Network

Figure 4b. Optimum Compensation Network

show compensation values that work with several combi-

nations of external components—use them as a starting

point. For complex cases or stubborn oscillations, contact

the LTC Applications Department.

External Schottky Diode

An external Schottky diode can be included across the

internal N-channel switch (Q2) to improve efficiency at

heavy loads. The diode carries the inductor current during

the nonoverlap time while the LTC1504 turns Q1 off and

Q2 on and prevents current from flowing in the intrinsic

body diode in parallel with Q2. This diode will improve

efficiency by a percentage point or two as output current

approaches 500mA and can help minimize erratic behav-

ior at very high peak current levels caused by excessive

parasitic current flow through Q2. A Motorola MBRS0530L

is usually adequate, with the cathode connected to SW and

the anode connected to GND. Note that this diode is not

required for normal operation and has a negligible effect

on efficiency at low (< 250mA) output currents.

Soft Start and Current Limit

Soft start and current limit are linked in the LTC1504. Soft

start works in a straightforward manner. An internal 12µA

current source connected to the SS pin will pull up an

external capacitor connected from SS to GND at a rate

determined by the capacitor value. COMP is clamped to a

voltage one diode drop above SS; as SS rises, COMP will

rise at the same rate. When COMP reaches roughly 2V

below V

, the duty cycle will slowly begin to increase until

the output comes into regulation. As SS continues to rise,

the feedback amplifier takes over at COMP, the clamp

releases and SS rises to V

. During a soft start cycle, the

MIN feedback comparator is disabled to prevent it from

overriding the COMP pin and forcing the output to maxi-

mum duty cycle.

Current limit operates by pulling down on the soft start pin

when it senses an overload condition at the output. The

current limit amplifier (I

LIM

) compares the voltage drop

across the internal P-channel switch (Q1) during its on

time to the voltage at the I

MAX

pin. I

MAX

includes an internal

12µA pull-down, allowing the voltage to be set by a single

resistor between V

and I

MAX

. When the IR drop across

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LTC1504IS8-3.3#PBF

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Description:

Switching Voltage Regulators 500mA L V Buck Sync Sw Reg

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