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LT3150

3150f

Table 1. Inductor Vendors

VENDOR

PHONE

URL

PART NUMBER

INDUCTANCE

DCR

ISAT

Murata

(404) 436-1300

www.murata.com

LQH31CN100K03K

µH

1.3

Ω

0.23A

LQH32CN100K23K

µH

0.44

Ω

0.3A

Panasonic

(800) 344-2112

www.panasonic.com

ELJPC100KF

µH

2.2

Ω

0.21A

Sumida

(847) 956-0666

www.sumida.com

CDRH3D16100

µH

0.21

Ω

0.55A

Taiyo Yuden

(800) 348-2496

www.t-yuden.com

LQLB2016T100M

µH

0.5

Ω

0.155A

LQLB2518T100M

µH

0.25

Ω

0.165A

Toko

(847) 297-0070

www.toko.com

LLM3225-100K

µH

1.7

Ω

0.22A

APPLICATIO S I FOR ATIO

VIN1 pin. ESR is not critical and in most cases, an inexpen-

sive tantalum or ceramic capacitor with a value from 1

µF

to 4.7

µF is appropriate.

Output Capacitor

The output capacitor choice is far more important. The

capacitor’s characteristics determines output voltage ripple.

The output capacitor must have enough capacitance to

satisfy the load under transient conditions and it must

shunt the switched component of current coming through

the diode. Output voltage ripple results because this

switched current passes through the capacitor’s finite

output impedance. The capacitor must have low imped-

ance at the 1.4MHz switching frequency of the LT3150. At

this frequency, the capacitor’s equivalent series resis-

tance (ESR) usually dominates the impedance. Choosing

a capacitor with lower ESR results in lower output voltage

ripple.

However, consider loop stability when choosing the out-

put capacitor because the LT3150 is internally compen-

sated and no access is provided to this internal

compensation. Small, low cost tantalum capacitors have

some ESR. This ESR enhances stability due to the addition

of a zero in the regulator feedback loop. Ceramic capaci-

tors are very popular, having attractive characteristics

such as near-zero ESR, small size and low cost. Replacing

the tantalum output capacitor with a ceramic unit de-

creases the phase margin, in some cases to unacceptable

levels. The addition of a phase lead capacitor and an

isolating resistor in the feedback divider network can be

used to stabilize the feedback loop, but the added compo-

nent count and cost makes the use of a tantalum output

capacitor the simpler and preferred choice.

The boost regulator output capacitor also serves as the

VIN2 input bypass capacitor. Place this capacitor physi-

cally close to the VIN2 pin. This capacitor supplies the

instantaneous current to slew the external MOSFET’s

gate capacitance quickly during an output load current

transient.

LINEAR REGULATOR COMPONENT SELECTION

Output Capacitors

The LT3150 linear regulator is stable with a wide range of

output capacitors (assuming the feedback loop is prop-

erly frequency compensated). However, using multiple,

low value, very low ESR ceramic capacitors (1

µFto4.7µF)

in parallel optimizes the load transient response of an

LT3150 feedback loop. As is discussed in the Frequency

Compensation section, the output capacitor value is criti-

cal because it sets the location of a pole in the feedback

loop that determines the unity-gain crossover frequency.

Therefore, the characteristics of ceramic capacitors war-

rant some discussion.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with different behavior across tempera-

ture and applied voltage. The most common dielectrics

are Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics

provide high C-V products in a small package at low cost,

but exhibit very strong voltage and temperature coeffi-

cients. The X5R and X7R dielectrics yield highly stable

characteristics and are more suitable for use as the output

capacitor at fractionally increased cost. The X5R and X7R

dielectrics both exhibit excellent voltage coefficient charac-

teristics. The X7R type works over a larger temperature

range and exhibits better temperature stability whereas

X5R is less expensive and is available in higher values.

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