13
LT3150
3150f
Table 1. Inductor Vendors
VENDOR
PHONE
URL
PART NUMBER
INDUCTANCE
DCR
I
SAT
Murata
(404) 436-1300
www.murata.com
LQH31CN100K03K
10礖
1.3&
0.23A
LQH32CN100K23K
10礖
0.44&
0.3A
Panasonic
(800) 344-2112
www.panasonic.com
ELJPC100KF
10礖
2.2&
0.21A
Sumida
(847) 956-0666
www.sumida.com
CDRH3D16100
10礖
0.21&
0.55A
Taiyo Yuden
(800) 348-2496
www.t-yuden.com
LQLB2016T100M
10礖
0.5&
0.155A
LQLB2518T100M
10礖
0.25&
0.165A
Toko
(847) 297-0070
www.toko.com
LLM3225-100K
10礖
1.7&
0.22A
APPLICATIO S I FOR ATIO
U
U
U
V
IN1
pin. ESR is not critical and in most cases, an inexpen-
sive tantalum or ceramic capacitor with a value from 1礔
to 4.7礔 is appropriate.
Output Capacitor
The output capacitor choice is far more important. The
capacitors 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 capacitors finite
output impedance. The capacitor must have low imped-
ance at the 1.4MHz switching frequency of the LT3150. At
this frequency, the capacitors 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
V
IN2
input bypass capacitor. Place this capacitor physi-
cally close to the V
IN2
pin. This capacitor supplies the
instantaneous current to slew the external MOSFETs
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礔 to 4.7礔)
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.