LTC1174
LTC1174-3.3/LTC1174-5
APPLICATIO S I FOR ATIO
Inductor Core Selection
With the value of L selected, the type of inductor must be
chosen. Basically there are two kinds of losses in an
inductor, core and copper
Core losses are dependent on the peak-to-peak ripple
current and the core material. However it is independent of
the physical size of the core. By increasing the inductance
the inductor’s peak-to-peak ripple current will decrease,
therefore reducing core loss. Utilizing low core loss mate-
rial, such as molypermalloy or Kool Mµ will allow users to
concentrate on reducing copper loss and preventing satu-
ration. Figure 1 shows the effect of different core material on
the efficiency of the LTC1174. The CTX core is Kool Mµ and
the CTXP core is powdered iron (material 52).
Although higher inductance reduces core loss, it increases
copper loss as it requires more windings. When space is not
100
CTX100-4
CTX100-4P
80
90
EFFICIENCY (%)
70
60
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
1
10
100
LOAD CURRENT (mA)
500
50
100
CTX50-4
CTX50-4P
80
90
EFFICIENCY (%)
70
60
V
IN
= 5V
V
OUT
= 3.3V
I
PGM
= V
IN
1
10
100
LOAD CURRENT (mA)
500
1174 F01
50
Figure 1. Efficiency Using Different Types of
Inductor Core Material
8
U
a premium larger gauge wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation.
C
IN
In continuous mode the source current of the P-channel
MOSFET is a square wave of duty cycle V
OUT
/V
IN
. To prevent
large voltage transients, a low ESR input capacitor sized for
the maximum RMS current must be used. The C
IN
RMS
current is given by:
I
RMS
≈
I
OUT
V
OUT
(
V
IN
−
V
OUT
)
V
IN
W
U U
[
]
1/ 2
(
A
RMS
)
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst case is commonly used for design
because even significant deviations do not offer much relief.
Note that ripple current directly affects capacitor’s lifetime.
DO NOT UNDERSPECIFY THIS COMPONENT. An additional
0.1µF ceramic capacitor is also required on V
IN
for high
frequency decoupling.
C
OUT
To avoid overheating, the output capacitor must be sized to
handle the ripple current generated by the inductor. The
worst case RMS ripple current in the output capacitor is
given by:
I
RMS
≈
I
PEAK
(
A
RMS
)
2
=
170 mA or 300mA
Although the output voltage ripple is determined by the
hysteresis of the voltage comparator, ESR of the output
capacitor is also a concern. Too high of an ESR will create
a higher ripple output voltage and at the same time cause the
LTC1174 to sleep less often. This will affect the efficiency of
the LTC1174. For a given technology, ESR is a direct
function of the volume of the capacitor. Several small-sized
capacitors can also be paralleled to obtain the same ESR as
one large can. Manufacturers such as Nichicon, Chemicon
and Sprague should be considered for high performance
capacitors. The OS-CON semiconductor dielectric capaci-
tor available from Sanyo has the lowest ESR for its size, at
a higher price.
1174fe
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
