LTC1174-3.3 Linear Technology, LTC1174-3.3 Datasheet - Page 7

• Manufacturer Part Number: LTC1174-3.3
• Description: High Efficiency Step-Down and Inverting DC/DC Converter
• Manufacturer: Linear Technology

OPERATIO
circuitry is shut off, reducing quiescent current from
0.45mA to 0.13mA. When the output capacitor discharges
by the amount of the hysteresis of the comparator A1, the
P-channel switch turns on again and the process repeats
itself.
Operating Frequency and Inductor
Since the LTC1174 utilizes a constant off-time architecture,
its operating frequency is dependent on the value of V
frequency of operation can be expressed as:
A
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 M 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 M 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
a premium larger gauge wire can be used to reduce the wire
resistance. This also prevents excessive heat dissipation.
C
In continuous mode the source current of the P-channel
MOSFET is a square wave of duty cycle V
large voltage transients, a low ESR input capacitor sized for
IN
PPLICATI
f
t
OFF
1
V
IN
V
IN
O
U
V
OUT
V
U
D
(Refer to Functional Diagram)
S
I FOR ATIO
U
Hz
W
OUT
/V
IN
. To prevent
U
IN
. The
where t
Note that the operating frequency is a function of the input
and ouput voltage.
Although the size of the inductor does not affect the fre-
quency, it does affect the ripple current. The peak-to-peak
ripple current is given by:
By choosing a smaller inductor, a low ESR output filter
capacitor has to be used (see C
loss will also increase (see Inductor Core Selection section)
due to higher ripple current.
I
RIPPLE
OFF
Figure 1. Efficiency Using Different Types of
Inductor Core Material
= 4 s and V
100
100
90
80
70
60
50
90
80
70
60
50
4 10
1
1
LTC1174-3.3/LTC1174-5
6
D
LOAD CURRENT (mA)
LOAD CURRENT (mA)
V
is the voltage drop across the diode.
OUT
10
10
L
CTX100-4P
CTX100-4
CTX50-4P
CTX50-4
IN
V
D
and C
V
V
I
V
V
I
PGM
100
PGM
100
IN
OUT
IN
OUT
OUT
= 5V
= 5V
A
= V
= V
= 3.3V
= 3.3V
P P
). Moreover, core
IN
IN
1174 F01
500
500
LTC1174
7