7

LTC1174

LTC1174-3.3/LTC1174-5

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:

I

VV

L

A

RIPPLE

OUT

D

PP

=×

+







−

−

410

6

By choosing a smaller inductor, a low ESR output filter

loss will also increase (see Inductor Core Selection section)

due to higher ripple current.

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,

frequency of operation can be expressed as:

f

t

VV

VV

Hz

OFF

IN

OUT

IN

D

=

−

+







1

S

APPLICATI

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 M

µ 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

µand

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.

In continuous mode the source current of the P-channel

large voltage transients, a low ESR input capacitor sized for

Figure 1. Efficiency Using Different Types of

Inductor Core Material

LOAD CURRENT (mA)

1

10

100

500

100

90

80

70

60

50

CTX50-4

CTX50-4P

1174 F01

LOAD CURRENT (mA)

1

10

100

500

100

90

80

70

60

50

CTX100-4

CTX100-4P