LTC1174 查看數據表（PDF） - Linear Technology

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LTC1174

High Efficiency Step-Down and Inverting DC/DC Converter

Linear Technology 

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

8

100

CTX100-4

90

CTX100-4P

80

70

60

50

1

100

90

80

VIN = 5V

VOUT = 3.3V

IPGM = VIN

10

100

500

LOAD CURRENT (mA)

CTX50-4

CTX50-4P

70

60

VIN = 5V

VOUT = 3.3V

IPGM = VIN

50

1

10

100

500

LOAD CURRENT (mA)

1174 F01

Figure 1. Efficiency Using Different Types of

Inductor Core Material

a premium larger gauge wire can be used to reduce the wire

resistance. This also prevents excessive heat dissipation.

CIN

In continuous mode the source current of the P-channel

MOSFET is a square wave of duty cycle VOUT/VIN. To prevent

large voltage transients, a low ESR input capacitor sized for

the maximum RMS current must be used. The CIN RMS

current is given by:

[ ] ( ) IRMS

≈ IOUT

VOUT

VIN − VOUT

VIN

1/ 2

(ARMS)

This formula has a maximum at VIN = 2VOUT, where IRMS =

IOUT/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 VIN for high

frequency decoupling.

COUT

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:

( ) IRMS

≈

IPEAK

2

A RMS

= 170mA 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

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