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LTC1474IS8-5

Low Quiescent Current High Efficiency Step-Down Converters

Linear Technology 

LTC1474/LTC1475

APPLICATIONS INFORMATION

If the LMIN calculated is not practical, a larger IPEAK should

be used. Although the above equation provides the mini-

mum, better performance (efficiency, line/load regulation,

noise) is usually gained with higher values. At higher

inductances, peak current and frequency decrease (im-

proving efficiency) and inductor ripple current decreases

(improving noise and line/load regulation). For a given

inductor type, however, as inductance is increased, DC

resistance (DCR) increases, increasing copper losses,

and current rating decreases, both effects placing an

upper limit on the inductance. The recommended range of

inductances for small surface mount inductors as a func-

tion of peak current is shown in Figure 3. The values in this

range are a good compromise between the trade-offs

discussed above. If space is not a premium, inductors with

larger cores can be used, which extends the recom-

mended range of Figure 3 to larger values.

1000

500

100

50

10

100

1000

PEAK INDUCTOR CURRENT (mA)

1474/75 F03

Figure 3. Recommended Inductor Values

Inductor Core Selection

Once the value of L is known, the type of inductor must be

selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite, molypermalloy

or Kool Mµ® cores. Actual core loss is independent of core

size for a fixed inductor value, but is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, as discussed in the previous

section, increased inductance requires more turns of wire

and therefore copper losses will increase.

Ferrite and Kool Mµ designs have very low core loss and

are preferred at high switching frequencies, so design

goals can concentrate on copper loss and preventing

saturation. Ferrite core material saturates “hard,” which

means that inductance collapses abruptly when the peak

design current is exceeded. This results in an abrupt

increase in inductor current above IPEAK and consequent

increase in voltage ripple. Do not allow the core to satu-

rate! Coiltronics, Coilcraft, Dale and Sumida make high

performance inductors in small surface mount packages

with low loss ferrite and Kool Mµ cores and work well in

LTC1474/LTC1475 regulators.

Catch Diode Selection

The catch diode carries load current during the off-time.

The average diode current is therefore dependent on the

P-channel switch duty cycle. At high input voltages the

diode conducts most of the time. As VIN approaches VOUT

the diode conducts only a small fraction of the time. The

most stressful condition for the diode is when the output

is short-circuited. Under this condition, the diode must

safely handle IPEAK at close to 100% duty cycle.

To maximize both low and high current efficiency, a fast

switching diode with low forward drop and low reverse

leakage should be used. Low reverse leakage current is

critical to maximize low current efficiency since the leak-

age can potentially approach the magnitude of the LTC1474/

LTC1475 supply current. Low forward drop is critical for

high current efficiency since loss is proportional to for-

ward drop. These are conflicting parameters (see Table 1),

but a good compromise is the MBR0530 0.5A Schottky

diode specified in the application circuits.

Table 1. Effect of Catch Diode on Performance

FORWARD

NO LOAD

DIODE (D1) LEAKAGE DROP SUPPLY CURRENT EFFICIENCY*

BAS85

200nA

0.6V

9.7µA

77.9%

MBR0530

1µA

0.4V

10µA

83.3%

MBRS130 20µA

0.3V

16µA

84.6%

*Figure 1 circuit with VIN = 15V, IOUT = 0.1A

8

Kool Mµ is a registered trademark of Magnetics, Inc.

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