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LTC1474/LTC1475

APPLICATIONS INFORMATION

ments. Lower peak currents have the advantage of lower

output ripple (

∆VOUT = IPEAK • ESR), lower noise, and less

stress on alkaline batteries and other circuit components.

Also, lower peak currents allow the use of inductors with

smaller physical size.

Peak currents as low as 10mA can be programmed with

the appropriate sense resistor. Increasing RSENSE above

Ω, however, gives no further reduction of IPEAK.

For RSENSE values less than 1Ω, it is recommended that

the user parallel standard resistors (available in values

≥

Ω) instead of using a special low valued shunt resistor.

Although a single resisor could be used with the desired

value, these low valued shunt resistor types are much

more expensive and are currently not available in case

sizes smaller than 1206. Three or four 0603 size standard

resistors require about the same area as one 1206 size

current shunt resistor at a fraction of the cost.

At higher supply voltages and lower inductances, the peak

currents may be slightly higher due to current comparator

overshoot and can be predicted from the second term in

the following equation:

PEAK

SENSE

OUT

()







−

()

−

025

25 10

(2)

Note that RSENSE only sets the maximum inductor current

peak. At lower dI/dt (lower input voltages and higher

inductances), the observed peak current at loads less than

IMAX may be less than this calculated peak value due to the

voltage comparator tripping before the current ramps up

high enough to trip the current comparator. This effect

improves efficiency at lower loads by keeping the I2R

losses down (see Efficiency Considerations section).

Inductor Value Selection

Once RSENSE and IPEAK are known, the inductor value can

be determined. The minimum inductance recommended

as a function of IPEAK and IMAX can be calculated from:

VV t

MIN

OUT

D OFF

PEAK

MAX

≥

()

−













075

(3)

where tOFF = 4.75µs.

MAXIMUM OUTPUT CURRENT (mA)

100

150

200

1474/75 F02

250

300

FOR LOWEST NOISE

FOR BEST EFFICIENCY

Figure 2. RSENSE Selection

The basic LTC1474/LTC1475 application circuit is shown

in Figure 1, a high efficiency step-down converter. External

component selection is driven by the load requirement and

begins with the selection of RSENSE. Once RSENSE is

known, L can be chosen. Finally D1, CIN and COUT are

selected.

RSENSE Selection

The current sense resistor (RSENSE) allows the user to

program the maximum inductor/switch current to opti-

mize the inductor size for the maximum load. The LTC1474/

LTC1475 current comparator has a maximum threshold of

100mV/(RSENSE + 0.25). The maximum average output

current IMAX is equal to this peak value less half the peak-

to-peak ripple current

∆IL.

Allowing a margin for variations in the LTC1474/LTC1475

and external components, the required RSENSE can be

calculated from Figure 2 and the following equation:

RSENSE = (0.067/IMAX) – 0.25

(1)

for 10mA < IMAX < 200mA.

For IMAX above 200mA, RSENSE is set to zero by shorting

Pins 6 and 7 to provide the maximum peak current of

400mA (limited by the fixed internal sense resistor). This

400mA default peak current can be used for lower IMAX if

desired to eliminate the need for the sense resistor and

associated decoupling capacitor. However, for optimal

performance, the peak inductor current should be set to no

more than what is needed to meet the load current require-

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