13

LTC1474/LTC1475

Figure 9. LTC1474/LTC1475 Layout Diagram (See Board Layout Checklist)

+

+

10pF

1000pF

R2

100k

LTC1474

R1

D1

LBO

LBI

GND

RUN

SENSE

SW

L

1474/75 F09

8

7

6

5

1

2

3

4

BOLD LINES INDICATE HIGH PATH CURRENTS

0.1

µF

OUTPUT DIVIDER REQUIRED WITH

ADJUSTABLE VERSION ONLY

5. Are the signal and power grounds segregated? The

the LTC1474/LTC1475 and the resistive divider. The

power ground consists of the Schottky diode anode,

6. Is a 100k resistor connected in series between RUN

(Pin 8) and the RUN control voltage? The resistor

should be as close as possible to Pin 8.

Selection)

this information, we can easily calculate all the important

components:

From the equation (1),

Using the standard resistors (1

Ω, 1Ω and 2Ω) in parallel

provides 0.4

Ω without having to use a more expensive

tion section).

calculated from (2), neglecting the second term, to be

150mA. The minimum inductance is, therefore, from the

L

s

H

+

()(

)

−

=

075 3 3 0 4 4 75

015 0 1

264

..

.

.

..

µ

µ

From Figure 3, an inductance of 270

µH is chosen from the

recommended region. The CDRH73-271 or CD54-271 is a

good choice for space limited applications.

For the feedback resistors, choose R1 = 1M to minimize

supply current. R2 can then be calculated from the equa-

tion (4) to be:

R

V

RM

OUT

2

123

11 1 43

=−









•=

.

.

For the catch diode, the MBR0530 will work well in this

application.

For the input and output capacitors, AVX 4.7

µF and 100µF,

respectively, low ESR TPS series work well and meet the

RMS current requirement of 100mA/2 = 50mA. They are

available in small “C” case sizes with 0.15

Ω ESR. The

0.15

Ω output capacitor ESR will result in 25mV of output

voltage ripple.

Figure 10 shows the complete circuit for this example.