13
LTC1474/LTC1475
APPLICATIONS INFORMATION
WUU
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Figure 9. LTC1474/LTC1475 Layout Diagram (See Board Layout Checklist)
+
+
V
OUT
V
IN
V
IN
R
SENSE
V
FB
C
OUT
C
IN
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
0.1µF
OUTPUT DIVIDER REQUIRED WITH
ADJUSTABLE VERSION ONLY
5. Are the signal and power grounds segregated? The
signal ground consists of the (–) plate of C
OUT
, Pin 4 of
the LTC1474/LTC1475 and the resistive divider. The
power ground consists of the Schottky diode anode,
the (–) plate of C
IN
and the 0.1µF decoupling capacitor.
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.
Design Example (Refer to R
SENSE
and Inductor
Selection)
As a design example, assume V
IN
= 10V, V
OUT
= 3V, and
a maximum average output current I
MAX
= 100mA. With
this information, we can easily calculate all the important
components:
From the equation (1),
R
SENSE
= (0.067/0.1) – 0.25 = 0.42Ω
Using the standard resistors (1Ω, 1Ω and 2Ω) in parallel
provides 0.4Ω without having to use a more expensive
low value current shunt type resistor (see R
SENSE
Selec-
tion section).
With R
SENSE
= 0.4Ω, the peak inductor current I
PEAK
is
calculated from (2), neglecting the second term, to be
150mA. The minimum inductance is, therefore, from the
equation (3) and assuming V
D
= 0.4V,
L
s
H
MIN
=
+
()()
−
=
07533 04 475
015 01
264
....
..
µ
µ
From Figure 3, an inductance of 270µH 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
11143=−
•=
.
.
For the catch diode, the MBR0530 will work well in this
application.
For the input and output capacitors, AVX 4.7µF 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 0.15Ω ESR. The
0.15Ω output capacitor ESR will result in 25mV of output
voltage ripple.
Figure 10 shows the complete circuit for this example.
