LTC1474CMS8 Linear Technology, LTC1474CMS8 Datasheet - Page 13

• Manufacturer Part Number: LTC1474CMS8
• Description: IC CONV STEP-DWN HI-EFF 8-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1474CMS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 750mA
• Voltage - Input: 3 ~ 18 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIONS
5. Are the signal and power grounds segregated? The
6. Is a 100k resistor connected in series between RUN
Design Example (Refer to R
Selection)
As a design example, assume V
a maximum average output current I
this information, we can easily calculate all the important
components:
From the equation (1),
Using the standard resistors (1 , 1 and 2 ) in parallel
provides 0.4 without having to use a more expensive
low value current shunt type resistor (see R
tion section).
With R
calculated from (2), neglecting the second term, to be
signal ground consists of the (–) plate of C
the LTC1474/LTC1475 and the resistive divider. The
power ground consists of the Schottky diode anode,
the (–) plate of C
(Pin 8) and the RUN control voltage? The resistor
should be as close as possible to Pin 8.
R
SENSE
SENSE
= (0.067/0.1) – 0.25 = 0.42
= 0.4 , the peak inductor current I
IN
U
and the 0.1 F decoupling capacitor.
INFORMATION
U
Figure 9. LTC1474/LTC1475 Layout Diagram (See Board Layout Checklist)
SENSE
V
OUT
IN
= 10V, V
+
and Inductor
W
BOLD LINES INDICATE HIGH PATH CURRENTS
MAX
C
OUT
= 100mA. With
R2
OUT
SENSE
OUT
R1
10pF
= 3V, and
U
, Pin 4 of
PEAK
Selec-
0.1 F
C
IN
1
2
3
4
+
OUTPUT DIVIDER REQUIRED WITH
ADJUSTABLE VERSION ONLY
is
V
LBO
LBI
GND
FB
LTC1474
150mA. The minimum inductance is, therefore, from the
equation (3) and assuming V
From Figure 3, an inductance of 270 H 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:
For the catch diode, the MBR0530 will work well in this
application.
For the input and output capacitors, AVX 4.7 F and 100 F,
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
0.15 output capacitor ESR will result in 25mV of output
voltage ripple.
Figure 10 shows the complete circuit for this example.
SENSE
RUN
L
R
D1
SW
V
IN
MIN
2
8
7
6
5
V
IN
100k
V
1 23
0 75 3 3 0 4 4 75
OUT
.
1000pF
1
0 15 0 1
R
SENSE
1474/75 F09
R
1 1 43
LTC1474/LTC1475
L
.
D
= 0.4V,
M
s
264
H
ESR. The
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