16
LTC1875
1875f
APPLICATIO S I FOR ATIO
WUUU
4. Place the small-signal components away from high
frequency switching nodes. In the layout shown in
Figure 8, all of the small-signal components have been
placed on one side of the IC and all of the power
components have been placed on the other.
5. For optimum load regulation and true sensing, the top
of the output resistor divider should connect indepen-
dently to the top of the output capacitor (Kelvin connec-
tion), staying away from any high dV/dt traces. Place
the divider resistors near the LTC1875 in order to keep
the high impedance FB node short.
Design Example
As a design example, assume the LTC1875 is used in a
single lithium-ion battery-powered cellular phone applica-
tion. The V
IN
will be operating from a maximum of 4.2V
down to about 2.65V. The load current requirement is a
maximum of 1.5A but most of the time it will be on standby
mode, requiring only 2mA. Efficiency at both low and high
load currents is important. Output voltage is 2.5V. With
this information we can calculate L using equation (1),
L
fI
V
V
V
L
OUT
OUT
IN
=
()
∆
()
1
1–
(3)
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, ∆I
L
= 450mA and
f = 550kHz in equation (3) gives:
L
V
kHz mA
V
V
H=
=µ
25
550 450
1
25
42
409
.
•
–
.
.
.
A 4.7µH inductor works well for this application. For good
efficiency choose a 2A inductor with less than 0.125Ω
series resistance.
C
IN
will require an RMS current rating of at least 0.75A at
temperature and C
OUT
will require an ESR of less than
0.125Ω. In most applications, the requirements for these
capacitors are fairly similar.
For the feedback resistors, choose R2 = 412k. R1 can then
be calculated from equation (2) to be:
R
V
R k use k
OUT
1
08
1 2 875 5 887=
=
.
–• .,
Figure 9 shows the complete circuit along with its effi-
ciency curve.
