LTC1877 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1877' PDF : 16 Pages View PDF
APPLICATIONS INFORMATION
CC2
OPTIONAL
RC CC1
LTC1877
1
RUN
8
PLL LPF
2
7
ITH SYNC/MODE
3
VFB
R2 VIN 6
4
GND
5
SW
BOLD LINES INDICATE
HIGH CURRENT PATHS
+
L1
+
R2 +
VOUT +
VIN
COUT
CIN
R1
–
–
1877 F07
LTC1877
Figure 7. LTC1877 Layout Diagram
LTC1877. The power ground traces should be kept
short, direct and wide. The signal ground and power
ground should converge to a common node in a star-
ground configuration.
2. Does the VFB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of COUT and signal ground.
3. Does the (+) plate of CIN connect to VIN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node SW away from sensitive small
signal nodes.
Design Example
As a design example, assume the LTC1877 is used in a
single lithium-ion battery-powered cellular phone applica-
tion. The input voltage will be operating from a maximum
of 4.2V down to about 2.7V. The load current requirement
is a maximum of 0.3A but most of the time it will be in
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
=
1
(f)(∆IL
)
VOUT
1−
VOUT
VIN
(3)
Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL=120mA and
f = 550kHz in equation (3) gives:
L
=
2.5V
550kHz(120mA)
1−
2.5V
4.2V
=
15.3µH
A 15µH inductor works well for this application. For best
efficiency choose a 1A inductor with less than 0.25Ω
series resistance.
CIN will require an RMS current rating of at least 0.15A at
temperature and COUT will require an ESR of less than
0.25Ω. In most applications, the requirements for these
capacitors are fairly similar.
For the feedback resistors, choose R1 = 412k. R2 can
then be calculated from equation (2) to be:
R2
=
VOUT
0.8
– 1 R1= 875.5k; use
887k
Figure 8 shows the complete circuit along with its effi-
ciency curve.
13
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