LTC3560_15 Datasheet, PDF (12/16 Pages) Linear Technology – 2.25MHz, 800mA Synchronous Step-Down Regulator in ThinSOT

LTC3560
APPLICATIONS INFORMATION
–
V_OUT
+
C_OUT
L1
1
6
RUN SYNC/MODE
LTC3560
2
GND
5
V_FB
3 _SW
V_IN ⁴
C_IN
R2
R1
C_FWD
V_IN
BOLD LINES INDICATE HIGH CURRENT PATHS
3560 F04
Figure 4. LTC3560 Layout Diagram
VIA TO GND
V_OUT
PIN 1
L1
SW
VIA TO V_IN
LTC3560
R1
R2
C_FWD
V_IN
VIA TO V_OUT
C_OUT
C_IN
GND
3560 F05
Figure 5. LTC3560 Suggested Layout
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 ) ₍ I_L
)
V_OUT


1

V_OUT
V_IN


(3)
Substituting V_OUT = 2.5V, V_IN = 4.2V, ΔI_L = 320mA and
f = 2.25MHz in equation (3) gives:
L
=
2.5V
2.25MHz(320mA)
₁

2.5V
4.2V



1.4μH
C_IN will require an RMS current rating of at least 0.4A ≅
I_LOAD(MAX)/2 at temperature and C_OUT will require an ESR
of less than 0.1Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
For the feedback resistors, choose R1 = 309k. R2 can
then be calculated from equation (2) to be:
R2 =


V_OUT
0.6

1^
R1=
978.5k;
use
976k
Figure 6 shows the complete circuit along with its ef-
ficiency curve.
A 1.5μH inductor works well for this application. For best
efficiency choose a 960mA or greater inductor with less
than 0.2Ω series resistance.
3560fb
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