LTC3406-1.5_15 Datasheet, PDF (12/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406
LTC3406-1.5/LTC3406-1.8
APPLICATIO S I FOR ATIO
–
V_OUT
+
C_OUT
L1
1
RUN
5
V_FB
LTC3406
2
GND
3 _SW
4
V_IN
C_IN
R2
R1
C_FWD
V_IN
BOLD LINES INDICATE HIGH CURRENT PATHS
3406 F05a
Figure 5a. LTC3406 Layout Diagram
VIA TO GND
1
RUN
–
V_OUT
+
C_OUT
L1
LTC3406-1.8
2
5
GND V_OUT
3 _SW
4
V_IN
C_IN
V_IN
3406 F05b
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 5b. LTC3406-1.8 Layout Diagram
V_OUT
PIN 1
L1 ^SW
VIA TO V_IN
LTC3406
R1
R2
C_FWD
V_IN
VIA TO V_OUT
V_OUT
VIA TO V_OUT
V_IN
VIA TO V_IN
PIN 1
L1
SW
LTC3406-1.8
C_OUT
C_IN
GND
3406 F06a
Figure 6a. LTC3406 Suggested Layout
C_OUT
C_IN
GND
3406 F06b
Figure 6b. LTC3406-1.8 Suggested Layout
Design Example
As a design example, assume the LTC3406 is used in a
single lithium-ion battery-powered cellular phone
application. The V_IN will be operating from a maximum of
4.2V down to about 2.7V. The load current requirement
is a maximum of 0.6A 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)(^∆I_L )
V_OUT ^{⎛⎝⎜}1−
V_OUT
V_IN
⎞
⎠⎟
(3)
Substituting V_OUT = 2.5V, V_IN = 4.2V, ∆I_L = 240mA and
f = 1.5MHz in equation (3) gives:
L
=
2.5V
1.5MHz(240mA)
⎛⎝⎜_1−
2.5V ⎞
4.2V ⎠⎟
=
2.81µH
A 2.2µH inductor works well for this application. For best
efficiency choose a 720mA or greater inductor with less
than 0.2Ω series resistance.
C_IN will require an RMS current rating of at least 0.3A ≅
I_LOAD(MAX)/2 at temperature and C_OUT will require an ESR
of less than 0.25Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
3406fa
12