LTC3406AB Datasheet, PDF (12/16 Pages) Linear Technology – 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406AB
APPLICATIONS INFORMATION
1
RUN
5
V_FB
LTC3406AB
R2
2
–
GND
V_OUT
+
C_OUT
L1
3
SW
4
V_IN
C_IN
BOLD LINES INDICATE HIGH CURRENT PATHS
R1
C_FWD
+
V_IN
–
3406AB F05a
Figure 3. LTC3406AB Layout Diagram
V_OUT
VIA TO V_IN
R1
PIN 1
L1 ^SW
LTC3406AB
R2
C_FWD
V_IN
VIA TO V_OUT
C_OUT
C_IN
GND
3406AB F06a
Figure 4. LTC3406AB Suggested Layout
Design Example
As a design example, assume the LTC3406AB 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 ) ( ΔI_L
)
V_OUT
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
(3)
Substituting V_OUT = 2.5V, V_IN = 4.2V, ΔI_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.
12
3406abfa