LTC3406B-2 Datasheet, PDF (12/16 Pages) Linear Technology – 2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOT

LTC3406B-2
APPLICATIO S I FOR ATIO
Design Example
As a design example, assume the LTC3406B-2 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 =}
f
1
∆I_L
V_OUT
⎛
⎝⎜^1−
V_OUT
V_IN
⎞
⎠⎟
(3)
Substituting V_OUT = 2.5V, V_IN = 4.2V, ∆I_L = 240mA and
f = 2.25MHz in equation (3) gives:
L
=
1.5MH2z.(52V40mA) ^{⎛⎝⎜1 −}
2.5V ^⎞
4.2V ⎠⎟
=
1.87µH
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.
For the feedback resistors, choose R1 = 316k. R2 can
then be calculated from equation (2) to be:
R2
=
⎛
⎝⎜
V_OUT
0.6
− 1^{⎞⎠⎟} R1= 1000k
Figure 7 shows the complete circuit along with its effi-
ciency curve.
V_IN
2.7V
TO 4.2V
C_IN^†
4.7µF
CER
4
V_IN
3
SW
LTC3406B-2
1
RUN
5
V_FB
GND
2
2.2µH*
22pF
1M
316k
3406B F07a
*MURATA LQH32CN2R2M33
** TAIYO YUDEN JHK316BJ106ML
† _{TAIYO YUDEN JMK212BJ475MG}
Figure 7a
V_OUT
2.5V
C_OUT**
10µF
CER
12
100
V_OUT = 2.5V
90 ^TA ^{= 25°C}
80
V_IN = 2.7V
70
V_IN = 4.2V
60
V_IN = 3.6V
50
40
30
20
10
0.1
1
10
100
OUTPUT CURRENT (mA)
1000
3406B G04
Figure 7b
sn3406b2 3406b2fs