LTC3410B_15 Datasheet, PDF (13/16 Pages) Linear Technology – 2.25MHz, 300mA Synchronous Step-Down Regulator in SC70

LTC3410B
APPLICATIO S I FOR ATIO
Design Example
As a design example, assume the LTC3410B 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 0.3A 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
=
(f
1
)(∆I_L
)
V_OUT
⎛
⎝⎜
1−
V_OUT
V_IN
⎞
⎠⎟
(3)
Substituting V_OUT = 2.5V, V_IN = 4.2V, ∆I_L = 100mA
and f = 2.25MHz in Equation (3) gives:
For best efficiency choose a 300mA or greater inductor
with less than 0.3Ω series resistance.
C_IN will require an RMS current rating of at least 0.125A ≅
I_LOAD(MAX)/2 at temperature and C_OUT will require an ESR
of less than 0.5Ω. In most cases, a ceramic capacitor will
satisfy this requirement.
For the feedback resistors, choose R1 = 412k. R2 can
then be calculated from equation (2) to be:
R2 =
⎛⎝⎜
V_OUT
0.8
− 1^{⎞⎠⎟} R1=
875.5k;
use
887k
Figure 6 shows the complete circuit along with its
efficiency curve.
L
=
2.5V
2.25MHz(100mA)
⎛⎝⎜_1−
2.5V
4.2V
⎞⎠⎟
=
4.5µH
100
90
80
70
60
50
40
30
20
10
0
1
V_IN
2.7V
TO 4.2V
C_IN^†
2.2µF
CER
4
V_IN
3
SW
LTC3410B
1
RUN
6
V_FB
GND
2, 5
4.7µH*
10pF
V_OUT
2.5V
C_OUT^†
2.2µF
CER
887k
412k
3410 F07a
†_{TAIYO YUDEN JMK212BJ225}
*MURATA LQH32CN4R7M23
Figure 6a
V_IN = 2.7V
V_IN = 3.6V
V_IN = 4.2V
10
100
1000
OUTPUT CURRENT (mA)
3410 F07b
Figure 6b
V_OUT
100mV/DIV
AC COUPLED
I_L
200mA/DIV
I_LOAD
200mA/DIV
V_IN = 3.6V
4µs/DIV
V_OUT = 2.5V
I_LOAD = 100mA TO 300mA
Figure 6c
3410 F07c
3410bfa
13