LTC1707_15 Datasheet, PDF (12/16 Pages) Linear Technology – High Efficiency Monolithic Synchronous Step-Down Switching Regulator

LTC1707
APPLICATIO S I FOR ATIO
Design Example
As a design example, assume the LTC1707 is used in a
single lithium-ion battery-powered cellular phone applica-
tion. The V_IN will be operating from a maximum of 4.2V
down to about 2.85V. 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 = 120mA and
f = 350kHz in equation (3) gives:
A 22µH inductor works well for this application. For best
efficiency choose a 1A inductor with less than 0.25Ω
series resistance.
C_IN will require an RMS current rating of at least 0.15A at
temperature and C_OUT will require an ESR of less than
0.25Ω. In most applications, the requirements for these
capacitors are fairly similar.
For the feedback resistors, choose R1 = 80.6k. R2 can then
be calculated from equation (2) to be:
R2
=


V_OUT
0.8
−

1_
R1 =
171k;
use
169k
Figure 8 shows the complete circuit along with its effi-
ciency curve.
( )( ) ^{L =}
2.5V
350kHz 120mA

¹
−
2.5V
4.2V


=
24.1µH
C_ITH
47pF
C_SS
0.1µF
1
I_TH
8
V_REF
2 _{RUN/SS SYNC/MODE} 7
3
LTC1707
V_FB
V_IN ⁶
4
GND
SW ⁵
22µH*
R2
169k
1%
R1
80.6k
1%
* SUMIDA CD54-220
† _{AVX TPSC107M006R0150}
†† _{AVX TPSC226M016R0375}
V_OUT
2.5V
0.3A
V_IN
2.85V TO
4.5V
+ C_OUT† + C_IN††
100µF
22µF
6.3V
16V
1707 F08a
100
V_IN = 3.6V
90
V_IN = 4.2V
80
70
60 _{VOUT = 2.5V}
L = 22µH
Burst Mode OPERATION
50
1
10
100
OUTPUT CURRENT (mA)
Figure 8. Single Lithium-Ion to 2.5V/0.3A Regulator from Design Example
1000
1707 F08b
12