LTC3448_15 Datasheet, PDF (14/20 Pages) Linear Technology – 1.5MHz/2.25MHz, 600mA Synchronous Step-Down Regulator with LDO Mode

LTC3448
APPLICATIO S I FOR ATIO
and high load currents is important. Output voltage is
1.8V. 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 = 1.8V, V_IN = 4.2V, ∆I_L = 240mA and
f = 1.5MHz in Equation (3) gives:
L
=
1.8V
1.5MHz(240mA)
⎛⎝⎜_1−
1.8V
4.2V
⎞⎠⎟
=
2.86 µH
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.
V_IN
2.7V
TO 5.5V
C_IN
4.7µF
CER
4
8
V_IN
RUN
5
SW
V_OUT ²
2.2µH*
LTC3448 ₃
MODE
6
1
FREQ
7 _SYNC
V_FB
22pF 632k
316k
GND
9
3448 F09a
C_IN: TAIYO YUDEN JMK212BJ475MG
C_OUT: TAIYO YUDEN JMK212BJ475MG
*MURATA LQH32CN2R2M11
Figure 9a
C_OUT
15µF
CER
V_OUT
1.8V
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 =
632k
Figure 9 shows the complete circuit along with its effi-
ciency curve.
100
V_IN = 3.6V
90 V_OUT = 1.8V
80 ^{TA = 25°C}
70
60
50
40
30
20
10
0
0.0001
0.001
0.01
0.1
LOAD CURRENT (A)
Figure 9b
1
3448 F09b
V_OUT
100mV/DIV
AC COUPLED
I_LOAD
100mA/DIV
I_L
500mA/DIV
V_IN = 3.6V
20µs/DIV
V_OUT = 1.8V
I_LOAD = 100µA TO 200mA
Figure 9c
3448 F09c
V_OUT
100mV/DIV
AC COUPLED
I_LOAD
250mA/DIV
I_L
500mA/DIV
V_IN = 3.6V
20µs/DIV
V_OUT = 1.8V
I_LOAD = 50mA TO 600mA
Figure 9d
3448 F09d
3448f
14