LTC3564_15 Datasheet, PDF (15/20 Pages) Linear Technology – 2.25MHz, 1.25A Synchronous Step-Down Regulator

LTC3564
APPLICATIO S I FOR ATIO
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC3564. These items are also illustrated graphically in
Figures 4 and 5. Check the following in your layout:
1. The power traces, consisting of the GND trace, the SW
trace and the V_IN trace should be kept short, direct and
wide.
2. Does the V_FB pin connect directly to the feedback
resistors? The resistive divider R1/R2 must be con-
nected between the (+) plate of C_OUT and ground.
3. Does the (+) plate of C_IN connect to V_IN as closely as
possible? This capacitor provides the AC current to the
internal power MOSFETs.
4. Keep the switching node, SW, away from the sensitive
V_FB node.
5. Keep the (–) plates of C_IN and C_OUT as close as possible.
Design Example
As a design example, assume the LTC3564 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 1.25A 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 = 500mA and
f = 2.25MHz in equation (3) gives:
L
=
2.5V
2.25MHz(500mA)
⎛
⎝⎜
1−
2.5V ⎞
4.2V ⎠⎟
=
0.9μH
A 1μH or 1.1μH inductor works well for this application.
For best efficiency choose a 1.5A or greater inductor with
less than 0.1Ω series resistance.
C_IN will require an RMS current rating of at least 0.6A ≅
I_LOAD(MAX)/2 at temperature and C_OUT will require an ESR
of less than 0.125Ω. 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 6 shows the complete circuit along with its effi-
ciency curve.
V_IN
2.7V
TO 4.2V
C_IN^**
22μF
CER
3
V_IN
4
SW
LTC3564
5
RUN
1
V_FB
GND
2
1.1μH*
22pF
1M
316k
3564 F06a
*TOKO A915AY-1R1M (D53LC SERIES)
** TAIYO YUDEN JMK316BJ226ML
Figure 6a. Typical Application
V_OUT
2.5V
C_OUT**
22μF
CER
100
95
90
85
80
75
70
V_IN = 2.7V
65
V_IN = 3.6V
60 ^{VOUT = 2.5V}
V_IN = 4.2V
0.1
1
10
100
1000
OUTPUT CURRENT (mA)
3564 F06b
Figure 6b. Efficiency vs Output Current
3564f
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