Datasheet

LTC1773
13
1773fb
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1773. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
1) Are the signal and power grounds segregated? The
LTC1773 signal ground consists of the resistive divider,
the compensation network and C
SS
. The power ground
consists of the (–) plate of C
IN
, the (–) plate of C
OUT
, the
source of the external synchronous NMOS, and Pin 5 of
the LTC1773. The power ground traces should be kept
short, direct and wide. Connect the synchronous
MOSFETs source directly to the input capacitor ground.
2) Does the V
FB
pin connect directly to the feedback
resistors? The resistive divider of R1 and R2 must be
connected between the (+) plate of C
OUT
and signal ground.
Be careful locating the feedback resistors too far away
from the LTC1773. The V
FB
line should not be routed close
to any other nodes with high slew rates.
3) Does the (+) terminal of C
IN
connect to V
IN
as closely as
possible? This capacitor provides the AC current to the
external power MOSFETs.
4) Keep the switching nodes SW, TG and BG away from
sensitive small-signal nodes, especially from the voltage
and current sensing feedback pins.
Design Example
As a design example, assume the LTC1773 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 2.7V. The load current requirement is a
maximum of 2A but most of the time it will be on 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 R
SENSE
to be around
33mΩ. For the inductor L, using equation (1),
L
fI
V
V
V
L
OUT
OUT
IN
=
()( )
⎛
⎝
⎜
⎞
⎠
⎟
1
1
∆
–
(3)
Substituting V
OUT
= 2.5V, V
IN
= 4.2V, ∆I
L
= 800mA and
f = 550kHz in equation (3) gives:
L
V
kHz mA
V
V
H=
⎛
⎝
⎜
⎞
⎠
⎟
=
25
550 800
1
25
42
23
.
()
–
.
.
. µ
A 2.5µH inductor works well for this application. For good
efficiency choose a 4A inductor with less than 0.1Ω series
resistance.
C
IN
will require an RMS current rating of at least 1A at
temperature and C
OUT
will require an ESR of less than
0.066Ω. In most applications, the requirements for these
capacitors are fairly similar.
Figure 6. LTC1773 Layout Diagram
APPLICATIONS INFORMATION
WUU
U
1773 F06
R1
R2
C
IN
1
2
3
4
5
10
9
8
7
6
R
SENSE
C
C2
C
C1
C
OUT
C
SS
R
C
V
OUT
+
–
+
–
V
IN
BOLD LINES INDICATE
HIGH CURRENT PATHS
LTC1773
RUN/SS
SYNC/FCB
GND
SENSE
–
V
IN
TG
I
TH
V
FB
BG
SW
L1
+
+
Q1
D1
Q2