LTC1773 LINER [Linear Technology], LTC1773 Datasheet - Page 13

• Manufacturer Part Number: LTC1773
• Description: Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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
consists of the (–) plate of C
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
resistors? The resistive divider of R1 and R2 must be
connected between the (+) plate of C
Be careful locating the feedback resistors too far away
from the LTC1773. The V
to any other nodes with high slew rates.
3) Does the (+) terminal of C
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.
FB
BOLD LINES INDICATE
HIGH CURRENT PATHS
pin connect directly to the feedback
U
FB
INFORMATION
U
R1
line should not be routed close
C
R
IN
C2
C
IN
connect to V
, the (–) plate of C
C
SS
SS
R2
OUT
W
C
. The power ground
C1
and signal ground.
1
2
3
4
5
I
V
RUN/SS
SYNC/FCB
GND
IN
TH
FB
Figure 6. LTC1773 Layout Diagram
as closely as
LTC1773
U
SENSE
OUT
SW
V
TG
BG
IN
, the
–
10
9
8
7
6
Design Example
As a design example, assume the LTC1773 is used in a
single lithium-ion battery powered cellular phone applica-
tion. The V
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
33mΩ. For the inductor L, using equation (1),
Substituting V
f = 550kHz in equation (3) gives:
A 2.5µH inductor works well for this application. For good
efficiency choose a 4A inductor with less than 0.1Ω series
resistance.
C
temperature and C
0.066Ω. In most applications, the requirements for these
capacitors are fairly similar.
IN
L
L
+
will require an RMS current rating of at least 1A at
=
=
C
OUT
( )( )
550
f
1
∆
IN
kHz
I
Q1
L1
R
L
will be operating from a maximum of 4.2V
2 5
SENSE
OUT
.
(
V
800
OUT
V
Q2
D1
= 2.5V, V
OUT
mA
⎛
⎜
⎝
1
–
will require an ESR of less than
)
⎛
⎜
⎝
V
1
V
OUT
–
IN
IN
+
2 5
4 2
= 4.2V, ∆I
⎞
⎟
⎠
.
.
V
V
C
IN
⎞
⎟ =
⎠
V
V
OUT
+
–
SENSE
–
+
IN
1773 F06
2 3
LTC1773
. µ
L
= 800mA and
to be around
H
13
1773fb
(3)