LTC1778 Linear Technology, LTC1778 Datasheet - Page 18

• Manufacturer Part Number: LTC1778
• Description: Wide Operating Range/ No RSENSE Step-Down Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1778/LTC1778-1
C
85 C. The output capacitors are chosen for a low ESR of
0.013 to minimize output voltage changes due to induc-
tor ripple current and load steps. The ripple voltage will be
only:
However, a 0A to 10A load step will cause an output
change of up to:
An optional 22 F ceramic output capacitor is included to
minimize the effect of ESL in the output ripple. The
complete circuit is shown in Figure 9.
PC Board Layout Checklist
When laying out a PC board follow one of the two sug-
gested approaches. The simple PC board layout requires
a dedicated ground plane layer. Also, for higher currents,
it is recommended to use a multilayer board to help with
heat sinking power components.
18
IN
is chosen for an RMS current rating of about 5A at
V
V
OUT(RIPPLE)
OUT(STEP)
C
C
L1: SUMIDA CEP125-1R8MC-H
IN
OUT1-2
14.0k
30.1k
R1
: UNITED CHEMICON THCR60EIHI06ZT
R2
: CORNELL DUBILIER ESRE181E04B
= I
11k
500pF
R3
C
6.8nF
C1
0.1 F
= I
= (5.1A) (0.013 ) = 66mV
C
C2
SS
100pF
LOAD
U
C
20k
39k
R
R4
C2
L(MAX)
C
(ESR) = (10A) (0.013 ) = 130mV
U
100k
R
R
1.4M
PG
ON
(ESR)
1
2
3
4
5
6
7
8
RUN/SS
PGOOD
V
FCB
I
SGND
I
V
TH
ON
RNG
FB
W
LTC1778
Figure 9. Design Example: 2.5V/10A at 250kHz
EXTV
INTV
BOOST
PGND
SW
V
TG
BG
CC
CC
IN
U
16
15
14
13
12
11
10
9
C
0.22 F
+
CMDSH-3
B
C
4.7 F
C
0.1 F
1
R
VCC
F
D
F
B
• The ground plane layer should not have any traces and
• Place C
• Place LTC1778 chip with pins 9 to 16 facing the power
• Use an immediate via to connect the components to
• Use compact plane for switch node (SW) to improve
• Use planes for V
• Flood all unused areas on all layers with copper. Flood-
it should be as close as possible to the layer with power
MOSFETs.
compact area. It may help to have some components on
the bottom side of the board.
components. Keep the components connected to pins
1 to 8 close to LTC1778 (noise sensitive components).
ground plane including SGND and PGND of LTC1778.
Use several bigger vias for power components.
cooling of the MOSFETs and to keep EMI down.
filtering and to keep power losses low.
ing with copper will reduce the temperature rise of
power component. You can connect the copper areas to
any DC net (V
your system).
M1
Si4884
M2
Si4874
IN
, C
OUT
1.8 H
D1
B340A
L1
IN
, MOSFETs, D1 and inductor all in one
, V
IN
OUT
and V
1778 F09
+
, GND or to any other DC rail in
OUT
C
10 F
35V
C
180 F
4V
IN
3
OUT1-2
2
to maintain good voltage
C
22 F
6.3V
X7R
OUT3
V
5V TO 28V
V
2.5V
10A
IN
OUT
1778fa