16
LTC3701
3701fa
APPLICATIO S I FOR ATIO
WUUU
2) Are the signal and power grounds kept separate? The
LTC3701 signal ground consists of the feedback resistor
divider, the I
TH
/RUN compensation network, and Pin 4.
The power ground consists of the (–) terminal of C
IN
, the
(–) terminals of C
OUT1,2
, the anodes of the Schottky
diodes, and Pin 13 of the LTC3701. The power ground
traces should be kept short, direct, and wide. Connect the
anode of the Schottky diodes directly to the input capacitor
ground.
3) Do the V
FB
pins connect directly to the feedback
resistors? Put the feedback resistors close to the V
FB
pins.
The traces connecting the top feedback resistors to the
corresponding output capacitor should to be Kelvin traces.
4) Are the SENSE
–
and SENSE
+
leads routed together with
minimum PC trace spacing? The (optional) filter capacitor
between SENSE
+
and SENSE
–
should be as close as
possible to the IC. Ensure accurate current sensing with
Kelvin connections at the sense resistor.
5) Keep the switching nodes (SW1, SW2) and top gate
nodes (PGATE1, PGATE2) away from small-signal nodes,
especially the opposite channel’s voltage and current
sensing feedback pins. All of these nodes have large and
fast moving signals and therefore should be keep on the
“output side” of the LTC3701 and occupy minimum PC
trace area.
Design Example
As a design example for one channel, assume V
IN
will be
operating from a maximum of 4.2V down to a minimum of
2.7V. Load current requirement is a maximum of 1.5A , but
most of the time it will be in a standby mode requiring only
2mA. Efficiency at both low and high load currents is
important. Burst Mode operation at light loads is desired.
Output voltage is 2.5V.
Maximum Duty Cycle =
V
OUT
+
+
=
V
VV
D
IN MIN D()
%93
From Figure 2, SF = 57%.
R
SF
I
SENSE
OUT
===Ω
12 7 100
057
12 7 1 5
003
.• •
.
.•.
.
In the application, a 0.03Ω resistor is used. The PLLLPF
pin will be left floating, so the LTC3701 will operate at its
default frequency of 550kHz. For continuous operation in
Burst Mode, the required minimum inductor value is:
L
VV
kHz
V
VV
VV
H
MIN
=
Ω
+
+
=µ
42 25
550
003
003
25 03
42 03
200
.–.
.
.
..
..
.
For the selection of the external MOSFET, the R
DS(ON)
must be guaranteed at 2.5V since the LTC3701 has to work
down to 2.7V. Let’s assume that the MOSFET dissipation
is to be limited to P
P
= 250mW and its thermal resistance
is 50°C/W. Hence, the junction temperature at T
A
= 25°C
will be 37.5°C and δp = 0.005 • (37.5 – 25) = 0.0625. The
required R
DS(ON)
is then given by:
R
P
DC I p
DS ON
P
OUT
()
•
.≈
+
()
=Ω
2
1
011
δ
The P-channel MOSFET requirement can be met by an
Si3443DV.
The requirement for the Schottky diode is the most strin-
gent when V
OUT
= 0V, i.e., short circuit. With a 0.03Ω
R
SENSE
resistor, the short-circuit current through the
Schottky is 0.1/0.03 = 3.3A. An MBRS340T3 Schottky
diode is chosen. With 3.3A flowing through, the diode is
rated with a forward voltage of 0.4V. Therefore, the worst-
case power dissipated by the diode is 1.32W. The addition
of D
FB1
and D
FB2
(Figure 6) will reduce the diode dissipa-
tion to approximately 0.66W
The input capacitor requires an RMS current rating of at
least 0.75A at temperature, and C
OUT
will require an ESR
of 0.1Ω for optimum efficiency.
