lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 18

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LT3724
APPLICATIO S I FOR ATIO
2. I
MOSFET, the inductor, the sense resistor, and the input
and output capacitors. In continuous conduction mode
the average output current flows through the inductor and
R
Schottky diode. The resistances of the MOSFET (R
and the R
with the resistances of the inductor and R
the total series resistance of the circuit. The total conduc-
tion power loss is proportional to this resistance and
usually accounts for between 2% to 5% loss in efficiency.
3. Transition losses of the MOSFET can be substantial with
input voltages greater than 20V. See MOSFET Selection
section.
4. The Schottky diode can be a major contributor of power
loss especially at high input to output voltage ratios (low
duty cycles) where the diode conducts for the majority of
the switch period. Lower V
oversizing the diode does not always help because as the
diode heats up the V
is decreased.
I
load currents. Other losses including C
dissipative losses and inductor core losses generally
account for less than 2% total additional loss in efficiency.
PCB Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation.
These items are illustrated graphically in the layout dia-
gram of Figure 9.
1. Keep the signal and power grounds separate. The signal
ground consists of the LT3724 SGND pin, the exposed pad
on the backside of the LT3724 IC and the (–) terminal of
V
contain any high, fast currents. The power ground con-
sists of the Schottky diode anode, the (–) terminal of the
18
2
OUT
SENSE
R losses and the Schottky diode loss dominate at high
2
R losses are calculated from the DC resistances of the
. The signal ground is the quiet ground and does not
but is chopped between the MOSFET and the
SENSE
multiplied by the duty cycle can be summed
U
f
is reduced and the diode loss term
U
f
reduces the losses. Note that
W
IN
SENSE
and C
U
to obtain
OUT
DS(ON)
ESR
)
input capacitor, and the ground return of the V
tor. This ground has very fast high currents and is consid-
ered the noisy ground. The two grounds are connected to
each other only at the (–) terminal of V
2. Use short wide traces in the loop formed by the
MOSFET, the Schottky diode and the input capacitor to
minimize high frequency noise and voltage stress from
parasitic inductance. Surface mount components are pre-
ferred.
3. Connect the V
independent of any other nodes, such as the SENSE
Connect the feedback resistors between the (+) and (–)
terminals of C
proximity to the LT3724 to keep the high impedance node,
V
4. Route the SENSE
as short as possible.
5. Locate the V
to the IC. These capacitors carry the MOSFET driver’s high
peak currents. Place the small signal components away
from high frequency switching nodes (BOOST, SW, and
TG). In the layout shown in Figure 9, place all the small
signal components on one side of the IC and all the power
components on the other. This helps to keep the signal and
power grounds separate.
6. A small decoupling capacitor (100pF) is sometimes
useful for filtering high frequency noise on the feedback
and sense nodes. If used, locate as close to the IC as
possible.
7. The LT3724 packaging will efficiently remove heat from
the IC through the exposed pad on the backside of the part.
The exposed pad is soldered to a copper footprint on the
PCB. Make this footprint as large as possible to improve
the thermal resistance of the IC case to ambient air. This
helps to keep the LT3724 at a lower temperature.
8. Make the trace connecting the gate of MOSFET M1 to the
TG pin of the LT3724 short and wide.
FB
, as short as possible.
OUT
CC
FB
and BOOST capacitors in close proximity
. Locate the feedback resistors in close
–
pin directly to the feedback resistors
and SENSE
+
traces together and keep
OUT
.
CC
capaci-
–
3724fb
pin.