LTC3701 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3701' PDF : 20 Pages View PDF
LTC3701
APPLICATIO S I FOR ATIO
2) Are the signal and power grounds kept separate? The
LTC3701 signal ground consists of the feedback resistor
divider, the ITH/RUN compensation network, and Pin 4.
The power ground consists of the (–) terminal of CIN, the
(–) terminals of COUT1,2, 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 VFB pins connect directly to the feedback
resistors? Put the feedback resistors close to the VFB 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 VIN 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
=
VOUT + VD
VIN(MIN) + VD
=
93%
From Figure 2, SF = 57%.
RSENSE
=
12.7
SF
• IOUT
• 100
=
0.57
12.7 •1.5
=
0.03Ω
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:
LMIN
=
4.2V – 2.5V
550kHz
0.03V
0.03Ω
2.5V
4.2V
+
+
00..33VV
=
2.00µH
For the selection of the external MOSFET, the RDS(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 PP = 250mW and its thermal resistance
is 50°C/W. Hence, the junction temperature at TA = 25°C
will be 37.5°C and δp = 0.005 • (37.5 – 25) = 0.0625. The
required RDS(ON) is then given by:
RDS(ON)
≈
DC
PP
• IOUT2(1+
δp)
=
0.11Ω
The P-channel MOSFET requirement can be met by an
Si3443DV.
The requirement for the Schottky diode is the most strin-
gent when VOUT = 0V, i.e., short circuit. With a 0.03Ω
RSENSE 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 DFB1 and DFB2 (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 COUT will require an ESR
of 0.1Ω for optimum efficiency.
3701fa
16
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