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LTC3861-1 Datasheet, PDF (31/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing
LTC3861-1
Applications Information
To set the output voltage equal to 1.2V:
RFB1 = 10k, RFB2 = 10k
The LTC4449 gate driver and external MOSFETs are chosen
for the power stage. DrMOSs from Fairchild, Infineon,
Vishay and others can also be used.
Printed Circuit Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the converter.
1. The connection between the SGND pin on the LTC3861-1
and all of the small-signal components surrounding the
IC should be isolated from the system power ground.
Place all decoupling capacitors, such as the ones on
VCC, between ISNSP and ISNSN etc., close to the IC. In
multiphase operation SGND should be Kelvin-connected
to the main ground node near the bottom terminal of
the input capacitor. In dual-channel operation, SGND
should be Kelvin-connected to the bottom terminal of
the output capacitor for channel 2, and channel 1 should
be remotely sensed using the remote sense differential
amplifier.
2. Place the small-signal components away from high fre-
quency switching nodes on the board. The LTC3861-1
contains remote sensing of output voltage and inductor
current and logic-level PWM outputs enabling the IC to
be isolated from the power stage.
3. The PCB traces for remote voltage and current sense
should avoid any high frequency switching nodes in
the circuit and should ideally be shielded by ground
planes. Each pair (VSNSP and VSNSN, ISNSP and
ISNSN) should be routed parallel to one another with
minimum spacing between them. If DCR sensing is
used, place the top resistor (Figure 5b, R1) close to
the switching node.
4. The input capacitor should be kept as close as possible
to the power MOSFETs. The loop from the input capaci-
tor’s positive terminal, through the MOSFETs and back
to the input capacitor’s negative terminal should also
be as small as possible.
5. If using discrete drivers and MOSFETs, check the
stress on the MOSFETs by independently measuring
the drain-to-source voltages directly across the device
terminals. Beware of inductive ringing that could exceed
the maximum voltage rating of the MOSFET. If this
ringing cannot be avoided and exceeds the maximum
rating of the device, choose a higher voltage rated
MOSFET.
6. When cascading multiple LTC3861-1 ICs, minimize
the capacitive load on the CLKOUT pin to minimize
phase error. Kelvin all the LTC3861-1 IC grounds to
the same point, typically SGND of the IC containing
the master.
38611f
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