LTC3808 Datasheet, PDF (23/28 Pages) Linear Technology – No RSENSE TM, Low EMI, Synchronous DC/DC Controller with Output Tracking

LTC3808
APPLICATIO S I FOR ATIO
The N-channel MOSFET in Si7540DP has 0.017Ω R_DS(ON).
The short circuit current is:
I_SC
=
90mV
0.017Ω
=
5.3A
So the inductor current rating should be higher than 5.3A.
The PLLLPF pin will be left floating, so the LTC3808 will
operate at its default frequency of 550kHz. For continuous
Burst Mode operation with 600mA I_RIPPLE, the required
minimum inductor value is:
L_MIN
=
1.8V
550kHz • 600mA
•
⎛⎝⎜_1−
1.8V
2.75V
⎞⎠⎟
=
1.88µH
A 6A 2.2µH inductor works well for this application.
C_IN will require an RMS current rating of at least 1A at
temperature. A C_OUT with 0.1 ESR will cause approxi-
mately 60mV output ripple. In most applications, the
requirements for these capacitors are fairly similar.
PC Board Layout Checklist
When laying out the printed circuit board, use the follow-
ing checklist to ensure proper operation of the LTC3808.
• The power loop (input capacitor, MOSFET, inductor,
output capacitor) should be as small as possible and
isolated as much as possible from LTC3808.
• Put the feedback resistors close to the V_FB pins. The I_TH
compensation components should also be very close to
the LTC3808.
• The current sense traces (SENSE⁺ and SENSE^–) should
be Kelvin connections right at the P-channel MOSFET
source and drain.
• Keeping the switch node (SW) and the gate driver nodes
(TG, BG) away from the small-signal components, espe-
cially the feedback resistors, and I_TH compensation
components.
3808f
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