LTC3729EG Linear Technology, LTC3729EG Datasheet - Page 23

• Manufacturer Part Number: LTC3729EG
• Description: IC SW REG SYNC STEP-DOWN 28-SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3729EG

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5 V
• Current - Output: 5A
• Frequency - Switching: 1.1MHz
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
which is much less than normal, full‑load conditions.
Incidentally, since the load no longer dissipates power in
the shorted condition, total system power dissipation is
decreased by over 99%.
The duty cycles when the peak RMS input current occurs
is at D = 0.25 and D = 0.75 according to Figure 4. Calculate
the worst‑case required RMS input current rating at the
input voltage, which is 5.5V, that provides a duty cycle
nearest to the peak.
From Figure 4, C
The output capacitor ripple current is calculated by using
the inductor ripple already calculated for each inductor and
multiplying by the factor obtained from Figure 3 along with
the calculated duty factor. The output ripple in continuous
mode will be highest at the maximum input voltage. From
Figure 3, the maximum output current ripple is:
Note that the PolyPhase technique will have its maximum
benefit for input and output ripple currents when the number
of phases times the output voltage is approximately equal
to or greater than the input voltage.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3729. These items are also illustrated graphically
in the layout diagram of Figure 11. Check the following
in your layout:
C
∆I
∆I
IN
COUT
COUTMAX
requiredI
=
V
OUT
fL
=
RMS
IN
(
300kHz
(
will require an RMS current rating of:
0.34
1.8 0.34
= 20A
= 4.6A
(
(
)
)
(
RMS
)
2µH
(
)
0.23
)
= 1A
)
1) Are the signal and power grounds segregated? The
LTC3729 signal ground pin should return to the (–) plate of
C
of the bottom N‑channel MOSFETs, anodes of the Schottky
diodes, and (–) plates of C
lead lengths as possible.
2) Does the LTC3729 V
of C
plate(s) of C
connected between the V
any feedforward capacitor across R1 should be as close
as possible to the LTC3729.
3) Are the SENSE
minimum PC trace spacing? The filter capacitors between
SENSE
possible to the LTC3729. Ensure accurate current sensing
with Kelvin connections to the sense resistors.
4) Do the (+) plates of C
MOSFETs as closely as possible? This capacitor provides
the AC current to the MOSFETs. Keep the input current path
formed by the input capacitor, top and bottom MOSFETs,
and the Schottky diode on the same side of the PC board in
a tight loop to minimize conducted and radiated EMI.
5) Is the INTV
nected closely between INTV
This capacitor carries the MOSFET driver peak currents.
A small value is used to allow placement immediately
adjacent to the IC.
6) Keep the switching nodes, SW1 (SW2), away from sen‑
sitive small‑signal nodes. Ideally the switch nodes should
be placed at the furthest point from the LTC3729.
7) Use a low impedance source such as a logic gate to drive
the PLLIN pin and keep the lead as short as possible.
8) Minimize the capacitive load on the CLKOUT pin to
minimize excess phase shift. Buffer if necessary with an
NPN emitter follower.
OUT
OUT
separately. The power ground returns to the sources
+
? Does the LTC3729 V
and SENSE
OUT
CC
? The resistive divider R1, R2 must be
–
1µF ceramic decoupling capacitor con‑
and SENSE
–
IN
OS
pin pairs should be as close as
connect to the drains of the topside
+
DIFFOUT
IN
pin connect to the (+) plate(s)
CC
, which should have as short
+
OS
and the power ground pin?
leads routed together with
–
and signal ground and
pin connect to the (–)
LTC3729
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