ltc1629-6 Linear Technology Corporation, ltc1629-6 Datasheet - Page 23

• Manufacturer Part Number: ltc1629-6
• Description: Polyphase, Synchronous Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The worst-case power disipated by the synchronous
MOSFET under short-circuit conditions at elevated ambi-
ent temperature and estimated 50 C junction temperature
rise is:
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 con-
tinuous 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 approxi-
mately equal to or greater than the input voltage.
P
C requiredI
SYNC
IN
I
I
COUT
COUTMAX
360
5 5
.
V
OUT
fL
V
mW
5 5
RMS
IN
.
300
V
will require an RMS current rating of:
U
1 8
0 34
1 8 0 34
.
.
.
kHz
V
4 6
20
.
U
5 28
.
A
A
.
2
RMS
0 23
A
H
.
2
1 48 0 013
W
1
.
A
.
U
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1629-6. These items are also illustrated graphically in
the layout diagram of Figure 11. Check the following in
your layout:
1) Are the signal and power grounds segregated? The
LTC1629-6 signal ground pin should return to the (–) plate
of C
sources of the bottom N-channel MOSFETs, anodes of the
Schottky diodes, and (–) plates of C
as short lead lengths as possible.
2) Does the LTC1629-6 V
of C
(–) plate(s) of C
connected between the V
any feedforward capacitor across R1 should be as close as
possible to the LTC1629-6.
3) Are the SENSE
minimum PC trace spacing? The filter capacitors between
SENSE
possible to the LTC1629-6. Ensure accurate current sens-
ing with Kelvin connections to the sense resistors.
4) Do the (+) plates of C
topside 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 adja-
cent to the IC.
6) Keep the switching nodes, SW1 (SW2), away from
sensitive small-signal nodes. Ideally the switch
nodes should be placed at the furthest point from the
LTC1629-6.
7) Use a low impedance source such as a logic gate to drive
the PLLIN pin and keep the lead as short as possible.
OUT
OUT
+
? Does the LTC1629-6 V
separately. The power ground returns to the
and SENSE
CC
OUT
–
1 F ceramic decoupling capacitor con-
and SENSE
? The resistive divider R1, R2 must be
–
pin pairs should be as close as
OS
IN
DIFFOUT
+
CC
pin connect to the (+) plate(s)
connect to the drains of the
+
and the power ground pin?
leads routed together with
OS
and signal ground and
IN
LTC1629-6
–
, which should have
pin connect to the
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