LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 19

• Manufacturer Part Number: LTC1929
• Description: 2-Phase, High Efficiency, Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The duty factors 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 current rating at the input
voltage that produces a duty cycle nearest to the peak.
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 since the duty factor is < 50%. The maximum
output current ripple is:
An alternate calculation just uses the equation for output
ripple current at D = 1.8V/5.5 = 0.33:
IN
C requiredI
V
V
will require an RMS current rating of:
IN
OUTRIPPLE
OUTRIPPLE
I
I
COUT
COUTMAX
I
RIPPLE
V
OUT
fL
RMS
0 97
20
0 99
20
310
310
.
.
U
m
m
1
2 1 8
kHz
A
A
kHz
1 8
4 76
20
.
V
.
.
0 97
0 99
V
OUT
U
V
IN
V
.
2
.
A
A
2
H
RMS
A
A
H
1 8
5 5
0 66
0 33
.
.
.
.
1 2 0 33 1 0 33
19 4
19 7
1
W
1 2 0 33
.
.
2
1
1 8
5 5
at
mV
mV
.
.
.
2
33
1
V
V
RMS
RMS
.
% . .
0 54
1 8
5 5
.
U
D F
.
.
1
.
2
1
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1929. 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
LTC1929 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 LTC1929 V
of C
plate(s) of C
connected between the V
any feedforward capacitor across R1 should be as close as
possible to the LTC1929.
3) Are the SENSE
minimum PC trace spacing? The filter capacitors between
SENSE
possible to the LTC1929. Ensure accurate current sensing
with Kelvin connections.
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 LTC1929.
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 LTC1929 V
separately. The power ground returns to the
and SENSE
OUT
CC
? The resistive divider R1, R2 must be
–
1 F ceramic decoupling capacitor con-
and SENSE
–
OS
pin pairs should be as close as
+
IN
DIFFOUT
pin connect to the (+) plate(s)
CC
connect to the drains of the
+
OS
and the power ground pin?
leads routed together with
–
and signal ground and
pin connect to the (–)
IN
, which should have
LTC1929
19