LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 8

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

APPLICATIO S I FOR ATIO
LTC1929
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 2. As the operating frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 310kHz.
Inductor Value Calculation and Output Ripple Current
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is efficiency. A higher
frequency generally results in lower efficiency because of
MOSFET gate charge and transition losses. In addition to
this basic tradeoff, the effect of inductor value on ripple
current and low current operation must also be consid-
ered. The PolyPhase approach reduces both input and
output ripple currents while optimizing individual output
stages to run at a lower fundamental frequency, enhancing
efficiency.
The inductor value has a direct effect on ripple current. The
inductor ripple current
decreases with higher inductance or frequency and in-
creases with higher V
where f is the individual output stage operating frequency.
8
I
L
V
Figure 2. Operating Frequency vs V
OUT
fL
2.5
2.0
1.5
1.0
0.5
0
120
1
U
V
OPERATING FREQUENCY (kHz)
V
OUT
IN
170
IN
or V
U
I
L
OUT
220
per individual section, N,
:
W
270
PLLFLTR
1929 F02
320
U
In a 2-phase converter, the net ripple current seen by the
output capacitor is much smaller than the individual
inductor ripple currents due to the ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Application Note 77.
Figure 3 shows the net ripple current seen by the output
capacitors for the 1- and 2-phase configurations. The
output ripple current is plotted for a fixed output voltage as
the duty factor is varied between 10% and 90% on the
x-axis. The output ripple current is normalized against the
inductor ripple current at zero duty factor. The graph can
be used in place of tedious calculations, simplifying the
design process.
Accepting larger values of I
inductances, but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is I
= 0.4(I
ber, the maximum I
voltage. The individual inductor ripple currents are deter-
mined by the inductor, input and output voltages.
Inductor Core Selection
Once the values for L1 and L2 are known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy, or Kool M
loss is independent of core size for a fixed inductor value,
Kool M is a registered trademark of Magnetics, Inc.
OUT
Figure 3. Normalized Output Ripple Current vs
Duty Factor [I
)/2, where I
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
0.2
RMS
OUT
0.3
DUTY FACTOR (V
L
0.3 ( I
is the total load current. Remem-
occurs at the maximum input
0.4
0.5
O(P–P)
L
allows the use of low
OUT
0.6
)]
®
/V
IN
0.7
cores. Actual core
1-PHASE
2-PHASE
)
0.8
1929 F03
0.9
L