LTC1929-PG LINER [Linear Technology], LTC1929-PG Datasheet - Page 12

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

APPLICATIO S I FOR ATIO
LTC1929/LTC1929-PG
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
internal compensation required to meet stability criterion
for buck regulators operating at greater than 50% duty
factor. A curve is provided to estimate this reduction in
peak output current level depending upon the operating
duty factor.
Operating Frequency
The LTC1929 uses a constant frequency, phase-lockable
architecture with the frequency determined by an internal
capacitor. This capacitor is charged by a fixed current plus
an additional current which is proportional to the voltage
applied to the PLLFLTR pin. Refer to Phase-Locked Loop
and Frequency Synchronization in the Applications Infor-
mation section for additional information.
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
12
Figure 2. Operating Frequency vs V
2.5
2.0
1.5
1.0
0.5
0
120
OPERATING FREQUENCY (kHz)
U
170
U
220
270
W
1929 F02
PLLFLTR
320
U
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 considered.
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.
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.
I
L
Figure 3. Normalized Output Ripple Current
vs Duty Factor [I
V
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
OUT
fL
0
0.1
0.2
1
0.3
DUTY FACTOR (V
V
V
OUT
IN
IN
RMS
0.4
or V
I
L
0.5
OUT
0.3 ( I
per individual section, N,
OUT
0.6
:
/V
IN
0.7
O(P–P)
1-PHASE
2-PHASE
)
0.8
1929 F03
)]
0.9