LTC1629 LINER [Linear Technology], LTC1629 Datasheet - Page 12

• Manufacturer Part Number: LTC1629
• Description: PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1629/LTC1629-PG
R
R
current. The LTC1629 current comparator has a maxi-
mum threshold of 75mV/R
mode range of SGND to 1.5( INTV
parator threshold sets the peak inductor current, yielding
a maximum average output current I
value less half the peak-to-peak ripple current, I
Allowing a margin for variations in the LTC1629 and
external component values yields:
where N = number of stages.
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
internal slope 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 LTC1629 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
SENSE
SENSE1, 2
R
SENSE
Selection For Output Current
= (50mV/I
are chosen based on the required output
U
MAX
)N
U
SENSE
and an input common
CC
W
MAX
). The current com-
equal to the peak
U
L
.
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.
In a PolyPhase 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 different 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. As shown in
Figure 3, the zero output ripple current is obtained when:
I
L
V
Figure 2. Operating Frequency vs V
OUT
fL
2.5
2.0
1.5
1.0
0.5
0
120
1
V
OPERATING FREQUENCY (kHz)
V
OUT
IN
170
IN
or V
I
L
OUT
220
per individual section, N,
:
270
PLLFLTR
1629 F02
320