LT8708-1
24
Rev 0
For more information
APPLICATIONS INFORMATION
OPERATING FREQUENCY SELECTION
The LT8708-1 uses a constant frequency architecture
operating between 100kHz and 400kHz. The LT8708-1
should be synchronized to the same frequency as the
LT8708 by connecting a clock signal to the SYNC pin.
An appropriate resistor must be placed from the RT pin
to ground. In general, use the same value R
T
resistor for
all the synchronized LT8708 and LT8708-1(s). See the
Operating Frequency Selection section of the LT8708 data
sheet on how to select the LT8708’s switching frequency.
C
IN
AND C
OUT
SELECTION
V
IN
and V
OUT
capacitance is necessary to suppress volt-
age ripple caused by discontinuous current moving in and
out of the regulator. A parallel combination of capacitors
is typically used to achieve high capacitance and low ESR
(equivalent series resistance). Dry tantalum, special poly-
mer, aluminum electrolytic and ceramic capacitors are all
available in surface mount packages. Capacitors with low
ESR and high ripple current ratings, such as OS-CON and
POSCAP are also available.
Ceramic capacitors should be placed near the regulator
input and output to suppress high frequency switching
spikes. A ceramic capacitor, of at least 1µF at the maxi-
mum V
INCHIP
operating voltage, should also be placed
from V
INCHIP
to GND as close to the LT8708-1 pins as
possible. Due to their excellent low ESR characteristics,
ceramic capacitors can significantly reduce input ripple
voltage and help reduce power loss in the higher ESR
bulk capacitors. X5R or X7R dielectrics are preferred, as
these materials retain their capacitance over wide volt-
age and temperature ranges. Many ceramic capacitors,
particularly 0805 or 0603 case sizes, have greatly reduced
capacitance at the desired operating voltage.
C
IN
and C
OUT
Selection: V
IN
Capacitance
Discontinuous V
IN
current is highest in the buck region
due to the M1 switch toggling on and off. Ensure that the
C
IN
capacitor network has low enough ESR and is sized
to handle the maximum RMS current. Figure 7 shows
the total input capacitor RMS ripple current for one to
six phases with the V
OUT
to V
IN
ratios in buck operation.
Figure 7. Normalized Total Input RMS Ripple Current vs V
OUT
/V
IN
for One to Six Phases in Buck Operation
V
OUT
/V
IN
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
87081 F06
I
(IN,RMS)
/I
OUT
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
The total input RMS ripple current I
(IN,RMS)
is normal-
ized against the total output current of the multiphase
system (I
OUT
). The graph can be used in place of tedious
calculations. From the graph, the minimum total input
RMS ripple current can be achieved when the product of
the number of phases (N) and the output voltage V
OUT
is approximately equal to integer multiples of the input
voltage V
IN
or:
V
OUT
/V
IN
= n/N
where n = 1, 2,…, N-1
Therefore, the number of phases can be chosen to mini-
mize the input capacitance for given input and output
voltages.
Figure 7 also shows the maximum total normalized input
RMS current for one to six phases. Choose an adequate
C
IN
capacitor network to handle this RMS current.
C
IN
is also necessary to reduce the V
IN
voltage ripple
caused by discontinuities and ripple of I
IN
. The effects of
ESR and the bulk capacitance must be considered when
choosing the correct capacitor for a given V
IN
ripple. A
low ESR input capacitor sized for the maximum RMS cur-
rent must be used. Add enough ceramic capacitance to
