LTC1709EG Linear Technology, LTC1709EG Datasheet - Page 12

• Manufacturer Part Number: LTC1709EG
• Description: IC REG SW 2PH SYNC STPDWN 36SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1709EG

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.3 ~ 3.5 V
• Current - Output: 3A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
LTC1709
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
MOSFET gate charge and transition losses increase di-
rectly with frequency. In addition to this basic tradeoff, the
effect of inductor value on ripple current and low current
operation must also be considered. The PolyPhase ap-
proach 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.
12
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
OPERATING FREQUENCY (kHz)
V
V
170
OUT
IN
IN
or V
U
I
L
OUT
220
per individual section, N,
:
W
270
PLLFLTR
1709 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 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
1709 F03
0.9
L