LTC1709EG-7 Linear Technology, LTC1709EG-7 Datasheet - Page 12

• Manufacturer Part Number: LTC1709EG-7
• Description: IC SW REG STEP-DOWN SYNC 36-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1709EG-7

• 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: -

LTC1709-7
APPLICATIO S I FOR ATIO
The basic LTC1709-7 application circuit is shown in
Figure 1 on the first page. External component selection
begins with the selection of the inductor(s) based on
ripple current requirements and continues with the
R
inductor current and/or maximum current limit. Next, the
power MOSFETs and D1 and D2 are selected. The oper-
ating frequency and the inductor are chosen based mainly
on the amount of ripple current. Finally, C
its ability to handle the input ripple current (that
PolyPhase
with low enough ESR to meet the output ripple voltage
and load step specifications (also minimized with
PolyPhase). Current mode architecture provides inherent
current sharing between output stages. The circuit shown
in Figure 1 can be configured for operation up to an input
voltage of 28V (limited by the external MOSFETs). Current
mode control allows the ability to connect the two output
stages to two different input power supply rails. A heavy
output load can take some power from each input supply
according to the selection of the R
R
R
current. The LTC1709-7 current comparator has a maxi-
mum threshold of 75mV/R
mode range of SGND to 1.1(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
Assuming a common input power source for each output
stage and allowing a margin for variations in the
LTC1709-7 and external component values yields:
Operating Frequency
The LTC1709-7 uses a constant frequency, phase-lock-
able 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 for addi-
tional information.
12
SENSE1, 2
SENSE
SENSE1, 2
R
SENSE
Selection For Output Current
= 2(50mV/I
TM
are chosen based on the required peak output
resistor selection using the calculated peak
operation minimizes) and C
U
MAX
)
U
SENSE
and an input common
CC
SENSE
W
MAX
). The current com-
equal to the peak
IN
resistors.
OUT
is selected for
is chosen
U
L
.
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 I
decreases with higher inductance or frequency and
increases with higher V
where f is the individual output stage operating frequency.
PolyPhase is a registered trademark of Linear Technology Corporation.
I
L
V
Figure 2. Operating Frequency vs V
2.5
2.0
1.5
1.0
0.5
OUT
fL
0
120
1
OPERATING FREQUENCY (kHz)
170
V
V
OUT
IN
IN
or V
220
L
OUT
per individual section, N,
:
270
17097 F02
PLLFLTR
320