LTC3701EGN#TR Linear Technology, LTC3701EGN#TR Datasheet - Page 10

• Manufacturer Part Number: LTC3701EGN#TR
• Description: IC DC/DC CNTRLR STPDN DUAL16SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3701EGN#TR

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 10 V
• Current - Output: 1A
• Frequency - Switching: 550kHz
• Voltage - Input: 2.5 ~ 10 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC3701EGNTR

APPLICATIO S I FOR ATIO
LTC3701
10
The basic LTC3701 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of L and R
Next, the power MOSFET M1 and the output diode D1 are
selected. Finally C
R
R
Since the current comparator monitors the voltage devel-
oped across R
determines the inductor’s peak current. The output cur-
rent that the LTC3701 can provide is given by:
where I
(see Inductor Value Calculation).
A reasonable starting point for setting ripple current is
I
yields:
However, for operation above 20% duty cycle, slope
compensation has to be taken into consideration to select
the appropriate value of R
amount of current. Using Figure 2, the value of R
For noise sensitive applications, a 1nF capacitor placed
between the SENSE
chip is suggested.
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC3701. The internal nominal frequency is
550kHz, but can be externally synchronized or set from
approximately 300kHz to 750kHz.
The operating frequency and inductor selection are inter-
related in that higher frequencies permit the use of a
RIPPLE
SENSE
SENSE
I
R
R
OUT
SENSE
SENSE
RIPPLE
Selection for Output Current
is chosen based on the required output current.
= (0.4)(I
R
0 095
SENSE
.
12 7 . •
is the inductor peak-to-peak ripple current
12 7
SENSE
OUT
.
IN
1
–
U
I
+
OUT
(C1) and C
). Rearranging the above equation
I
I
, the threshold of the comparator
SF
OUT
RIPPLE
and SENSE
2
U
for Duty Cycle < 20%
SENSE
100
OUT
to provide the required
–
W
pins very close to the
(C2) are chosen.
U
SENSE
SENSE
is:
.
smaller inductor for the same amount of inductor ripple
current. However, this is at the expense of efficiency due
to an increase in MOSFET gate charge and switching
losses.
The inductance value also has a direct effect on ripple
current. The ripple current, I
inductance or frequency. The inductor’s peak-to-peak
ripple current is:
where f is the operating frequency and V
voltage drop of the external Schottky diode. Accepting
larger values of I
but results in higher output voltage ripple and greater core
losses. A reasonable starting point for setting ripple cur-
rent is I
occurs at the maximum input voltage.
With Burst Mode operation selected on the LTC3701, the
ripple current is normally set such that the inductor
current is continuous during the burst periods. Therefore,
the peak-to-peak ripple current must not exceed:
This implies a minimum inductance of:
A smaller value than L
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
Kool M is a registered trademark of Magnetics, Inc.
I
I
L
RIPPLE
RIPPLE
MIN
RIPPLE
®
f
V
Use V
(0.03)/R
IN
cores. Actual core loss is independent of core
V
R
IN
0 03
–
SENSE
= 0.4(I
RIPPLE
.
f L
–
V
IN
•
OUT
V
OUT
SENSE
V
OUT(MAX)
allows the use of low inductances,
IN MAX
MIN
V
(
V
V
OUT
IN
V
OUT
could be used in the circuit,
RIPPLE
IN
)
V
). The maximum I
V
D
V
D
V
, decreases with higher
D
D
D
is the forward
RIPPLE
3701fa