LTC3417 Linear Technology, LTC3417 Datasheet - Page 9

• Manufacturer Part Number: LTC3417
• Description: Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator
• Manufacturer: Linear Technology

OPERATIO
APPLICATIO S I FOR ATIO
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100%. In this
dropout condition, the PMOS switch is turned on continu-
ously with the output voltage being equal to the input
voltage minus the voltage drops across the internal P-
channel MOSFET and inductor.
A general LTC3417 application circuit is shown in
Figure 4. External component selection is driven by the
load requirement, and begins with the selection of the
inductors L1 and L2. Once L1 and L2 are chosen, C
C
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency, f
by pulling the FREQ pin to V
connecting an external resistor from FREQ to ground. The
value of the resistor sets the ramp current that is used to
charge and discharge an internal timing capacitor within
the oscillator and can be calculated by using the following
equation:
for 0.6MHz ≤ f
select the value for R
The maximum operating frequency is also constrained by
the minimum on-time and duty cycle. This can be calcu-
lated as:
OUT1
R
T
=
and C
1 61 10
.
OUT2
f
•
O
O
U
≤ 4MHz. Alternatively, use Figure 1 to
can be selected.
11
U
( )
T
Ω
.
–
O
U
16 586
, of the LTC3417 is determined
IN
.
for 1.5MHz operation or by
k
W
Ω
U
IN
,
Low Supply Operation
The LTC3417 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below about 2.07V to prevent unstable operation.
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R
Inductor Selection
Although the inductor does not influence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current, ∆I
higher inductance and increases with higher V
Accepting larger values of ∆I
inductances, but results in higher output voltage ripple,
greater core losses and lower output current capability.
f
∆ =
O MAX
I
(
L
V
f
)
160
140
120
100
O
80
60
40
20
OUT
≈
0
•
0
6 67
L
.
⎛
⎜
⎝
0.5 1.0
Figure 1. Frequency vs R
– 1
⎛
⎜
⎝
V
V
IN MAX
V
V
OUT
1.5
FREQUENCY (MHz)
OUT
(
IN
2.0
⎞
⎟
⎠
)
⎞
⎟
⎠
2.5
(
L
MHz
3.0
www.DataSheet4U.com
allows the use of low
3.5
)
T
4.0
L
3417 F01
, decreases with
LTC3417
T
4.5
.
IN
or V
3417fb
OUT
9
.