LTC3417 Datasheet, PDF (9/20 Pages) Linear Technology – Dual Synchronous 1.4A/800mA 4MHz Step-Down DC/DC Regulator

U
OPERATIO
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.
LTC3417
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.
APPLICATIO S I FOR ATIO
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_IN,
C_OUT1 and C_OUT2 can be selected.
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_O, of the LTC3417 is determined
by pulling the FREQ pin to V_IN for 1.5MHz operation or by
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:
R_T
=
1.61• 10¹¹
f_O
(Ω)
–
16.586kΩ
for 0.6MHz ≤ f_O ≤ 4MHz. Alternatively, use Figure 1 to
select the value for R_T.
The maximum operating frequency is also constrained by
the minimum on-time and duty cycle. This can be calcu-
lated as:
160
140
120
100
80
60
40
20
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
FREQUENCY (MHz)
3417 F01
Figure 1. Frequency vs R_T
( ) ^f_O(MAX)
≈
⎛
6.67_⎝⎜
V_OUT
V_IN(MAX)
⎞
⎠⎟
MHz
The minimum frequency is limited by leakage and noise
coupling due to the large resistance of R_T.
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_L, decreases with
higher inductance and increases with higher V_IN or V_OUT.
∆I_L
=
V_OUT
f_O • L
⎛⎝⎜_1–
V_OUT
V_IN
⎞
⎠⎟
Accepting larger values of ∆I_L allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses and lower output current capability.
3417fb
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