Datasheet

13
LTC3727/LTC3727-1
3727fc
Figure 1 on the first page is a basic LTC3727 /LTC3727-1
application circuit. External component selection is driven
by the load requirement, and begins with the selection of
R
SENSE
and the inductor value. Next, the power MOSFETs
and D1 are selected. Finally, C
IN
and C
OUT
are selected.
The circuit shown in Figure 1 can be configured for
operation up to an input voltage of 28V (limited by the
external MOSFETs).
R
SENSE
Selection For Output Current
R
SENSE
is chosen based on the required output current.
The LTC3727 current comparator has a maximum thresh-
old of 135mV/R
SENSE
and an input common mode range
of SGND to 14V. The current comparator threshold sets
the peak of the inductor current, yielding a maximum
average output current I
MAX
equal to the peak value less
half the peak-to-peak ripple current, ΔI
L
.
Allowing a margin for variations in the LTC3727 and
external component values yields:
R
mV
I
SENSE
MAX
=
90
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability crite-
rion for buck regulators operating at greater than 50%
duty factor. A curve is provided to estimate this reducton
in peak output current level depending upon the operating
duty factor.
Operating Frequency
The LTC3727 uses a constant frequency phase-lockable
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 in the Applications Infor-
mation section for additional information.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 5. As the operating frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 550kHz.
Inductor Value Calculation
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 of
MOSFET gate charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current. The
inductor ripple current ΔI
L
decreases with higher induc-
tance or frequency and increases with higher V
IN
:
ΔI
fL
V
V
V
L OUT
OUT
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
1
1
()( )
–
Accepting larger values of ΔI
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI
L
= 0.3(I
MAX
). The maximum ΔI
L
occurs at the maximum input voltage.
APPLICATIO S I FOR ATIO
WUUU
Figure 5. PLLFLTR Pin Voltage vs Frequency
OPERATING FREQUENCY (kHz)
200 250 300 350 550400 450 500
PLLFLTR PIN VOLTAGE (V)
3727 F05
2.5
2.0
1.5
1.0
0.5
0