Datasheet

LTC3727A-1
13
3727a1fa
Figure 1 on the fi rst page is a basic LTC3727A-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 confi gured 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 LTC3727A-1 current comparator has a maximum
threshold 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 LTC3727A-1 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
criterion 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 LTC3727A-1 uses a constant frequency phase-lockable
architecture with the frequency determined by an internal
capacitor. This capacitor is charged by a fi xed 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
Information 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
APPLICATIONS INFORMATION
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
is increased the gate charge losses will be higher, reducing
effi ciency (see Effi ciency 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 effi ciency. A higher
frequency generally results in lower effi ciency 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
inductance 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.