LTC3728LX Linear Technology, LTC3728LX Datasheet - Page 13

• Manufacturer Part Number: LTC3728LX
• Description: 2-Phase Synchronous Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Figure 1 on the first page is a basic LTC3728L/LTC3728LX
application circuit. External component selection is driven
by the load requirement, and begins with the selection of
R
and D1 are selected. Finally, C
The circuit shown in Figure 1 can be configured for
operation up to an input voltage of 28V (limited by the
external MOSFETs).
R
R
The current comparator has a maximum threshold of
75mV/R
to 1.1(INTV
peak of the inductor current, yielding a maximum average
output current I
peak-to-peak ripple current, I
Allowing a margin for variations in the IC and external
component values yields:
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 reduction
in peak output current level depending upon the operating
duty factor.
Operating Frequency
The IC uses a constant frequency phase-lockable architec-
ture with the frequency determined by an internal capaci-
tor. 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
SENSE
SENSE
SENSE
R
SENSE
and the inductor value. Next, the power MOSFETs
Selection For Output Current
is chosen based on the required output current.
SENSE
CC
50
). The current comparator threshold sets the
and an input common mode range of SGND
I
MAX
MAX
mV
U
equal to the peak value less half the
U
L
IN
.
and C
W
OUT
are selected.
U
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 I
tance or frequency and increases with higher V
Accepting larger values of I
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is I
occurs at the maximum input voltage.
I
L
Figure 5. PLLFLTR Pin Voltage vs Frequency
( )( )
f L
2.5
2.0
1.5
1.0
0.5
1
0
200
LTC3728L/LTC3728LX
V
OUT
OPERATING FREQUENCY (kHz)
300
1
–
V
L
L
V
=0.3(I
OUT
400
decreases with higher induc-
IN
L
www.DataSheet4U.com
MAX
allows the use of low
500
). The maximum I
3728 F05
600
IN
13
:
3728lxfa
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