LTC3728LXCUH#TR Linear Technology, LTC3728LXCUH#TR Datasheet - Page 16

• Manufacturer Part Number: LTC3728LXCUH#TR
• Description: IC CTRLR SW REG 2-PH SYNC 32-QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3728LXCUH#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 7 V
• Current - Output: 3A
• Frequency - Switching: 250kHz ~ 550kHz
• Voltage - Input: 4.5 ~ 28 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS INFORMATION
LTC3728L/LTC3728LX
Figure 1 on the fi rst 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 confi gured 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, ΔI
Allowing a margin for variations in the IC and external
component values yields:
Because of possible PCB layout-induced noise in the
current sensing loop, the AC current sensing ripple of
ΔV
design to get good signal-to-noise ratio. In general, for
a reasonably good PCB layout, a 15mV ΔV
is recommended as a conservative design starting point.
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 reduction in
peak output current level depending upon the operating
duty factor.
Operating Frequency
The IC uses a constant-frequency, phase-lockable ar-
chitecture 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
16
SENSE
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
= ΔI • R
=
CC
50mV
). The current comparator threshold sets the
I
and an input common mode range of SGND
MAX
MAX
SENSE
equal to the peak value less half the
also needs to be checked in the
L
IN
.
and C
OUT
SENSE
are selected.
voltage
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
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
inductance or frequency and increases with higher V
Accepting larger values of ΔI
ductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ΔI = 30% of maximum output current or
higher for good load transient response and suffi cient
ripple current signal in the current loop.
I
L
=
Figure 5. PLLFLTR Pin Voltage vs Frequency
(f)(L)
1
2.5
2.0
1.5
1.0
0.5
0
200
V
OUT
OPERATING FREQUENCY (kHz)
300
1–
V
V
OUT
400
IN
L
allows the use of low in-
L
decreases with higher
500
3728 F05
600
3728lxfe
IN
: