LTC3728 Linear Technology, LTC3728 Datasheet - Page 13

• Manufacturer Part Number: LTC3728
• Description: 2-Phase Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Figure 1 on the first page is a basic LTC3728 application
circuit. External component selection is driven by the
load requirement, and begins with the selection of R
and the inductor value. Next, the power MOSFETs and D1
are selected. Finally, C
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 LTC3728 current comparator has a maximum thresh-
old of 75mV/R
SGND to 1.1(INTV
sets the peak of the inductor current, yielding a maximum
average output current I
half the peak-to-peak ripple current, ∆I
Allowing a margin for variations in the LTC3728 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 reducton
in peak output current level depending upon the operating
duty factor.
Operating Frequency
The LTC3728 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
SENSE
SENSE
R
SENSE
Selection For Output Current
is chosen based on the required output current.
=
50
I
SENSE
MAX
mV
CC
U
and an input common mode range of
). The current comparator threshold
IN
MAX
U
and C
equal to the peak value less
OUT
W
are selected. The
L
.
U
SENSE
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
V
OUT
250
OPERATING FREQUENCY (kHz)
⎛
⎜
⎝
300
1
–
350
V
L
L
V
OUT
=0.3(I
IN
decreases with higher induc-
400
⎞
⎟
⎠
L
www.DataSheet4U.com
MAX
allows the use of low
450
). The maximum ∆I
500
LTC3728
3728 F05
550
IN
13
:
3728fb
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