LTC1628CG-SYNC View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1628CG-SYNC' PDF : 32 Pages View PDF
LTC1628-SYNC
APPLICATIO S I FOR ATIO
Figure 1 on the first page is a basic LTC1628-SYNC
application circuit. External component selection is driven
by the load requirement, and begins with the selection of
RSENSE and the inductor value. Next, the power MOSFETs
and D1 are selected. Finally, CIN and COUT 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).
RSENSE Selection For Output Current
RSENSE is chosen based on the required output current.
The LTC1628-SYNC current comparator has a maximum
threshold of 75mV/RSENSE and an input common mode
range of SGND to 1.1(INTVCC). The current comparator
threshold sets the peak of the inductor current, yielding a
maximum average output current IMAX equal to the peak
value less half the peak-to-peak ripple current, ∆IL.
Allowing a margin for variations in the LTC1628-SYNC and
external component values yields:
RSENSE
=
50mV
IMAX
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 LTC1628-SYNC 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 Appli-
cations 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
2.5
2.0
1.5
1.0
0.5
0
120
170
220
270
320
OPERATING FREQUENCY (kHz)
1628 F05
Figure 5. PLLFLTR Pin Voltage vs Frequency
is increased the gate charge losses will be higher, reducing
efficiency (see Efficiency Considerations). The maximum
switching frequency is approximately 310kHz.
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 ∆IL decreases with higher induc-
tance or frequency and increases with higher VIN:
∆IL
=
1
(f)(L)
VOUT 1–
VOUT
VIN
Accepting larger values of ∆IL 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 ∆IL=0.3(IMAX). The maximum ∆IL
occurs at the maximum input voltage.
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