LTC3728_15 Datasheet, PDF (15/36 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIONS INFORMATION
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_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 configured 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
LTC3728 current comparator has a maximum threshold
of 75mV/R_SENSE and an input common mode range of
SGND to 1.1(INTV_CC). 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 LTC3728 and external
component values yields:
R_SENSE
=
50mV
I_MAX
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ΔV_SENSE = ΔI • R_SENSE
also needs to be checked in the design to get good sig-
nal-to-noise ratio. In general, for a reasonable good PCB
layout, a 15mV ΔV_SENSE voltage is recommended as a
conservative number to start with.
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 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
2.5
2.0
1.5
1.0
0.5
0
200 250 300 350 400 450 500 550
OPERATING FREQUENCY (kHz)
3728 F05
Figure 5. PPLFLTR Pin Voltage vs Frequency
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
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 ΔI_L decreases with higher
inductance or frequency and increases with higher V_IN:

I_L
=
1
(f)(L)
V_OUT


1–
V_OUT
V_IN


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) or higher for good
3728fg
15