Datasheet

LTC3615/LTC3615-1
15
3615fb
For more information www.linear.com/LTC3615
Operating Frequency
Selection of the operating frequency is a trade-off between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency of the LTC3615 is determined
by an external resistor that is connected between pin R
T
/
SYNC and ground. The value of the resistor sets the ramp
current that is used to charge and discharge an internal
timing capacitor within the oscillator and can be calculated
by using the following equation:
R
T
=
4 • 10
11
ΩHz
f
OSC
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3615 imposes a minimum
limit on the operating duty cycle. The minimum on-time
applicaTions inForMaTion
is typically 60ns, therefore, the minimum duty cycle is
equal to 60ns • 100% • f
OSC
(Hz)
Tying the R
T
/SYNC pin to SV
IN
sets the default internal
operating frequency to 2.25MHz ±20%.
Frequency Synchronization
The LTC3615’s internal oscillator can be synchronized to
an external frequency by applying a square wave clock
signal to the R
T
/SYNC pin. During synchronization, the
top MOSFET turn-on of channel 1 is locked to the rising
edge of the external frequency source. The synchronization
frequency range is 400kHz to 4MHz. The internal slope
compensation is automatically adapted to the external
clock frequency.
In the signal path from the R
T
/SYNC clock input to the
SW output, the LTC3615 is processing the external clock
frequency through an internal PLL.
After detecting an external clock on the first rising edge
of R
T
/SYNC the PLL starts up with the internal default of
2.25MHz. The internal PLL then requires a certain number
Figure 3. Soft-Start and Compensation for Channel 1 Externally Programmed,
Soft-Start and Compensation for Channel 2 Internally Programmed
(2×) SW1
FB1
MODELTC3615
3615 F03
SGND
PGND
RUN1
TRACK/SS1
R
T
, 200k
ITH1
PHASE
RUN2
TRACK/SS2
PGOOD2
ITH2
PGOOD1
R
T
/SYNC
SRLIM
V
IN
3.3V
SV
IN
(2×) PV
IN1
(2×) PV
IN2
1µF47µF47µF
0.47µH
47µF
V
OUT1
1.8V/3A
R1
422k
R2
29.4k
R3
178k
R
SS
4.7M
C
SS
10nF
R
C
15k
C
C
1000pF
(2×) SW2
FB2
0.47µH
47µF
V
OUT2
2.5V/3A
R5
665k
R4
210k
10pF
R
SRLIM
40.2k
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