Datasheet

LTC3618
12
3618fc
For more information www.linear.com/LTC3618
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.
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 LTC3618 is determined
by an external resistor that is connected between the RT
pin 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 LTC3618 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 80ns, therefore, the minimum duty cycle is
equal to 80ns • 100% • f
OSC
(Hz)
applicaTions inForMaTion
Tying the RT pin to SV
IN
sets the default internal operating
frequency to 2.25MHz.
The minimum on-time also limits the sinking current
capability for high switching frequency applications.
Figure 2 shows the
sinking current vs switching frequency
at different input voltages.
Figure 3. Soft-Start and Compensation for V
DDQ
Externally Programmed,
Compensation for V
TT
Internally Programmed
Figure 2. Sinking Current vs Switching Frequency
FREQUENCY (MHz)
–2.0
PEAK INDUCTOR CURRENT (A)
–5.0
–1.5
–1.0
–4.5
–3.0
–4.0
–3.5
–2.5
3618 G03
–0.5
5V
IN
3.3V
IN
2.5V
IN
2.5
0 1 1.5 20.5
V
TT
= 0.9V
(2×) SW1
FB1
VDDQIN
LTC3618
3618 F03
SGND
PGND
RUN1
TRACK/SS1
R
T
, 392k
ITH1
PHASE
RUN2
PGOOD2
ITH2
PGOOD1
RT
MODE/SYNC
VTTR
V
IN
3.3V
SV
IN
(2×) PV
IN1
(2×) PV
IN2
1µF47µF47µF
1µH
47µF
V
DDQ
1.8V/3A
R1
845k
R2
422k
R
SS
4.7M
C
SS
10nF
R
C
15.8k
C
C
470pF
(2×) SW2
FB2
1µH
47µF
V
TT
0.9V/±3A
10pF
0.01µF
L: VISHAY 1HLP2525BDERIROMO
10pF