Datasheet

LTC3617
10
3617fa
applications inForMation
The basic LTC3617 application circuit is shown in Figure 1.
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 voltage ripple.
The operating frequency of the LTC3617 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
=
3.82 • 10
11
Hz
f
OSC
Hz
( )
Ω – 16kΩ
Although frequencies as high as 4MHz are possible, the
minimum on-time of the LTC3617 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 • f
OSC
(Hz)•100%.
RUN
VDDQIN
RT
PGOOD
PGOOD
ITH
SGND
PGND
V
IN
2.5V
VTTR
SV
IN
LTC3617
SW
PV
IN
C
IN
22µF
×4
C
C
680pF
L1
0.33µH
3617 F01
SYNC
V
FB
C
OUT
100µF
V
OUT
1.25V
±6A
0.1µF
VTTR
1.25V
±10mA
R
C
10k
R
T
365k
R
PG
100k
C
C1
10pF
(OPT)
Figure 1. 1.25V, ±6A at 1MHz from 2.5V
Tying the RT pin to SV
IN
sets the default internal operating
frequency to 2.25MHz ±20%.
Frequency Synchronization
The LTC3617’s internal oscillator can be synchronized to
an external frequency by applying a square wave clock
signal to the SYNC pin. During synchronization, the top
switch turn-on is locked to the falling edge of the external
frequency source. The synchronization frequency range
is 300kHz to 4MHz.
The frequency set by the resistor on the RT pin should be
the same as the external clock frequency to ensure the
internal oscillator properly adjusts when the clock signal
is applied or removed.
LTC3617
SV
IN
V
IN
SYNC
SGND
LTC3617
SV
IN
f
OSC
2.25MHz
f
OSC
1/T
P
f
OSC
∝1/R
T
LTC3617
SV
IN
V
IN
SYNC
SGND
V
IN
SYNC
SGND
T
P
1.2V
0.3V
3617 F02
RT
RT
RT
0.4V
R
T
R
T
Figure 2. Setting Switching Frequency