LTC3617_15 Datasheet, PDF (10/20 Pages) Linear Technology – 6A Monolithic Synchronous Step-Down Regulator for DDR Termination

LTC3617
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 • 10¹¹Hz
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%.
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.
V_{IN LTC3617}
SV_IN
RT
SYNC
SGND
f_OSC
2.25MHz
V_IN
0.4V
LTC3617
SV_IN
RT
SYNC
R_T
SGND
f_OSC ∝1/R_T
V_IN
2.5V
R_PG
100k
PGOOD
R_C
10k
C_C
680pF
R_T
365k
SV_IN
RUN
VDDQIN
RT
PV_IN
VTTR
LTC3617
PGOOD
SW
SGND
C_IN
22µF
×4
L1
0.33µH
VTTR
1.25V
0.1µF ^±10mA
V_OUT
1.25V
C_{OUT ±6A}
100µF
C_C1
10pF
(OPT)
ITH
SYNC
PGND
V_FB
3617 F01
Figure 1. 1.25V, ±6A at 1MHz from 2.5V
V_{IN LTC3617}
R_T
SV_IN
RT
f_OSC
1/T_P
SYNC
SGND
1.2V
0.3V
T_P
3617 F02
Figure 2. Setting Switching Frequency
3617fa
10