LTC3815_15 Datasheet, PDF (20/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Applications Information
The basic LTC3815 application circuit is shown in Figure 8.
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 out-
put ripple voltage. The operating frequency of the LTC3815
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:
( ) ^RT
=
1.15 •
f_OSC
10¹¹
1.11
Ω
Frequencies as high as 3MHz are possible, as long as the
minimum on-time requirement is met (see next section).
Tying the RT pin to V_IN sets the default internal operating
frequency to 1MHz ±15%.
The LTC3815’s internal oscillator can be synchronized to an
external frequency by applying a square wave clock signal
to the MODE/SYNC pin. During synchronization, the top
switch turn-on is locked to the rising edge of the external
frequency source. The synchronization frequency range is
300kHz to 3MHz. During synchronization, discontinuous
operation is disabled.
The internal PLL has a synchronization range of ±30%
around its programmed frequency. Therefore, during
external clock synchronization be sure that the external
clock frequency is within this ±30% range of the R_T pro-
grammed frequency.
When using the RT pin to program the oscillator frequency,
a square wave clock that is running 180° out-of-phase with
the internal oscillator is available at the CLKOUT pin for
connection to a second LTC3815 for 2-phase operation
(see the 2-Phase section).
V_IN
2.25V TO 5.5V
PMBUS
C_SS
15nF
C_SLEW
1nF
R_T
11.8k, 1%
R_REF2
3.7k
0.5%
R_REF1
14.3k
0.5%
PV_IN
V_IN
SCL, SDA, ALERT MODE/SYNC
MARGIN
WP
PGOOD
CLKOUT
SS/TRACK
RUN_MSTR
RUN_STBY
LTC3815
SW
V_{CC_SENSE}
CSLEW
V_{SS_SENSE}
RT
PGLIM
REF
ASEL
DA_OUT
V_FB
I_TH
PGFD
R_ASEL
R_PGFD
L
300nH
R_C1
1k
R_C2
10k
C_C2
1nF
C_C1
47pF
Figure 8. 1.8V, 6A Step-Down Regulator
C_IN
22µF
×4
C_OUT
100µF
×2
V_OUT
1.8V
6A
3815 F08
3815p
20
For more information www.linear.com/LTC3815