LTC3816_15 Datasheet, PDF (28/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs

LTC3816
APPLICATIONS INFORMATION
V_OUT
100mV/DIV
VID5
1V/DIV
DPRSLPVR
5V/DIV
V_IN = 12V
0.1ms/DIV
V_OUT = 0.75V TO 1.15V
C_SLEW = 47pF
IMVP-6 CONFIGURATION
3816 F15
Figure 15. Programmable VID Slew Rate
the controller reduces the I_CSLEW pull-up current from 40µA
to 10µA (deeper sleep mode). This effectively reduces the
VID DAC slew rate to 1/4 of its original value. If IMVP-6.5
is selected, the slow slew rate function is disabled.
Pulse-Skipping and Forced Continuous Mode
Operation
The LTC3816 can operate in one of two modes select-
able with the MODE/SYNC pin: pulse-skipping mode
or forced continuous mode. Shorting the MODE/SYNC
pin to INTV_CC selects pulse-skipping mode. Pulse-skip-
ping mode is selected when high efficiency at very light
loads is desired. In this mode, when the inductor current
reverses, the bottom MOSFET turns off to minimize the
efficiency loss due to reverse current flow. This reduces
the conduction loss and slightly improves the efficiency.
As the load reduces, the top gate duty cycle shrinks to
maintain regulation. The LTC3816 is capable of operating
at extremely low duty cycles; hence, TG will continue to
run at a constant switching frequency until the top gate
on-time is less than 40ns to 50ns. When the load decreases
beyond this point, the LTC3816 TG begins to skip cycles
to maintain regulation. The driver switching frequency
drops, which further improves efficiency by minimizing
gate charge losses.
Forcing the MODE/SYNC pin low enables forced continuous
mode operation. In forced continuous mode, the bottom
MOSFET is always on when the top MOSFET is off, allowing
the inductor current to reverse at low currents. This mode
is less efficient due to conduction and switching losses,
but has the advantage of better transient response at low
currents, constant frequency operation, and the ability to
maintain regulation when sinking current.
During soft-start, the LTC3816 forces the controller to
operate in pulse-skipping mode until the switcher output
voltage reaches its V_BOOT power good threshold. During
VID code transitions, however, the controller always oper-
ates in forced continuous mode to allow the switcher to
sink current.
Operating Frequency/Frequency
Synchronization
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency operation
increases efficiency by reducing MOSFET switching losses,
but requires a larger inductance and/or capacitance to
maintain low output voltage ripple. For converters with
high step-down V_IN-to-V_OUT ratios, another consideration
is the minimum on-time of the converter.
t_ON(MIN)
=
V_OUT
V_IN(MAX) •
f_OSC
If the MODE/SYNC pin is not driven by an external clock,
the RFREQ pin voltage configures the LTC3816 free-run-
ning switching frequency. Floating or shorting the RFREQ
pin to INTV_CC allows the controller to run at the nominal
400kHz frequency. Connecting the RFREQ pin to GND
selects 210kHz. Tying RFREQ to a potential between 2.5V
and 3.5V selects 580kHz. The RFREQ pin has an internal
3816f
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