LTC3717 Datasheet, PDF (8/20 Pages) Linear Technology – Wide Operating Range, No RSENSE Step-Down Controller for DDR/QDR Memory Termination

LTC3717
U
OPERATIO
Main Control Loop
The LTC3717 is a current mode controller for DC/DC
step-down converters. In normal operation, the top
MOSFET is turned on for a fixed interval determined by a
one-shot timer OST. When the top MOSFET is turned off,
the bottom MOSFET is turned on until the current com-
parator I_CMP trips, restarting the one-shot timer and initi-
ating the next cycle. Inductor current is determined by
sensing the voltage between the PGND and SW pins using
the bottom MOSFET on-resistance . The voltage on the I_TH
pin sets the comparator threshold corresponding to in-
ductor valley current. The error amplifier EA adjusts this
I_TH voltage by comparing 2/3 of the feedback signal V_FB
from the output voltage with a reference equal to 1/3 of the
V_REF voltage. If the load current increases, it causes a drop
in the feedback voltage relative to the reference. The I_TH
voltage then rises until the average inductor current again
matches the load current. As a result in normal DDR
operation V_OUT is equal to 1/2 of the V_REF voltage.
The operating frequency is determined implicitly by the
top MOSFET on-time and the duty cycle required to
maintain regulation. The one-shot timer generates an on-
time that is proportional to the ideal duty cycle, thus
holding frequency approximately constant with changes
in V_IN. The nominal frequency can be adjusted with an
external resistor R_ON.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ±10% window around the regulation point.
Furthermore, in an overvoltage condition, M1 is turned off
and M2 is turned on and held on until the overvoltage
condition clears.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Releasing
the pin allows an internal 1.2µA current source to charge
up an external soft-start capacitor C_SS. When this voltage
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I_TH voltage clamped at approximately
0.6V below the RUN/SS voltage. As C_SS continues to
charge, the soft-start current limit is removed.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
INTV_CC pin. The top MOSFET driver is powered from a
floating bootstrap capacitor C_B. This capacitor is re-
charged from INTV_CC through an external Schottky diode
D_B when the top MOSFET is turned off. When the EXTV_CC
pin is grounded, an internal 5V low dropout regulator
supplies the INTV_CC power from V_CC. If EXTV_CC rises
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV_CC to INTV_CC. This allows
a high efficiency source connected to EXTV_CC, such as an
external 5V supply or a secondary output from the
converter, to provide the INTV_CC power. Voltages up to
7V can be applied to EXTV_CC for additional gate drive. If
the V_CC voltage is low and INTV_CC drops below 3.4V,
undervoltage lockout circuitry prevents the power
switches from turning on.
APPLICATIO S I FOR ATIO
The basic LTC3717 application circuit is shown in
Figure 1. External component selection is primarily deter-
mined by the maximum load current and begins with the
selection of the sense resistance and power MOSFET
switches. The LTC3717 uses the on-resistance of the syn-
chronous power MOSFET for determining the inductor
current. The desired amount of ripple current and operating
frequency largely determines the inductor value. Finally, C_IN
is selected for its ability to handle the large RMS current into
the converter and C_OUT is chosen with low enough ESR to
meet the output voltage ripple and transient specification.
8
Maximum Sense Voltage and V_RNG Pin
Inductor current is determined by measuring the voltage
across a sense resistance that appears between the PGND
and SW pins. The maximum sense voltage is set by the
voltage applied to the V_RNG pin and is equal to approxi-
mately (0.13)V_RNG for sourcing current and (0.17)V_RNG for
sinking current. The current mode control loop will not
allow the inductor current valleys to exceed (0.13)V_RNG/
R_SENSE for sourcing and (0.17)V_RNG/R_SENSE for sinking. In
practice, one should allow some margin for variations in
sn3717 3717fs