Datasheet

8
LTC3717
sn3717 3717fs
OPERATIO
U
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
WUUU
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.
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