ltc3717-1 Linear Technology Corporation, ltc3717-1 Datasheet - Page 8

• Manufacturer Part Number: ltc3717-1
• Description: Wide Operating Range, No Rsense Step-down Controller For Ddr/qdr Memory Termination
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3717-1
Main Control Loop
The LTC3717-1 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
ating the next cycle. Inductor current is determined by
sensing the voltage between the SENSE
using the bottom MOSFET on-resistance . The voltage on
the I
to inductor valley current. The error amplifier EA adjusts
this I
V
of the V
a drop in the feedback voltage relative to the reference. The
I
again matches the load current. As a result in normal DDR
operation V
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
external resistor R
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a
8
TH
FB
voltage then rises until the average inductor current
from the output voltage with a reference equal to 1/3
IN
TH
TH
. The nominal frequency can be adjusted with an
pin sets the comparator threshold corresponding
REF
voltage by comparing 2/3 of the feedback signal
CMP
10% window around the regulation point.
voltage. If the load current increases, it causes
OUT
trips, restarting the one-shot timer and initi-
is equal to 1/2 of the V
U
ON
.
+
REF
and SENSE
voltage.
–
pins
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 1.2 A current source to charge
up an external soft-start capacitor C
reaches 1.5V, the controller turns on and begins switch-
ing, but with the I
0.6V below the RUN/SS voltage. As C
charge, the soft-start current limit is removed.
INTV
Power for the top and bottom MOSFET drivers and most
of the internal controller circuitry is derived from the
INTV
floating bootstrap capacitor C
charged from INTV
D
pin is grounded, an internal 5V low dropout regulator
supplies the INTV
above 4.7V, the internal regulator is turned off, and an
internal switch connects EXTV
a high efficiency source connected to EXTV
external 5V supply or a secondary output from the
converter, to provide the INTV
7V can be applied to EXTV
the V
undervoltage lockout circuitry prevents the power
switches from turning on.
B
when the top MOSFET is turned off. When the EXTV
CC
CC
CC
/EXTV
pin. The top MOSFET driver is powered from a
voltage is low and INTV
CC
Power
TH
CC
CC
through an external Schottky diode
voltage clamped at approximately
power from V
CC
for additional gate drive. If
CC
CC
B
. This capacitor is re-
to INTV
power. Voltages up to
CC
SS
CC
drops below 3.4V,
. When this voltage
. If EXTV
SS
CC
CC
continues to
. This allows
, such as an
sn37171 37171fs
CC
rises
CC