LTC1909-8 LINER [Linear Technology], LTC1909-8 Datasheet - Page 14

• Manufacturer Part Number: LTC1909-8
• Description: Wide Operating Range,No RSENSE TM Step-Down DC/DC Controller with SMBus Programming
• Manufacturer: LINER [Linear Technology]

OPERATIO
LTC1909-8
The LTC1909-8 consists of two independent sections: a
current mode controller for the DC/DC step-down con-
verter and a SMBus VID voltage programmer. It simplifies
the design of SMBus controlled power supplies.
Current Mode Controller
In normal operation, the top MOSFET of the current mode
controller 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 PGND and SENSE
using either the bottom MOSFET on-resistance or a sepa-
rate sense resistor. The voltage on the I
comparator threshold corresponding to inductor valley
current. The error amplifier EA adjusts this voltage by
comparing the feedback signal V
with an internal 0.8V reference. The feedback voltage is
derived from the output voltage by a resistive divider in the
SMBus VID programmer. If the load current increases, it
causes a drop in the feedback voltage relative to the
reference. The I
inductor current again matches the load current.
At low load currents, the inductor current can drop to zero
and become negative. This is detected by current reversal
comparator I
discontinuous operation. Both switches will remain off
with the output capacitor supplying the load current until
the I
initiate another cycle. Discontinuous mode operation is
disabled by comparator F when the FCB pin is brought
below 0.8V, forcing continuous synchronous operation.
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 7.5% window around the regulation point. Fur-
thermore, in an overvoltage condition, M1 is turned off and
14
IN
TH
. The nominal frequency can be adjusted with an
voltage rises above the zero current level (0.8V) to
CMP
trips, restarting the one-shot timer and initi-
REV
TH
U
which then shuts off M2, resulting in
ON
voltage then rises until the average
.
(Refer to Functional Diagram)
FB
from the output voltage
TH
pin sets the
+
pins
M2 is turned on and held on until the overvoltage condition
clears.
Foldback current limiting is provided if the output is
shorted to ground. As V
threshold voltage I
level set by Q4 and Q6. This reduces the inductor valley
current level to one sixth of its maximum value as V
approaches 0V.
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. Shorting
the RUN/SS pin to the CPUON pin of the SMBus VID
programmer puts the regulator under software control.
The open-drain CPUON pin does not interfere with the
soft-start cycle.
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 and V
dropout regulator supplies the INTV
EXTV
off, and an internal switch connects EXTV
This allows a high efficiency source connected to EXTV
such as an external 5V supply or a secondary output from
the converter, to provide the INTV
7V can be applied to EXTV
input voltage is low and INTV
undervoltage lockout circuitry prevents the power switches
from turning on.
SMBus VID Voltage Programmer
The SMBus interface is used to program the divider (to set
the output voltage of the DC/DC converter) and to shut
down the current mode controller or allow it to soft-start.
B
when the top MOSFET is turned off. When the EXTV
CC
CC
CC
/EXTV
pin. The top MOSFET driver is powered from a
rises above 4.7V, the internal regulator is turned
CC
Power
THB
TH
CC
through an external Schottky diode
voltage clamped at approximately
is pulled down by clamp Q3 to a 1V
RUN/SS
CC
FB
for additional gate drive. If the
drops, the buffered current
>1.5V, an internal 5V low
B
. This capacitor is re-
CC
CC
SS
power. Voltages up to
CC
drops below 3.5V,
. When this voltage
power from V
SS
CC
continues to
to INTV
IN
19098f
CC
CC
. If
CC
FB
.
,