S
INGLE
-C
HIP
P
ROGRAMMABLE
PWM C
ONTROLLERS
WITH
5-B
IT
DAC
LX1662/62A, LX1663/63A
PRODUCT DATABOOK 1996/1997
Copyright © 1999
Rev. 1.2a, 11/04
8
P
RODUCTION DATA SHEET
THEORY OF OPERATION
IC OPERATION
Referring to the block diagram and typical application circuit, the
output turns ON the top MOSFET, allowing the inductor current
to increase. At the error comparator threshold, the PWM latch is
reset, the top MOSFET turns OFF and the synchronous MOSFET
turns ON. The OFF-time capacitor C
T
is now allowed to discharge.
At the valley voltage, the synchronous MOSFET turns OFF and the
top MOSFET turns on. A special break-before-make circuit
prevents simultaneous conduction of the two MOSFETs.
The V
CC_CORE
pin is offset by +40mV to enhance transient
response. The INV pin is connected to the positive side of the
current sense resistor, so the controller regulates the positive side
of the sense resistor. At light loads, the output voltage will be
regulated above the nominal setpoint voltage. At heavy loads, the
output voltage will drop below the nominal setpoint voltage. To
minimize frequency variation with varying output voltage, the
OFF-time is modulated as a function of the voltage at the V
CC_CORE
pin.
ERROR VOLTAGE COMPARATOR
The error voltage comparator compares the voltage at the positive
side of the sense resistor to the set voltage plus 40mV. An external
filter is recommended for high-frequency noise.
CURRENT LIMIT
Current limiting is done by sensing the inductor current. Exceed-
ing the current sense threshold turns the output drive OFF and
latches it OFF until the PWM latch Set input goes high again. See
Current Limit Section in "Using The LX1662/63 Devices" later in
this data sheet.
OFF-TIME CONTROL TIMING SECTION
The timing capacitor C
T
allows programming of the OFF-time. The
timing capacitor is quickly charged during the ON time of the top
MOSFET and allowed to discharge when the top MOSFET is OFF.
In order to minimize frequency variations while providing differ-
ent supply voltages, the discharge current is modulated by the
voltage at the V
CC_CORE
pin. The OFF-time is inversely proportional
to the V
CC_CORE
voltage.
UNDER VOLTAGE LOCKOUT SECTION
The purpose of the UVLO is to keep the output drive off until the
input voltage reaches the start-up threshold. At voltages below
the start-up voltage, the UVLO comparator disables the internal
biasing, and turns off the output drives, and the SS (Soft-Start) pin
is pulled low.
SYNCHRONOUS CONTROL SECTION
The synchronous control section incorporates a unique break-
before-make function to ensure that the primary switch and the
synchronous switch are not turned on at the same time. Approxi-
mately 100 nanoseconds of deadtime is provided by the break-
before-make circuitry to protect the MOSFET switches.
PROGRAMMING THE OUTPUT VOLTAGE
The output voltage is set by means of a 5-bit digital Voltage
Identification (VID) word (See Table 1). The VID code may be
incorporated into the package of the processor or the output
voltage can be set by means of a DIP switch or jumpers. For a low
or '0' signal, connect the VID pin to ground (DIP switch ON/
CLOSED); for a high or '1' signal, leave the VID pin open (DIP
switch OFF/OPEN).
The five VID pins on the LX166x series are designed to
interface directly with a Pentium Pro or Pentium II processor.
Therefore, all inputs are expected to be either ground or floating.
Any floating input will be pulled high by internal connections. If
using a Socket 7 processor, or other load, the VID code can be set
directly by connecting jumpers or DIP switches to the VID[0:4]
pins.
The VID pins are not designed to take TTL inputs, and
should not be connected high. Unpredictable output voltages
may result. If the LX166x devices are to be connected to a logic
circuit, such as BIOS, for programming of output voltage, they
should be buffered using a CMOS gate with open-drain, such as
a 74HC125 or 74C906.
POWER GOOD SIGNAL (LX1663 only)
An open collector output is provided which presents high
impedance when the output voltage is between 90% and 117% of
the programmed VID voltage, measured at the SS pin. Outside this
window the output presents a low impedance path to ground.
The Power Good function also toggles low during OVP operation.
OVER-VOLTAGE PROTECTION
The controller is inherently protected from an over-voltage
condition due to its constant OFF-time architecture. However,
should a failure occur at the power switch, an over-voltage drive
pin is provided (on the LX1663 only) which can drive an external
SCR crowbar (Q
3
), and so blow a fuse (F
1
). The fault condition
must be removed and power recycled for the LX1663 to resume
normal operation (See Figure 9).
