LX1662/62A, LX1663/63A

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.1 11/99

8

THEOR Y OF OPERA TION

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

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

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

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

to the V

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

The fault condition

must be removed and power recycled for the LX1663 to resume

normal operation (See Figure 9).