LX1670

PRODUCT DA T ABOOK 1996/1997

Copyright © 1997

Rev. 1.0

7/97

6

THEOR Y OF OPERA TION

UNDER-VOLTAGE LOCKOUT

This function keeps order to the IC until the power supply voltage

reaches 85% of its nominal value. The output voltage is pulled low

and P

Once the power supply

voltage crosses 85%, the output voltage, VIDO, is released, and

P

GOOD

continues to be pulled low.

When V

SENSE

(the

microprocessor's power supply) is within ±7% of VIDO, P

released.

The comparator has hysteresis to prevent transient

oscillation.

4-BIT DAC and 5th-BIT SCALER

This function generates a precise voltage from a binary, 5-bit

digital word VID [0:4]. It uses a 4-bit, binary weighted current

DAC, driving a precise current to voltage converter. This voltage

is then op-amp buffered and sent out. The scaling is accomplished

by multiplexing the buffer op-amp. The resulting transfer function

is:

VIDO (D0,D1,D2,D3) =

(2.00 + 0.1 * (1*D0 + 2*D1 + 4*D2 + 8*D3))

for D4 = 1

VIDO (D0,D1,D2,D3) =

(2.00 + 0.6 + 0.1 * (1*D0 + 2*D1 + 4*D2 + 8*D3)) * 0.5

for D4 = 0

for the code VIDO < 1.8V will be blocked and VIDO = 0.

The zero-scale output voltage, full-scale output voltage and the

scale factor are all trimmed on-chip using fusable links.

OVER- / UNDER-VOLTAGE COMPARATORS

This function generates an open collector Power Good signal

when V

OVER-VOLTAGE PROTECTION

This function generates an OV

higher than VIDO. This voltage can be used to drive an external

SCR.

CHIP ENABLE

This function disables the chip when low. VIDO, P

are all pulled low. When ENABLE is high, the chip's state is then

determined by the other functions.

BLANK ENABLE

This function will disable VIDO for codes with voltages below

1.80V when this pin is high. When pulled low, this function will

allow VIDO to accept codes for voltages of 1.30 to 3.50V.