4

SuperLite™

SY55856U

Micrel, Inc.

M9999-011207

hbwhelp@micrel.com or (408) 955-1690

FUNCTIONAL DESCRIPTION

Establishing Static Logic Inputs

The true pin of a CML input pair is internally biased to

ground through a 75k

Ω resistor. The complement pin of a

CML input pair is internally biased halfway between V

CC

and ground by a voltage divider consisting of two 75k

Ω

resistors. To keep a CML input at static logic zero at V

3.0V, leave both inputs unconnected. For V

connect the complement input to V

input unconnected. To make an input static logic one,

connect the true input to V

input unconnected. These are the only safe ways to cause

CML inputs to be at a static value. In particular, no CML

input should be directly connected to ground. All NC pins in

the figures below should be left unconnected.

VT (Variable Threshold) Inputs

Five inputs to SY55856U, CINV, DELAY_SEL, S0, S1, and

S2, are variable threshold inputs. The LVL input determines

the Voltage threshold that differentiates logic high from logic

low for these five inputs only. If LVL is left unconnected, the

VT inputs will switch at about

V

GND

2

or V

whichever is higher. To obtain a logic switching threshold

different from this, the LVL input must be driven with the

actual desired threshold voltage. The user may drive the

LVL pin with any voltage between V

For example, driving LVL with a voltage set at Vcc – 1.3V

causes the VT inputs to accept single ended PECL outputs

and switch appropriately.

Note that VT inputs are internally clamped so that the

threshold will not fall below VTCL Volts. Since driving the

LVL input to ground causes the threshold to be somewhere

between V

user will keep the Voltage at the LVL pin at or above V

TCL

(max). Please refer to Figure 3 for clarification.

NC

IN

/IN

NC

NC

IN

/IN

NC

VCC

IN

/IN

Operating

Range

LVL

Input

Logic

Switching

Threshold

Figure 3a. Logic Switching Threshold

3

3.0V

≤ V

SY55856

909

Ω

1.10k

S0, S1, S2

LVL

TTL

Driver

Figure 3b. Interfacing TTL-to-CML Select

(CINV, DELAY_SEL, S0, S1, S2)

Note 1.

IN is either the DATA_IN or the CLK_IN input. /IN is either the /

DATA_IN or the /CLK_IN input.