
       SMM764 
Preliminary Information 
Summit Microelectronics, Inc 
2098  1.1  6/29/2005 
14 
APPLICATIONS INFORMATION 
DEVICE OPERATION 
POWER SUPPLY 
The SMM764 can be powered by either a 12V input 
through the 12VIN pin or by a 3.3V or 5.0V input 
through the VDD pin.  The 12VIN pin feeds an internal 
programmable regulator that internally generates 
either 5.5V or 3.6V.  A voltage arbitration circuit allows 
the device to be powered by the highest voltage from 
either the regulator output or the VDD input.  This 
voltage arbitration circuit continuously checks for these 
voltages to determine which will power the SMM764.  
The resultant internal power supply rail is connected to 
the VDD_CAP pin that allows both filtering and hold-
up of the internal power supply. To ensure that the 
input voltage is high enough for reliable operation, an 
under voltage lockout circuit holds the controlled 
supplies off until the UVLO thresholds are met. When 
multiple Sequence-Link devices are connected, the 
same VDD and/or 12VIN supplies must power all 
devices. 
MODES OF OPERATION 
The SMM764 has four basic modes of operation 
(shown in Figures 5 through 8): power-on sequencing 
mode, ongoing operations-monitoring mode, supply 
margining mode, and power-on sequencing mode.  In 
addition, there are two features:      
ADOC and force-shutdown, which can be used during 
monitoring and margining mode. A detailed description 
of each mode and feature follows. 
ACTIVE DC OUTPUT CONTROL (ADOC
TM
) 
The SMM764 can actively control the DC output 
voltage of bricks or DC/DC converters that have a trim 
pin during monitoring and margining mode.  The 
converter may be an off-the shelf compact device, or 
may be a “roll your own” circuit on the application 
board. In either case, the SMM764 dramatically 
improves voltage accuracy (down to 0.2%) by 
implementing closed-loop ADOC active control. This 
utilizes the DC-DC’s “trim” pin as shown in Figure 12, 
or an equivalent output voltage feedback adjustment 
“VADJ”,  “FB”, or “Sense” node in a user’s custom 
circuit, Figure 13. Each of the TRIM
X
 pins on the 
SMM764 is connected to the trim input pins on the 
power supply converters.  A sense line from the 
channel’s point-of-load connects to the corresponding 
VM input.  The ADOC function cycles through all 4 
channels (A-D) every 1.7ms making slight adjustments 
to the voltage on the associated TRIM
X
 output pins 
based on the voltage inputs on the VM
X
 pins.  These 
voltage adjustments allow the SMM764 to control the 
output voltage of power supply converters to within 
±0.2% when using a ±0.1% external voltage reference.
Figure 7 - Waveform shows four SMM764 channels 
exhibiting Sequence-on to Nominal voltage, Margin 
High or Low, Nominal voltage and then sequence-off 
Ch 1 = 3.3V DC-DC converter output (Yellow trace) 
Ch 2 = 2.5V DC-DC converter output (Blue trace) 
Ch 3 = 2.0V DC-DC converter output (Purple trace)  
Ch 4 = 1.8V DC-DC converter output (Green trace) 
Figure 8 - Waveform shows three SMM764 channels 
Sequencing-on to Nominal voltage, Margin High and 
Low, and then sequence-off.  Channel 4 shows the 
HEALTHY signal.
Ch 1 = 3.3V DC-DC converter output (Yellow trace) 
Ch 2 = 2.5V DC-DC converter output (Blue trace) 
Ch 2 = 2.0V DC-DC converter output (Purple trace)       
Ch 4 = Healthy signal output (Green trace)