SMM151/152
GENERAL DESCRIPTION
The SMM151 and SMM152 are capable of margining the
DC output voltage of LDOs or DC/DC converters that
use a trim/adjust pin. The Margin function is
programmable over a standard 2-wire I
2
C serial data
interface and is used to set the margin low/high DC
output voltages. The devices are also capable of
monitoring the differential output voltage and the input
current of DC/DC converters, thereby providing real-time
power information to the system.
In margining mode the user communicates with the
SMM151/152 via the I
2
C serial data bus to select the
desired values for margining. This allows the part to
margin the supplies up or down to these set values
either through asserting the MUP and MDN pins or by
writing to the margin register directly. The margin high
and margin low voltage settings can range from 0.3V to
VDD around the converter’s nominal output voltage
setting depending on the specified margin range of the
DC-DC converter and/or system components, usually
±10%.
The SMM151/152 also feature a margin nominal
command and set-point. This can be used to set the
converter's nominal output voltage to a more accurate
value than is attainable without margining due to
inaccuracy of the standalone converter and its voltage
setting components.
When the SMM151/152 receive the command to margin,
the TRIM output will begin adjusting the supply to the
selected margin voltage. This is accomplished by
incrementing (or decrementing) an internal counter
based on the digital comparison between the voltage
margin target value and that read by the ADC from the
VM+ - VM- differential input. This operation is repeated
until the 2 values are equal, after which the
SMM151/152 hold the TRIM output pin at the voltage
required to maintain the margin setting. An I
2
C command
or de-assertion of the MUP/MDN pin will return the TRIM
output pin to a high impedance state thus allowing the
converter to return to its nominal operating voltage.
The SMM151 and SMM152 sense converter input
current using a Kelvin connected sense resistor in series
with the converter supply whose terminals are connected
to the CS+ and CS- pins. The internal ADC, also used
for measuring the converter’s output voltage, is used to
measure the converter’s input current using the voltage
dropped across the current sense resistor R
S
(see
Figure 1).
The SMM151/152 have two additional input pins and
one additional output pin. The input pins, COMP1 and
COMP2, are high impedance inputs, each connected to
a comparator and compared against the internal
reference (VREF: 0.5V or 1.25V). Each comparator can
be independently programmed to monitor for UV or OV.
When either of the COMP1 or COMP2 inputs are in fault
the open-drain FAULT# output will be pulled low. A
configuration option exists to disable the FAULT# output
during margining.
The SMM152 also provides four programmable general-
purpose inputs/outputs. The power-on state of these
I/Os is determined via non-volatile (NV) memory. Volatile
programming allows the user to select the logic level
(HIGH or LOW) of each I/O, which can also be read via
a status register.
Programming of the SMM151/152 is performed over the
industry standard I
2
C 2-wire serial data interface. A
status register is available to read the state of the part
and a Write Protect (WP) pin is available to prevent
writing to the configuration registers and EE memory.
Summit Microelectronics, Inc
2131 3.0 1/20/2010
2
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
