Application Note
2 September 2008
PDF: margin-volt_prog_an.pdf
Application Guidelines for Non-Isolated Converters
AN04-005: Margining and Voltage Programming
Introduction
The Austin Lynx
TM
, Lynx II, TLynx and Naos Raptor
series of non-isolated modules all have the ability to
adjust their output voltage by interfacing through the Trim
pin. In addition to the output voltage set function, two
other types of output voltage adjustment are frequently
used. The first is margining, where the output voltage is
adjusted a fixed percentage (typically ±5 or 10%) off the
nominal during system level test. This adjustment is
generally used for checking the operational margin of
circuits and thereby assuring the robustness of the
application. The second is voltage programming where
an external voltage is provided to either provide more
accurate adjustment of the output voltage of the module
or to implement a margining-like function. This
application note describes circuits and provides design
guidelines for implementing margining or voltage
programming.
To understand how margining or voltage programming
are done, it is beneficial to understand the circuit used to
scale down the output voltage in the Austin Lynx and
Lynx II series of modules. Figure 1 shows a portion of the
output voltage divider and feedback circuitry that is used
in all Lynx and Lynx II series modules (except the fixed-
output voltage 5/3.3V and 12V input Lynx modules –
these come in different output voltage versions and do
not have an wide-output voltage adjust capability). The
Austin Lynx or
Lynx II Series Module
PWM
Controller
-
R1
output voltage is scaled down by a divider consisting of
resistors R1 and R2. The mid point of the divider is
connected to the inverting input of the op-amp which is
used to process the error voltage between the internal
reference voltage Vref (0.7V) and the divider midpoint
voltage. The voltage control loop of the module regulates
the output voltage so as to maintain the voltage at the
mid point of the divider equal to the reference voltage.
The trim pin is connected through a 1KΩ resistor to the
mid point of the divider. By connecting an external
resistor between the Trim pin and GND, the divider ratio
is changed thus allowing adjustment of the output
voltage. In both margining and voltage programming, the
output voltage is adjusted by in essence effecting a
change in the scaling between the output and voltage
divider mid point voltages.
In the TLynx and Naos Raptor series of modules, a
simpler circuit is used to divide down the voltage as
shown in Fig. 2. However, the same functionality is
obtained is obtained through the TRIM pin.
TLynx or
Naos Raptor Module
PWM
Controller
-
Vout
R1
AMP
+
VREF
Trim
Vout
GND
R3
Trim
R2
AMP
+
VREF
3.3Vin TLynx
12Vin TLynx
3A/6A/10A/20A Naos Raptor
40A/50A/60A Naos Raptor
R1
2K
10K
2K
2K
VREF
0.6V
0.591V
0.591V
0.6V
GND
Fig. 2. Circuit diagram of the output voltage divider
and feedback circuitry used in the TLynx and Naos
Raptor series of modules.
VREF
0.7V
0.7V
0.8V
R1
5Vin Austin Lynx or
Lynx II
12Vin Austin Lynx
or Lynx II
Austin MegaLynx
30.1K
15K
1.5K
R2
402K
200K
Open
R3
1K
1K
100
Implementing Margining
Margining is implemented by essentially changing the
divider by small amounts to obtain the desired margining
voltages. One method to accomplish this is shown in Fig.
3. This scheme connects a pre-determined resistor with a
small FET in series between the trim pin and ground (to
margin up or slightly increase the output voltage), and a
similar FET+resistor combination in series between TRIM
Fig. 1. Circuit diagram of the output voltage divider
and feedback circuitry used in the Austin Lynx and
Lynx II series of modules (except fixed-output 5/3.3V
in Austin Lynx and 12V in Austin Lynx modules).
LINEAGE
POWER
1
LINEAGEPOWER [ LINEAGE POWER CORPORATION ]
