SMM151/152
Preliminary Datasheet
APPLICATIONS INFORMATION (CONTINUED)
Maximizing Accuracy
Figure 7 below displays the internal trimming circuit for
a typical isolated DC-DC converter. In this example, the
converter uses positive trimming, i.e., an increase in
voltage at the TRIM pin causes an increase in output
voltage.
Maximum margining accuracy is obtained by placing a
resistor between the SMM151/152 TRIM output and the
TRIM input of the converter. From the manufacturer’s
data sheet obtain the value of the internal voltage
reference and equivalent TRIM input series resistance.
V+
+S
VREF
L
O
A
D
R1
DC-DC
Converter
TRIM
RTRIM
R2
V-
-S
SMM151/152
TRIM Pin
VREF
Figure 7 - Simplified TRIM circuit of an isolated DC-DC converter connects to SMM151/152 TRIM output
For this example RTRIM is found:
The next example applies to most non-isolated DC-DC
converters, LDO’s and in-system designed converters
using monolithic PWM controllers. Figure 8 is a
simplified schematic showing the resistor divider
network used to close the loop from the output to the
circuit’s feedback node. These type circuits employ
negative trimming, meaning any decrease in voltage
into the feedback node cause an increase in output
voltage.
VREF×k -0.3
(
)
R2×
(VREF-0.3)
R
TRIM
=
k×VREF-0.3
(
)
1-
VREF-0.3
(
)
Where:
R1× VREF-0. 3
)
R
TRIM
=
V
V
Low
k =
VNom× k-1
(
)
Nom
V
V
High
k =
0.3 = SMM151/152 TRIM output saturation voltage
VLow = Margin Low target voltage
Nom
0.3 = SMM151/152 TRIM output saturation voltage
VHIGH = Margin Low target voltage
VNom = Nominal (non-trimmed output voltage)
VREF = Converter internal reference voltage
VNom = Nominal (non-trimmed output voltage)
VREF = Converter internal reference voltage
Summit Microelectronics, Inc
2131 2.1 8/15/2008
14
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
