SMM150
Preliminary Information
APPLICATIONS INFORMATION (CONTINUED)
Maximizing Accuracy
Maximum margining accuracy is obtained by placing a
resistor between the SMM150 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.
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.
V+
+S
VREF
L
R1
R2
O
A
D
DC-DC
Converter
TRIM
RTRIM
V-
-S
SMM150
TRIM Pin
VREF
Figure 7 - Simplified TRIM circuit of an isolated DC-DC converter connects to SMM150 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)
RTRIM
=
k×VREF-0.3
(
)
1-
VREF-0.3
(
)
Where:
R1× VREF-0.3
)
R
TRIM
=
VM arg(Low)
VNom× k-1
k =
(
)
VNom
VM arg(High)
k =
VNom
0.3 = TRIM output saturation voltage
Vnom = Nominal (non-trimmed output voltage)
0.3 = TRIM output saturation voltage
Vnom = Nominal (non-trimmed output voltage)
VOUT
R1
RTRIM
SMM150
TRIM Pin
To FB node
(VREF)
R2
Figure 8 - Simplified TRIM circuit of a non-isolated DC-DC converter connects to SMM150 TRIM output
Summit Microelectronics, Inc
2075 2.6 05/13/05
12
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
