SMM151 View Datasheet(PDF) - Summit Microelectronics
Part Name
Description
MFG CO.
'SMM151' PDF : 23 Pages View PDF
SMM151/152
Preliminary Datasheet
APPLICATIONS INFORMATION (CONTINUED)
Maximizing Accuracy
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.
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+
VREF
+S
R1
L
DC-DC
O
Converter
A
D
RTRIM
SMM151/152
TRIM
R2
VREF
V-
TRIM Pin
-S
Figure 7 - Simplified TRIM circuit of an isolated DC-DC converter connects to SMM151/152 TRIM output
For this example RTRIM is found:
RTRIM
=
R2×
1-
(VREF×k) -0.3
(VREF -0.3)
(k×VREF -0.3)
(VREF -0.3)
Where:
k = VLow
VNom
0.3 = SMM151/152 TRIM output saturation voltage
VLow = Margin Low target voltage
VNom = Nominal (non-trimmed output voltage)
VREF = Converter internal reference voltage
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.
RTRIM
=
R1×(VREF -0.3)
VNom× ( k -1)
k = VHigh
VNom
0.3 = SMM151/152 TRIM output saturation voltage
VHIGH = Margin Low target voltage
VNom = Nominal (non-trimmed output voltage)
VREF = Converter internal reference voltage
Summit Microelectronics, Inc
2131 2.1 8/15/2008
14
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