BCD20007-G Rev AD, 4-Nov-09
Page 10 of 15
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IMX7/IMS7 DC-DC Series Data Sheet
7-Watt DC-DC Converters
Table 7: Vext for Vo = 75 to 105% Vo nom;
typical values (Vi nom, Io1 = Io2 = 0.5 Io1/2 nom)
Vo [% Vo nom]
Vext [V]
≥105
0
102
1.6
95
4.5
85
9
75
13
Single-output models with R-input:
The R input (pin 13) is referenced to the secondary side of the
converter. Adjustment of the output voltage is possible by
means of either an external resistor or a voltage source.
Fig. 13
Output voltage control with external resistor
R
Vo+
Vo–
Vi+
Vi–
R
ext2
R
ext1
06131
Fig. 14
Output voltage adjust for single-output models
R
Vo+
Vo-
+
V
ext
-
4 k
Ω
Vref = 2.5 V
Control
logic
R
ext1
R
ext 2
06029b
Vi–
Vi+
a) Adjustment by means of an external resistor:
Depending upon the value of the required output voltage,
a resistor Rext1 or Rext2 shall be connected as shown in the
figure below:
Either: Rext1 between the R pin and Vo– to achieve an
output voltage adjustment range of Vo = 75 to 100% of
Vo nom (85 to 100% for 3.3 V outputs):
Vo
Rext1
≈ 4 kΩ –––––––––
Vo nom – Vo
or: Rext2 between the R pin and Vo+ to achieve an output
voltage range of approximately Vo = 100 to 105 % of Vo nom:
(Vo – 2.5 V)
Rext2
≈ 4 kΩ ––––––––––––––––––
2.5 V (Vo/Vo nom – 1)
b) Adjustment by means of an external voltage Vext between
Vo– and R pin.
The control voltage is 1.96 to 2.62 V and allows for
adjustment in the range of 75 to 105% of Vo nom.
Vo 2.5 V
Vext
≈ ––––––––
Vo nom
Caution: Applying an external voltage >2.75 V may damage the
converter.
Note: Attempting to adjust the output below this range will cause
the converter to shut down (hiccup mode).
Trim
Vo1+
Vo2–
+
V
ext
–
Vi+
Vi–
R
ext
Vo1–
Vo2+
06089a
Control
circuit
Vref 2.5 V
Fig. 15
Output voltage adjust for double-output models
Double-output models with Trim input:
The Trim input (pin 4) of double-output models is referenced
to the primary side. The figure below shows the circuitry.
a) Adjustment by means of an external resistor Rext.
Programming of the output voltage by means of an
external resistor Rext1 is possible within a limited range of
100 to 105% Vo nom. Rext should be connected between pin
4 and Vi –. The following table indicates suitable resistor
values
for
typical
output
voltages
under
nominal
conditions (Vi nom, Io = 0.5 Io nom), with paralleled outputs or
equal load conditions on each output.
Caution: Connection of Rext to Vi+ may damage the converter.
Table 6: Rext for Vo > Vo nom
approximate values (Vi nom, Io1 = Io2 = 0.5 Io1/2 nom)
Vo [% Vo nom]
Rext [k
Ω
Ω]
105 to 108 (107 typically)
0
105
1.5
104
5.6
103
12
102
27
101
68
100
∞
Adjustment of the output voltage is possible in the range of
100 to 105% of Vo nom using an external resistor, or in the range
of 75 to 105% of Vo nom using an external voltage source.
b) Adjustment by means of an external voltage source Vext.
For external output voltages in the range of 75 to 105% of
Vo nom a voltage source Vext (0 to 20 V) is required,
connected to the Trim-input (pin 4) and Vi–. The table
below indicates typical Vo versus Vext values under
nominal conditions (Vi nom, Io = 0.5 Io nom), with paralleled
outputs or equal load conditions on each output. Direct
paralleling of the Trim-inputs of converters connected in
parallel is feasible.
Note: Applying a control voltage greater than 20 V will set the
converter into a hiccup mode.