
Board Mountable
DC-DC Converters
IMX 35 Series
Edition 3/4.2000
12/18
Synchronisation (W)
This logic input can be used to synchronise the oscillator to
an external frequency source. This pin is edge triggered
with TTL thresholds, and requires a source frequency of
490...540 kHz (duty cycle 10...90%). The external source
frequency is internally divided by 2 to define the switching
frequency for the converter. If unused, this pin can be con-
nected to V1– (pin 2) or left open-circuit.
Reference (Ref)
This option provides a stable 5 V (
±0.1 V) reference signal
on pin Ref. It is protected by a 1.33 k
resistor. This signal
may be used also in conjunction with the Trim input pin 5
(primary side) as a limited external voltage reference. We
recommend to connect a filter capacitor (0.1
F) between
Ref and Vi–, if Vref is used.
a) Adjustment by means of an external resistor
Rext:
Adjustment of the output voltage by means of an external
resistor
Rext is possible within the range of 100...105% of
Uo nom. Rext should be connected between the Trim pin 5
and Vi–. Connection of
Rext to Vi+ may damage the con-
verter. The following table indicates suitable resistor val-
ues for typical output voltages under nominal conditions
(
Ui nom, Io = 0.5 Io nom).
b) Adjustment by means of an external voltage source
Uext.
For external output voltage adjustment in the range
80...105% of
Uo nom a (0...20 V) source Uext is required,
connected to the Trim pin 5 and Vi–. The table below indi-
cates typical
Uo versus Uext values. Applying a control
voltage 15...20 V will set the converter into a hiccup
mode. Direct paralleling of the Trim pins of units of the
same type connected in parallel is feasible.
Table 11: Uo versus Uext for Uo = 80...105% Uo nom;
typical values (Ui nom, Io = 0.5 Io nom)
Uo [% Uo nom]
Uext [V]
≥105
0
102
1.8
100
2.5
95
4.25
85
8.25
80
10.2
Table 10: Rext1 for Uo > Uo nom;
approximate values (Ui nom, Io = 0.5 Io nom)
Uo [% Uo nom]
Rext [k]
105...108 (107 typically)
0
105
3.3
104
6.2
103
10
102
18
101
39
100
∞
The block diagram 2 shows the triple output units. They of-
fer a Trim1 input (pin 18) on the secondary side to adjust
Uo1. The other outputs remain unchanged. Pin 18 is not
connected. Refer to Fig. 15 for the schematic and Table 8 to
determine
Rext.
The quadruple output units are shown in block diagram 3.
All types with equal output voltage have the Trim function
connected to pin 5 referenced to the primary side which in-
fluences all outputs simultaneously. The schematics are
shown in fig. 16, the values of the adjust resistor
Rext in Ta-
ble 10 and the external voltage source in Table 11.