SI88240ED-IS Datasheet, PDF (20/48 Pages) Silicon Laboratories – QUAD DIGITAL ISOLATORS WITH DC-DC CONVERTER

Si88x4x
2.3. DC-DC Converter Application Information
The Si88xx isolated dc-dc converter is based on a modified fly-back topology and uses an external transformer and
Schottky rectifying diode for low cost and high operating efficiency. The PWM controller operates in closed-loop,
peak current mode control and generates isolated output voltages with 2 W average output power at 5.0 V. Options
are available for 24 Vdc input or output operation and externally configured switching frequency.
The dc-dc controller modulates a pair of internal primary-side power switches (see Figure 8) to generate an
isolated voltage at external diode D1 cathode. Closed-loop feedback is provided by a compensated error amplifier,
which compares the voltage at the VSNS pin to an internal voltage reference. The resulting error voltage is fed
back through the isolation barrier via an internal feedback path to the controller, thus completing the control loop.
For higher input supply voltages than 5 V, an external FET Q2 is modulated by a driver pin ESW as shown in (see
Figure 9). A shunt resistor based voltage sense pin RSN provides current sensing capability to the controller.
Additional features include an externally-triggered shutdown of the converter functionality using the SH pin and a
programmable soft start configured by a capacitor connected to the SS pin. The Si88xx can be used in low- or high-
voltage configurations. These features and configurations are explained in more detail below.
2.3.1. Shutdown
This feature allows the operation of the dc-dc converter to be shut down when asserted high. This function is
provided by pin 6 (labeled “SH” on the Si882xx) and pin 7 (labeled “SH_FC” on the Si883xx and Si886xx). This
feature is not available on the Si884xx. Pin 6 or pin 7 provide the exact same functionality and shut down the dc-dc
converter when asserted high. For normal operation, pins 6 and 7 should be connected to ground.
2.3.2. Soft-Start
The dc-dc controller has an internal timer that controls the power conversion start-up to limit inrush current. There
is also the Soft Start option where users can program the soft start up by an external capacitor connected to the SS
pin. This feature is available on the Si883xx and the Si886xx.
2.3.3. Programmable Frequency
The frequency of the PWM modulator is set to a default of 250 kHz for Si882xx/4xx. Users can program their
desired frequency within a given band of 200 kHz to 800 kHz by controlling the time constant of an external RC
connected to the SH_FC and SS pins for Si883xx/6xx.
2.3.4. External Transformer Driver
The dc-dc controller has internal switches (VSW) for driving the transformer with up-to a 5.5 V voltage supply. For
higher voltages on the primary side, a driver output (ESW) is provided that can drive an external NMOS power
transistor for driving the transformer. When this configuration is used, a shunt resistor based voltage sense pin
(RSN) provides current sensing to the controller.
2.3.5. VREGA, VREGB
For supporting voltages greater than 5.5 V, an internal voltage regulator (VREGA, VREGB) needs to be used in
conjunction with an external NPN transistor, a resistor and a capacitor to provide regulated voltage to the IC.
2.3.6. Output Voltage Control
The isolated output voltage (VOUT) is sensed by a resistor divider that provides feedback to the controller through
the VSNS pin. The voltage error is encoded and transmitted back to the primary side controller across the isolation
barrier, which in turn changes the duty cycle of the transformer driver. The equation for VOUT is as follows:
VOUT
=
VSN
S



1
+
RR_-----12_{--}
+ R1  I_OFFSET
20
Preliminary Rev. 0.6