ML1565 datasheet(19/21 Pages) MINILOGIC

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P19/21

Rev. C, Sep 2005

ML1565

Applications Information (2)

Multiple Output Flyback Circuits

Some applications require multiple voltages from a single

converter channel. This is often the case when generating

voltages for CCD bias or LCD power. Figure 8 shows a

two-output flyback configuration with AUX_ controller. The

controller drives an external MOSFET that switches the

transformer

primary.

Two

transformer

secondaries

generate the output voltages. Only one positive output

voltage can be feedback, so the other voltages are set by

the turn ratio of the transformer secondaries. The load

stability of the other secondary voltages depends on

transformer leakage inductance and winding resistance.

Voltage regulation is best when the load on the secondary

with feedback is small when compared to the load on the

one that is. Regulation also improves if the load current

range is limited. Consult the transformer manufacturer for

the proper design for a given application.

Figure 8. +15V and -7.5V CCD Bias with Transformer

Boost with Charge Pump for Positive and Negative

Outputs

Negative output voltages can be produced without a

transformer, using a charge-pump circuit with an auxiliary

controller as shown in Figure 9. When MOSFET Q1 turns

off, the voltage at its drain rises to supply current to VOUT+.

At the same time, C1 charges to the voltage VOUT+

through D1.

Figure 9. ±15V Output Using a Boost with

Charge-Pump Inversion

When the MOSFET turns on, C1 discharges through D3,

thereby charging C3 to VOUT- minus the drop across D3 to

create roughly the same voltage as VOUT+ at VOUT - but with

inverted polarity. If different magnitudes are required for the

positive and negative voltages, a linear regulator can be

used at one of the outputs to achieve the desired voltages.

ML1565

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