RDR-131 POWERINT [Power Integrations, Inc.], RDR-131 Datasheet - Page 8

• Manufacturer Part Number: RDR-131
• Description: Reference Design Report for 3 W Non- Isolated Constant Current LED Driver Using LNK306DN
• Manufacturer: POWERINT [Power Integrations, Inc.]

This voltage is divided by R7 and R1 so the voltage at the FB pin is 1.65 V when the
output voltage reaches ~12 V. Due to the interaction with the current sense circuit and
the small output capacitor value the actual peak no-load voltage is limited to ~18 V.
Current feedback is provided by sensing the voltage drop across R8 and R10, which is
filtered by R4 and C6. Once the voltage drop exceeds the VBE of Q1, both Q1 and Q2
turn on, feeding additional current into the FB pin of U1 from C2. By adjusting the ratio
of enabled to disabled cycles the average output current is controlled. As the VBE of
Q1 varies with temperature the circuit exhibits a negative output current temperature
coefficient.
Resistor R11 provides a minimum load to ensure correct operation at zero load.
This relatively complicated current sense can be simplified by using the FB pin directly
to sense the voltage drop across the sense resistors. However as the FB pin has a
voltage of 1.65 V this resulted in unacceptable dissipation (0.3 A x 1.65 V = 0.5 W)
inside the GU10 enclosure. However in less thermally challenging designs this
approach may be used.
Power Integrations, Inc.
Tel: [phone] Fax: [phone]
www.powerint.com
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