RDK-138 Power Integrations, RDK-138 Datasheet - Page 8

• Manufacturer Part Number: RDK-138
• Description: KIT REF DESIGN 1.2W PS TN FAMILY
• Manufacturer: Power Integrations

Specifications of RDK-138

• Mfg Application Notes: LinkSwitch-TN Design Guide, AN-37
• Main Purpose: AC/DC, Non-Isolated
• Outputs And Type: 2, Non-Isolated
• Power - Output: 1.2W
• Voltage - Output: 12V, 5V
• Current - Output: 80mA, 50mA
• Voltage - Input: 85 ~ 265VAC
• Regulator Topology: Buck
• Frequency - Switching: 66kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LNK304
• Product: Accessories & Kits
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Other names: 596-1197

4.5 Linear Regulator
LinkSwitch-TN regulates the voltage across C5 (J5 to J3) to be 12 V. To create a second
regulated output, a linear regulator was used.
The linear regulator acts to keep the voltage at J4 centered such that the voltage from J4
to J5 is 6.9 V (12 V - 5.1 V), and -5.1 V from J4 to J5.
Zener diode VR1 defines the voltage on the emitter of Q2 (via Q1), and therefore J4, to
be 5.1 V above the voltage on J5. Should this difference in voltage across J4 and J5
reduce, then the voltage on the base of Q2 rises, increasing collector and load current
delivered to the -7 V output (J4 to J5) and maintaining the voltage difference.
Maximum dissipation within the linear regulator occurs when the +5 V (J4 to J5) is fully
loaded. At full load of 50 mA, this dissipation can be estimated as
P
P
Resistor R3 sets the bias current through VR1 to be ~ 4 mA, which also acts as a pre-
load for the 12 V output (J5 to J3). For better efficiency and lower no-load consumption,
a low test current Zener diode should be used.
An optional pre-load resistor, R5, is placed on the output to maintain better than ±5%
voltage regulation on this output. As the transistors Q1 and Q2 are physically close to
one another, they provide better thermal tracking by cancelling V
helping to maintain good regulation.
DQ2
DQ2
= V
= 5.9 V x 50 mA = 295 mW
CEQ2
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x I
CQ2
BE
variations, thereby
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