LNK302/304-306
Topology
Low-Side
Basic Circuit Schematic
Key Features
±. Output referenced to input
Buck-Boost –
Optocoupler
Feedback
2. Positive output (VO) with respect to +VIN
3. Step up/down – VO > VIN or VO < VIN
4. Optocoupler feedback
+
LinkSwitch-TN
- Accuracy only limited by reference choice
- Low cost non-safety rated optocoupler
- No pre-load required
VIN
VO
BP
FB
+
1. Fail-safe – output is not subjected to input
voltage if the internal power MOSFET fails
6. Minimum no-load consumption
D
S
PI-3756-041509
Table 2 (cont). Common Circuit Configurations Using LinkSwitch-TN.
should not exceed the rated ripple voltage divided by the typical
current limit of the chosen LinkSwitch-TN.
Component Selection
Referring to Figure 1, the following considerations may be
helpful in selecting components for a LinkSwitch-TN design.
Feedback Resistors R1 and R3
The values of the resistors in the resistor divider formed by R±
and R3 are selected to maintain ±.61 V at the FEEDBACK pin. It
is recommended that R3 be chosen as a standard ±5 resistor
of 2 kΩ. This ensures good noise immunity by biasing the
feedback network with a current of approximately 0.8 mA.
Freewheeling Diode D1
Diode D± should be an ultrafast type. For MDCM, reverse
recovery time tRR ≤71 ns should be used at a temperature of
70 °C or below. Slower diodes are not acceptable, as continuous
mode operation will always occur during startup, causing high
leading edge current spikes, terminating the switching cycle
prematurely, and preventing the output from reaching regulation.
If the ambient temperature is above 70 °C then a diode with tRR
≤31 ns should be used.
Feedback Capacitor C3
Capacitor C3 can be a low cost general purpose capacitor. It
provides a “sample and hold” function, charging to the output
voltage during the off time of LinkSwitch-TN. Its value should
be ±0 mF to 22 mF; smaller values cause poorer regulation at
light load conditions.
For CCM an ultrafast diode with reverse recovery time tRR ≤31 ns
should be used. A slower diode may cause excessive leading
edge current spikes, terminating the switching cycle prematurely
and preventing full power delivery.
Pre-Load Resistor R4
In high-side, direct feedback designs where the minimum load
is <3 mA, a pre-load resistor is required to maintain output
regulation. This ensures sufficient inductor energy to pull the
inductor side of the feedback capacitor C3 to input return via
D2. The value of R4 should be selected to give a minimum
output load of 3 mA.
Fast and slow diodes should never be used as the large reverse
recovery currents can cause excessive power dissipation in the
diode and/or exceed the maximum drain current specification
of LinkSwitch-TN.
In designs with an optocoupler the Zener or reference bias
current provides a ± mA to 2 mA minimum load, preventing
“pulse bunching” and increased output ripple at zero load.
Feedback Diode D2
Diode D2 can be a low-cost slow diode such as the ±N400X
series, however it should be specified as a glass passivated
type to guarantee a specified reverse recovery time. To a first
order, the forward drops of D± and D2 should match.
LinkSwitch-TN Layout Considerations
In the buck or buck-boost converter configuration, since the
SOURCE pins in LinkSwitch-TN are switching nodes, the
copper area connected to SOURCE should be minimized to
minimize EMI within the thermal constraints of the design.
Inductor L1
Choose any standard off-the-shelf inductor that meets the
design requirements. A “drum” or “dog bone” “I” core inductor
is recommended with a single ferrite element due to its low cost
and very low audible noise properties. The typical inductance
value and RMS current rating can be obtained from the
LinkSwitch-TN design spreadsheet available within the PI Expert
design suite from Power Integrations. Choose L± greater than
or equal to the typical calculated inductance with RMS current
rating greater than or equal to calculated RMS inductor current.
In the boost configuration, since the SOURCE pins are tied to
DC return, the copper area connected to SOURCE can be
maximized to improve heat sinking.
The loop formed between the LinkSwitch-TN, inductor (L±),
freewheeling diode (D±), and output capacitor (C2) should be
kept as small as possible. The BYPASS pin capacitor C±
(Figure 6) should be located physically close to the SOURCE (S)
and BYPASS (BP) pins. To minimize direct coupling from
switching nodes, the LinkSwitch-TN should be placed away
Capacitor C2
The primary function of capacitor C2 is to smooth the inductor
current. The actual output ripple voltage is a function of this
capacitor’s ESR. To a first order, the ESR of this capacitor
7
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Rev. J 06/13
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