AN611 datasheet(3/10 Pages) SILABS | USING THE ISOVOLT DC/DC CONVERTER REFERENCE DESIGN

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AN611

Rev. 0.2

1.2. Active Clamp

The active clamp shown in Figure 3 protects the linear output regulator from excessive input voltages when the

converter is operating at an output current of 50 mA or less. At these low currents, the voltage at point “A”

increases. The R18, D3, and R20 network provides a threshold at the base of Q8 such that, as the voltage on point

A exceeds 6.8 Vdc, Q8 gradually turns-on, causing current to flow through R19 to ground and limiting the voltage

excursion at point A.

Figure 3. Active Clamp

The minimum voltage at point A must be greater than the sum of Zener voltage D3, base-emitter voltage of Q8, and

the drop across resistor R18 to turn on transistor Q8 so that a minimum load current is absorbed by resistor R19

and the transistor Q8. For example, the reference design board uses the Torex XC6220B as its 5 V output linear

regulator (maximum input voltage specification of 6.5 V). The active clamp is used to limit the regulator input

voltage to less than 6.5 V under light load conditions. The Zener diode has a value of 5.1 V, and resistor R18 is

chosen to limit the current through the Zener diode once its clamp voltage is exceeded. Current is sunk through

transistor Q8 such that the voltage at the regulator input remains below 6.5 V. Resistor R19 is appropriately

selected for power dissipation.

R19

100

R20

10K

R18

8.25K

Active Clamp

From Rectifiers and

Bulk Capacitor

To Output

Regulator

AN611 Datasheet(PDF) 3 Page - Silicon Laboratories