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AN611 Datasheet(PDF) 3 Page - Silicon Laboratories

Part No. AN611
Description  USING THE ISOVOLT DC/DC CONVERTER REFERENCE DESIGN
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Maker  SILABS [Silicon Laboratories]
Homepage  http://www.silabs.com
Logo SILABS - Silicon Laboratories

AN611 Datasheet(HTML) 3 Page - Silicon Laboratories

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AN611
Rev. 0.2
3
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
D3
Q8
 Active Clamp
From Rectifiers and 
Bulk Capacitor
To Output 
Regulator
A


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