TNY274-280
In a typical implementation, the EN/UV pin is driven by an
optocoupler. The collector of the optocoupler transistor is
connected to the EN/UV pin and the emitter is connected to the
SOURCE pin. The optocoupler LED is connected in series with
a Zener diode across the DC output voltage to be regulated.
When the output voltage exceeds the target regulation voltage
level (optocoupler LED voltage drop plus Zener voltage), the
optocoupler LED will start to conduct, pulling the EN/UV pin
low. The Zener diode can be replaced by a TL43ꢀ reference
circuit for improved accuracy.
ON/OFF Operation with Current Limit State Machine
The internal clock of the TinySwitch-III runs all the time. At the
beginning of each clock cycle, it samples the EN/UV pin to
decide whether or not to implement a switch cycle, and based
on the sequence of samples over multiple cycles, it determines
the appropriate current limit. At high loads, the state machine
sets the current limit to its highest value. At lighter loads, the
state machine sets the current limit to reduced values.
V
V
EN
EN
CLOCK
CLOCK
DC
DC
MAX
MAX
I
I
DRAIN
DRAIN
V
V
DRAIN
DRAIN
PI-2749-082305
PI-2667-082305
Figure 6. Operation at Near Maximum Loading.
Figure 7. Operation at Moderately Heavy Loading.
V
V
EN
EN
CLOCK
CLOCK
DC
DC
MAX
MAX
I
I
DRAIN
DRAIN
V
V
DRAIN
DRAIN
PI-2661-082305
PI-2377-082305
Figure 9. Operation at Very Light Load.
Figure 8. Operation at Medium Loading.
5
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Rev. I 01/09
POWERINT [ Power Integrations ]
