TNY284-290
Enable Function
TinySwitch-4 senses the ENABLE/UNDERVOLTAGE pin to
determine whether or not to proceed with the next switching
cycle. The sequence of cycles is used to determine the current
limit. Once a cycle is started, it always completes the cycle
(even when the ENABLE/UNDERVOLTAGE pin changes state
half way through the cycle). This operation results in a power
supply in which the output voltage ripple is determined by the
output capacitor, amount of energy per switch cycle and the
delay of the feedback.
The ENABLE/UNDERVOLTAGE pin signal is generated on the
secondary by comparing the power supply output voltage with
a reference voltage. The ENABLE/UNDERVOLTAGE pin signal
is high when the power supply output voltage is less than the
reference voltage. In a typical implementation, the ENABLE/
UNDERVOLTAGE pin is driven by an optocoupler. The collector
of the optocoupler transistor is connected to the ENABLE/
UNDERVOLTAGE pin and the emitter is connected to the
SOURCE pin. The optocoupler LED is connected in series with
V
EN
CLOCK
DC
V
EN
CLOCK
DC
MAX
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2749-082305
PI-2667-082305
Figure 7.
Operation at Near Maximum Loading.
Figure 8.
Operation at Moderately Heavy Loading.
V
EN
CLOCK
DC
V
EN
CLOCK
DC
MAX
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2377-082305
PI-2661-082305
Figure 9.
Operation at Medium Loading.
Figure 10. Operation at Very Light Load.
5
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Rev. A 09/12
POWERINT [ POWER INTEGRATIONS, INC. ]
